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diff --git a/Documentation/vm/transhuge.rst b/Documentation/vm/transhuge.rst deleted file mode 100644 index a8cf6809e36e..000000000000 --- a/Documentation/vm/transhuge.rst +++ /dev/null @@ -1,197 +0,0 @@ -.. _transhuge: - -============================ -Transparent Hugepage Support -============================ - -This document describes design principles Transparent Hugepage (THP) -Support and its interaction with other parts of the memory management. - -Design principles -================= - -- "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) - -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). - -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. |