diff options
| -rw-r--r-- | Documentation/vm/transhuge.txt | 151 |
1 files changed, 96 insertions, 55 deletions
diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt index 8a282687ee06..21cf34f3ddb2 100644 --- a/Documentation/vm/transhuge.txt +++ b/Documentation/vm/transhuge.txt @@ -35,10 +35,10 @@ miss is going to run faster. == Design == -- "graceful fallback": mm components which don't have transparent - hugepage knowledge fall back to breaking a transparent hugepage and - working on the regular pages and their respective regular pmd/pte - mappings +- "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 @@ -221,9 +221,18 @@ 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_split is incremented every time a huge page is split into base +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 is incremented if kernel fails to split huge + page. This can happen if the page was pinned by somebody. + +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 @@ -274,10 +283,8 @@ 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 (while serializing properly against -split_huge_page() to avoid the head and tail pages to disappear from -under it, see the futex code to see an example of that, hugetlbfs also -needed special handling in futex code for similar reasons). +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 @@ -312,9 +319,9 @@ unaffected. libhugetlbfs will also work fine as usual. == Graceful fallback == Code walking pagetables but unware about huge pmds can simply call -split_huge_page_pmd(vma, addr, pmd) where the pmd is the one returned by +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_page_pmd where +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 @@ -323,7 +330,8 @@ 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. +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: @@ -335,14 +343,14 @@ diff --git a/mm/mremap.c b/mm/mremap.c return NULL; pmd = pmd_offset(pud, addr); -+ split_huge_page_pmd(vma, addr, pmd); ++ 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_page_pmd() has a cost. +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 @@ -351,47 +359,80 @@ 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 -mm->page_table_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_page can run in parallel to 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 should run pmd_trans_splitting on the pmd. In -case pmd_trans_splitting returns true, it means split_huge_page is -already in the middle of splitting the page. So if pmd_trans_splitting -returns true it's enough to drop the page_table_lock and call -wait_split_huge_page and then fallback the old code paths. You are -guaranteed by the time wait_split_huge_page returns, the pmd isn't -huge anymore. If pmd_trans_splitting returns false, you can proceed to -process the huge pmd and the hugepage natively. Once finished you can -drop the page_table_lock. - -== compound_lock, get_user_pages and put_page == +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 ->_count. + + - ->_count 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). + +PageDoubleMap() indicates that ->_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. + +We set PG_double_map when a PMD of the page got split for the first time, +but still have PMD mapping. The addtional references go away with last +compound_mapcount. 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 do that easily for refcounts taken by huge pmd -mappings. But the GUI API as created by hugetlbfs (that returns head -and tail pages if running get_user_pages on an address backed by any -hugepage), requires the refcount to be accounted on the tail pages and -not only in the head pages, if we want to be able to run -split_huge_page while there are gup pins established on any tail -page. Failure to be able to run split_huge_page if there's any gup pin -on any tail page, would mean having to split all hugepages upfront in -get_user_pages which is unacceptable as too many gup users are -performance critical and they must work natively on hugepages like -they work natively on hugetlbfs already (hugetlbfs is simpler because -hugetlbfs pages cannot be split so there wouldn't be requirement of -accounting the pins on the tail pages for hugetlbfs). If we wouldn't -account the gup refcounts on the tail pages during gup, we won't know -anymore which tail page is pinned by gup and which is not while we run -split_huge_page. But we still have to add the gup pin to the head page -too, to know when we can free the compound page in case it's never -split during its lifetime. That requires changing not just -get_page, but put_page as well so that when put_page runs on a tail -page (and only on a tail page) it will find its respective head page, -and then it will decrease the head page refcount in addition to the -tail page refcount. To obtain a head page reliably and to decrease its -refcount without race conditions, put_page has to serialize against -__split_huge_page_refcount using a special per-page lock called -compound_lock. +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->_count and +page->_mapcount. + +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 has zero ->_count until atomic_add(). It prevent scanner +from geting reference to tail page up to the point. After the atomic_add() +we don't care about ->_count value. We already known how many references +with should uncharge from 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 unmap happens during +exit(2) if an THP crosses 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. |