From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964843AbbFCRHd (ORCPT ); Wed, 3 Jun 2015 13:07:33 -0400 Received: from mga01.intel.com ([192.55.52.88]:52355 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933250AbbFCRHJ (ORCPT ); Wed, 3 Jun 2015 13:07:09 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.13,548,1427785200"; d="scan'208";a="704747398" From: "Kirill A. Shutemov" To: Andrew Morton , Andrea Arcangeli , Hugh Dickins Cc: Dave Hansen , Mel Gorman , Rik van Riel , Vlastimil Babka , Christoph Lameter , Naoya Horiguchi , Steve Capper , "Aneesh Kumar K.V" , Johannes Weiner , Michal Hocko , Jerome Marchand , Sasha Levin , linux-kernel@vger.kernel.org, linux-mm@kvack.org, "Kirill A. Shutemov" Subject: [PATCHv6 36/36] thp: update documentation Date: Wed, 3 Jun 2015 20:06:07 +0300 Message-Id: <1433351167-125878-37-git-send-email-kirill.shutemov@linux.intel.com> X-Mailer: git-send-email 2.1.4 In-Reply-To: <1433351167-125878-1-git-send-email-kirill.shutemov@linux.intel.com> References: <1433351167-125878-1-git-send-email-kirill.shutemov@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The patch updates Documentation/vm/transhuge.txt to reflect changes in THP design. Signed-off-by: Kirill A. Shutemov --- Documentation/vm/transhuge.txt | 124 +++++++++++++++++++++++------------------ 1 file changed, 69 insertions(+), 55 deletions(-) diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt index 6b31cfbe2a9a..2352b12cae93 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 nesessary, 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 @@ -200,9 +200,18 @@ 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 @@ -253,10 +262,8 @@ 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 @@ -291,9 +298,9 @@ 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 @@ -302,7 +309,8 @@ 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: @@ -314,14 +322,14 @@ 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 @@ -330,47 +338,53 @@ 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 relevent 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. + +Note that split_huge_pmd() doesn't have any limitation on refcounting: +pmd can be split at any point and never fails. -- 2.1.4