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diff --git a/Documentation/vm/transhuge.rst b/Documentation/vm/transhuge.rst new file mode 100644 index 000000000000..569d182cc973 --- /dev/null +++ b/Documentation/vm/transhuge.rst @@ -0,0 +1,573 @@ +.. _transhuge: + +============================ +Transparent Hugepage Support +============================ + +Objective +========= + +Performance critical computing applications dealing with large memory +working sets are already running on top of libhugetlbfs and in turn +hugetlbfs. Transparent Hugepage Support is an alternative means of +using huge pages for the backing of virtual memory with huge pages +that supports the automatic promotion and demotion of page sizes and +without the shortcomings of hugetlbfs. + +Currently it only works for anonymous memory mappings and tmpfs/shmem. +But in the future it can expand to other filesystems. + +The reason applications are running faster is because of two +factors. The first factor is almost completely irrelevant and it's not +of significant interest because it'll also have the downside of +requiring larger clear-page copy-page in page faults which is a +potentially negative effect. The first factor consists in taking a +single page fault for each 2M virtual region touched by userland (so +reducing the enter/exit kernel frequency by a 512 times factor). This +only matters the first time the memory is accessed for the lifetime of +a memory mapping. The second long lasting and much more important +factor will affect all subsequent accesses to the memory for the whole +runtime of the application. The second factor consist of two +components: 1) the TLB miss will run faster (especially with +virtualization using nested pagetables but almost always also on bare +metal without virtualization) and 2) a single TLB entry will be +mapping a much larger amount of virtual memory in turn reducing the +number of TLB misses. With virtualization and nested pagetables the +TLB can be mapped of larger size only if both KVM and the Linux guest +are using hugepages but a significant speedup already happens if only +one of the two is using hugepages just because of the fact the TLB +miss is going to run faster. + +Design +====== + +- "graceful fallback": mm components which don't have transparent hugepage + knowledge fall back to breaking huge pmd mapping into table of ptes and, + if necessary, split a transparent hugepage. Therefore these components + can continue working on the regular pages or regular pte mappings. + +- if a hugepage allocation fails because of memory fragmentation, + regular pages should be gracefully allocated instead and mixed in + the same vma without any failure or significant delay and without + userland noticing + +- if some task quits and more hugepages become available (either + immediately in the buddy or through the VM), guest physical memory + backed by regular pages should be relocated on hugepages + automatically (with khugepaged) + +- it doesn't require memory reservation and in turn it uses hugepages + whenever possible (the only possible reservation here is kernelcore= + to avoid unmovable pages to fragment all the memory but such a tweak + is not specific to transparent hugepage support and it's a generic + feature that applies to all dynamic high order allocations in the + kernel) + +Transparent Hugepage Support maximizes the usefulness of free memory +if compared to the reservation approach of hugetlbfs by allowing all +unused memory to be used as cache or other movable (or even unmovable +entities). It doesn't require reservation to prevent hugepage +allocation failures to be noticeable from userland. It allows paging +and all other advanced VM features to be available on the +hugepages. It requires no modifications for applications to take +advantage of it. + +Applications however can be further optimized to take advantage of +this feature, like for example they've been optimized before to avoid +a flood of mmap system calls for every malloc(4k). Optimizing userland +is by far not mandatory and khugepaged already can take care of long +lived page allocations even for hugepage unaware applications that +deals with large amounts of memory. + +In certain cases when hugepages are enabled system wide, application +may end up allocating more memory resources. An application may mmap a +large region but only touch 1 byte of it, in that case a 2M page might +be allocated instead of a 4k page for no good. This is why it's +possible to disable hugepages system-wide and to only have them inside +MADV_HUGEPAGE madvise regions. + +Embedded systems should enable hugepages only inside madvise regions +to eliminate any risk of wasting any precious byte of memory and to +only run faster. + +Applications that gets a lot of benefit from hugepages and that don't +risk to lose memory by using hugepages, should use +madvise(MADV_HUGEPAGE) on their critical mmapped regions. + +sysfs +===== + +Transparent Hugepage Support for anonymous memory can be entirely disabled +(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE +regions (to avoid the risk of consuming more memory resources) or enabled +system wide. This can be achieved with one of:: + + echo always >/sys/kernel/mm/transparent_hugepage/enabled + echo madvise >/sys/kernel/mm/transparent_hugepage/enabled + echo never >/sys/kernel/mm/transparent_hugepage/enabled + +It's also possible to limit defrag efforts in the VM to generate +anonymous hugepages in case they're not immediately free to madvise +regions or to never try to defrag memory and simply fallback to regular +pages unless hugepages are immediately available. Clearly if we spend CPU +time to defrag memory, we would expect to gain even more by the fact we +use hugepages later instead of regular pages. This isn't always +guaranteed, but it may be more likely in case the allocation is for a +MADV_HUGEPAGE region. + +:: + + echo always >/sys/kernel/mm/transparent_hugepage/defrag + echo defer >/sys/kernel/mm/transparent_hugepage/defrag + echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag + echo madvise >/sys/kernel/mm/transparent_hugepage/defrag + echo never >/sys/kernel/mm/transparent_hugepage/defrag + +always + means that an application requesting THP will stall on + allocation failure and directly reclaim pages and compact + memory in an effort to allocate a THP immediately. This may be + desirable for virtual machines that benefit heavily from THP + use and are willing to delay the VM start to utilise them. + +defer + means that an application will wake kswapd in the background + to reclaim pages and wake kcompactd to compact memory so that + THP is available in the near future. It's the responsibility + of khugepaged to then install the THP pages later. + +defer+madvise + will enter direct reclaim and compaction like ``always``, but + only for regions that have used madvise(MADV_HUGEPAGE); all + other regions will wake kswapd in the background to reclaim + pages and wake kcompactd to compact memory so that THP is + available in the near future. + +madvise + will enter direct reclaim like ``always`` but only for regions + that are have used madvise(MADV_HUGEPAGE). This is the default + behaviour. + +never + should be self-explanatory. + +By default kernel tries to use huge zero page on read page fault to +anonymous mapping. It's possible to disable huge zero page by writing 0 +or enable it back by writing 1:: + + echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page + echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page + +Some userspace (such as a test program, or an optimized memory allocation +library) may want to know the size (in bytes) of a transparent hugepage:: + + cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size + +khugepaged will be automatically started when +transparent_hugepage/enabled is set to "always" or "madvise, and it'll +be automatically shutdown if it's set to "never". + +khugepaged runs usually at low frequency so while one may not want to +invoke defrag algorithms synchronously during the page faults, it +should be worth invoking defrag at least in khugepaged. However it's +also possible to disable defrag in khugepaged by writing 0 or enable +defrag in khugepaged by writing 1:: + + echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag + echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag + +You can also control how many pages khugepaged should scan at each +pass:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan + +and how many milliseconds to wait in khugepaged between each pass (you +can set this to 0 to run khugepaged at 100% utilization of one core):: + + /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs + +and how many milliseconds to wait in khugepaged if there's an hugepage +allocation failure to throttle the next allocation attempt:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs + +The khugepaged progress can be seen in the number of pages collapsed:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed + +for each pass:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/full_scans + +``max_ptes_none`` specifies how many extra small pages (that are +not already mapped) can be allocated when collapsing a group +of small pages into one large page:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none + +A higher value leads to use additional memory for programs. +A lower value leads to gain less thp performance. Value of +max_ptes_none can waste cpu time very little, you can +ignore it. + +``max_ptes_swap`` specifies how many pages can be brought in from +swap when collapsing a group of pages into a transparent huge page:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap + +A higher value can cause excessive swap IO and waste +memory. A lower value can prevent THPs from being +collapsed, resulting fewer pages being collapsed into +THPs, and lower memory access performance. + +Boot parameter +============== + +You can change the sysfs boot time defaults of Transparent Hugepage +Support by passing the parameter ``transparent_hugepage=always`` or +``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` +to the kernel command line. + +Hugepages in tmpfs/shmem +======================== + +You can control hugepage allocation policy in tmpfs with mount option +``huge=``. It can have following values: + +always + Attempt to allocate huge pages every time we need a new page; + +never + Do not allocate huge pages; + +within_size + Only allocate huge page if it will be fully within i_size. + Also respect fadvise()/madvise() hints; + +advise + Only allocate huge pages if requested with fadvise()/madvise(); + +The default policy is ``never``. + +``mount -o remount,huge= /mountpoint`` works fine after mount: remounting +``huge=never`` will not attempt to break up huge pages at all, just stop more +from being allocated. + +There's also sysfs knob to control hugepage allocation policy for internal +shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount +is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or +MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem. + +In addition to policies listed above, shmem_enabled allows two further +values: + +deny + For use in emergencies, to force the huge option off from + all mounts; +force + Force the huge option on for all - very useful for testing; + +Need of application restart +=========================== + +The transparent_hugepage/enabled values and tmpfs mount option only affect +future behavior. So to make them effective you need to restart any +application that could have been using hugepages. This also applies to the +regions registered in khugepaged. + +Monitoring usage +================ + +The number of anonymous transparent huge pages currently used by the +system is available by reading the AnonHugePages field in ``/proc/meminfo``. +To identify what applications are using anonymous transparent huge pages, +it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages fields +for each mapping. + +The number of file transparent huge pages mapped to userspace is available +by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``. +To identify what applications are mapping file transparent huge pages, it +is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields +for each mapping. + +Note that reading the smaps file is expensive and reading it +frequently will incur overhead. + +There are a number of counters in ``/proc/vmstat`` that may be used to +monitor how successfully the system is providing huge pages for use. + +thp_fault_alloc + is incremented every time a huge page is successfully + allocated to handle a page fault. This applies to both the + first time a page is faulted and for COW faults. + +thp_collapse_alloc + is incremented by khugepaged when it has found + a range of pages to collapse into one huge page and has + successfully allocated a new huge page to store the data. + +thp_fault_fallback + is incremented if a page fault fails to allocate + a huge page and instead falls back to using small pages. + +thp_collapse_alloc_failed + is incremented if khugepaged found a range + of pages that should be collapsed into one huge page but failed + the allocation. + +thp_file_alloc + is incremented every time a file huge page is successfully + allocated. + +thp_file_mapped + is incremented every time a file huge page is mapped into + user address space. + +thp_split_page + is incremented every time a huge page is split into base + pages. This can happen for a variety of reasons but a common + reason is that a huge page is old and is being reclaimed. + This action implies splitting all PMD the page mapped with. + +thp_split_page_failed + is incremented if kernel fails to split huge + page. This can happen if the page was pinned by somebody. + +thp_deferred_split_page + is incremented when a huge page is put onto split + queue. This happens when a huge page is partially unmapped and + splitting it would free up some memory. Pages on split queue are + going to be split under memory pressure. + +thp_split_pmd + is incremented every time a PMD split into table of PTEs. + This can happen, for instance, when application calls mprotect() or + munmap() on part of huge page. It doesn't split huge page, only + page table entry. + +thp_zero_page_alloc + is incremented every time a huge zero page is + successfully allocated. It includes allocations which where + dropped due race with other allocation. Note, it doesn't count + every map of the huge zero page, only its allocation. + +thp_zero_page_alloc_failed + is incremented if kernel fails to allocate + huge zero page and falls back to using small pages. + +As the system ages, allocating huge pages may be expensive as the +system uses memory compaction to copy data around memory to free a +huge page for use. There are some counters in ``/proc/vmstat`` to help +monitor this overhead. + +compact_stall + is incremented every time a process stalls to run + memory compaction so that a huge page is free for use. + +compact_success + is incremented if the system compacted memory and + freed a huge page for use. + +compact_fail + is incremented if the system tries to compact memory + but failed. + +compact_pages_moved + is incremented each time a page is moved. If + this value is increasing rapidly, it implies that the system + is copying a lot of data to satisfy the huge page allocation. + It is possible that the cost of copying exceeds any savings + from reduced TLB misses. + +compact_pagemigrate_failed + is incremented when the underlying mechanism + for moving a page failed. + +compact_blocks_moved + is incremented each time memory compaction examines + a huge page aligned range of pages. + +It is possible to establish how long the stalls were using the function +tracer to record how long was spent in __alloc_pages_nodemask and +using the mm_page_alloc tracepoint to identify which allocations were +for huge pages. + +get_user_pages and follow_page +============================== + +get_user_pages and follow_page if run on a hugepage, will return the +head or tail pages as usual (exactly as they would do on +hugetlbfs). Most gup users will only care about the actual physical +address of the page and its temporary pinning to release after the I/O +is complete, so they won't ever notice the fact the page is huge. But +if any driver is going to mangle over the page structure of the tail +page (like for checking page->mapping or other bits that are relevant +for the head page and not the tail page), it should be updated to jump +to check head page instead. Taking reference on any head/tail page would +prevent page from being split by anyone. + +.. note:: + these aren't new constraints to the GUP API, and they match the + same constrains that applies to hugetlbfs too, so any driver capable + of handling GUP on hugetlbfs will also work fine on transparent + hugepage backed mappings. + +In case you can't handle compound pages if they're returned by +follow_page, the FOLL_SPLIT bit can be specified as parameter to +follow_page, so that it will split the hugepages before returning +them. Migration for example passes FOLL_SPLIT as parameter to +follow_page because it's not hugepage aware and in fact it can't work +at all on hugetlbfs (but it instead works fine on transparent +hugepages thanks to FOLL_SPLIT). migration simply can't deal with +hugepages being returned (as it's not only checking the pfn of the +page and pinning it during the copy but it pretends to migrate the +memory in regular page sizes and with regular pte/pmd mappings). + +Optimizing the applications +=========================== + +To be guaranteed that the kernel will map a 2M page immediately in any +memory region, the mmap region has to be hugepage naturally +aligned. posix_memalign() can provide that guarantee. + +Hugetlbfs +========= + +You can use hugetlbfs on a kernel that has transparent hugepage +support enabled just fine as always. No difference can be noted in +hugetlbfs other than there will be less overall fragmentation. All +usual features belonging to hugetlbfs are preserved and +unaffected. libhugetlbfs will also work fine as usual. + +Graceful fallback +================= + +Code walking pagetables but unaware about huge pmds can simply call +split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by +pmd_offset. It's trivial to make the code transparent hugepage aware +by just grepping for "pmd_offset" and adding split_huge_pmd where +missing after pmd_offset returns the pmd. Thanks to the graceful +fallback design, with a one liner change, you can avoid to write +hundred if not thousand of lines of complex code to make your code +hugepage aware. + +If you're not walking pagetables but you run into a physical hugepage +but you can't handle it natively in your code, you can split it by +calling split_huge_page(page). This is what the Linux VM does before +it tries to swapout the hugepage for example. split_huge_page() can fail +if the page is pinned and you must handle this correctly. + +Example to make mremap.c transparent hugepage aware with a one liner +change:: + + diff --git a/mm/mremap.c b/mm/mremap.c + --- a/mm/mremap.c + +++ b/mm/mremap.c + @@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru + return NULL; + + pmd = pmd_offset(pud, addr); + + split_huge_pmd(vma, pmd, addr); + if (pmd_none_or_clear_bad(pmd)) + return NULL; + +Locking in hugepage aware code +============================== + +We want as much code as possible hugepage aware, as calling +split_huge_page() or split_huge_pmd() has a cost. + +To make pagetable walks huge pmd aware, all you need to do is to call +pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the +mmap_sem in read (or write) mode to be sure an huge pmd cannot be +created from under you by khugepaged (khugepaged collapse_huge_page +takes the mmap_sem in write mode in addition to the anon_vma lock). If +pmd_trans_huge returns false, you just fallback in the old code +paths. If instead pmd_trans_huge returns true, you have to take the +page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the +page table lock will prevent the huge pmd to be converted into a +regular pmd from under you (split_huge_pmd can run in parallel to the +pagetable walk). If the second pmd_trans_huge returns false, you +should just drop the page table lock and fallback to the old code as +before. Otherwise you can proceed to process the huge pmd and the +hugepage natively. Once finished you can drop the page table lock. + +Refcounts and transparent huge pages +==================================== + +Refcounting on THP is mostly consistent with refcounting on other compound +pages: + + - get_page()/put_page() and GUP operate in head page's ->_refcount. + + - ->_refcount in tail pages is always zero: get_page_unless_zero() never + succeed on tail pages. + + - map/unmap of the pages with PTE entry increment/decrement ->_mapcount + on relevant sub-page of the compound page. + + - map/unmap of the whole compound page accounted in compound_mapcount + (stored in first tail page). For file huge pages, we also increment + ->_mapcount of all sub-pages in order to have race-free detection of + last unmap of subpages. + +PageDoubleMap() indicates that the page is *possibly* mapped with PTEs. + +For anonymous pages PageDoubleMap() also indicates ->_mapcount in all +subpages is offset up by one. This additional reference is required to +get race-free detection of unmap of subpages when we have them mapped with +both PMDs and PTEs. + +This is optimization required to lower overhead of per-subpage mapcount +tracking. The alternative is alter ->_mapcount in all subpages on each +map/unmap of the whole compound page. + +For anonymous pages, we set PG_double_map when a PMD of the page got split +for the first time, but still have PMD mapping. The additional references +go away with last compound_mapcount. + +File pages get PG_double_map set on first map of the page with PTE and +goes away when the page gets evicted from page cache. + +split_huge_page internally has to distribute the refcounts in the head +page to the tail pages before clearing all PG_head/tail bits from the page +structures. It can be done easily for refcounts taken by page table +entries. But we don't have enough information on how to distribute any +additional pins (i.e. from get_user_pages). split_huge_page() fails any +requests to split pinned huge page: it expects page count to be equal to +sum of mapcount of all sub-pages plus one (split_huge_page caller must +have reference for head page). + +split_huge_page uses migration entries to stabilize page->_refcount and +page->_mapcount of anonymous pages. File pages just got unmapped. + +We safe against physical memory scanners too: the only legitimate way +scanner can get reference to a page is get_page_unless_zero(). + +All tail pages have zero ->_refcount until atomic_add(). This prevents the +scanner from getting a reference to the tail page up to that point. After the +atomic_add() we don't care about the ->_refcount value. We already known how +many references should be uncharged from the head page. + +For head page get_page_unless_zero() will succeed and we don't mind. It's +clear where reference should go after split: it will stay on head page. + +Note that split_huge_pmd() doesn't have any limitation on refcounting: +pmd can be split at any point and never fails. + +Partial unmap and deferred_split_huge_page() +============================================ + +Unmapping part of THP (with munmap() or other way) is not going to free +memory immediately. Instead, we detect that a subpage of THP is not in use +in page_remove_rmap() and queue the THP for splitting if memory pressure +comes. Splitting will free up unused subpages. + +Splitting the page right away is not an option due to locking context in +the place where we can detect partial unmap. It's also might be +counterproductive since in many cases partial unmap happens during exit(2) if +a THP crosses a VMA boundary. + +Function deferred_split_huge_page() is used to queue page for splitting. +The splitting itself will happen when we get memory pressure via shrinker +interface. |
