The patch titled
Subject: thp: update documentation
has been added to the -mm tree. Its filename is
thp-update-documentation.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/thp-update-documentation.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/thp-update-documentation.patch
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From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Subject: thp: update documentation
The patch updates Documentation/vm/transhuge.txt to reflect changes in
THP design.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Acked-by: Jerome Marchand <jmarchan@xxxxxxxxxx>
Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Steve Capper <steve.capper@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/vm/transhuge.txt | 149 +++++++++++++++++++------------
1 file changed, 95 insertions(+), 54 deletions(-)
diff -puN Documentation/vm/transhuge.txt~thp-update-documentation Documentation/vm/transhuge.txt
--- a/Documentation/vm/transhuge.txt~thp-update-documentation
+++ a/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
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 t
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
== 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 (kh
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.
+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.
-== compound_lock, get_user_pages and put_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.
_
Patches currently in -mm which might be from kirill.shutemov@xxxxxxxxxxxxxxx are
rcu-force-alignment-on-struct-callback_head-rcu_head.patch
mm-make-optimistic-check-for-swapin-readahead-fix.patch
mm-make-swapin-readahead-to-improve-thp-collapse-rate-fix.patch
mm-make-swapin-readahead-to-improve-thp-collapse-rate-fix-2.patch
mm-make-swapin-readahead-to-improve-thp-collapse-rate-fix-3.patch
mm-drop-page-slab_page.patch
slab-slub-use-page-rcu_head-instead-of-page-lru-plus-cast.patch
zsmalloc-use-page-private-instead-of-page-first_page.patch
mm-pack-compound_dtor-and-compound_order-into-one-word-in-struct-page.patch
mm-make-compound_head-robust.patch
mm-make-compound_head-robust-fix.patch
mm-use-unsigned-int-for-page-order.patch
mm-use-unsigned-int-for-compound_dtor-compound_order-on-64bit.patch
page-flags-trivial-cleanup-for-pagetrans-helpers.patch
page-flags-move-code-around.patch
page-flags-introduce-page-flags-policies-wrt-compound-pages.patch
page-flags-introduce-page-flags-policies-wrt-compound-pages-fix.patch
page-flags-introduce-page-flags-policies-wrt-compound-pages-fix-fix.patch
page-flags-introduce-page-flags-policies-wrt-compound-pages-fix-3.patch
page-flags-define-pg_locked-behavior-on-compound-pages.patch
page-flags-define-behavior-of-fs-io-related-flags-on-compound-pages.patch
page-flags-define-behavior-of-lru-related-flags-on-compound-pages.patch
page-flags-define-behavior-slb-related-flags-on-compound-pages.patch
page-flags-define-behavior-of-xen-related-flags-on-compound-pages.patch
page-flags-define-pg_reserved-behavior-on-compound-pages.patch
page-flags-define-pg_reserved-behavior-on-compound-pages-fix.patch
page-flags-define-pg_swapbacked-behavior-on-compound-pages.patch
page-flags-define-pg_swapcache-behavior-on-compound-pages.patch
page-flags-define-pg_mlocked-behavior-on-compound-pages.patch
page-flags-define-pg_uncached-behavior-on-compound-pages.patch
page-flags-define-pg_uptodate-behavior-on-compound-pages.patch
page-flags-look-at-head-page-if-the-flag-is-encoded-in-page-mapping.patch
mm-sanitize-page-mapping-for-tail-pages.patch
mm-proc-adjust-pss-calculation.patch
rmap-add-argument-to-charge-compound-page.patch
memcg-adjust-to-support-new-thp-refcounting.patch
mm-thp-adjust-conditions-when-we-can-reuse-the-page-on-wp-fault.patch
mm-adjust-foll_split-for-new-refcounting.patch
mm-handle-pte-mapped-tail-pages-in-gerneric-fast-gup-implementaiton.patch
thp-mlock-do-not-allow-huge-pages-in-mlocked-area.patch
khugepaged-ignore-pmd-tables-with-thp-mapped-with-ptes.patch
thp-rename-split_huge_page_pmd-to-split_huge_pmd.patch
mm-vmstats-new-thp-splitting-event.patch
mm-temporally-mark-thp-broken.patch
thp-drop-all-split_huge_page-related-code.patch
mm-drop-tail-page-refcounting.patch
futex-thp-remove-special-case-for-thp-in-get_futex_key.patch
ksm-prepare-to-new-thp-semantics.patch
mm-thp-remove-compound_lock.patch
arm64-thp-remove-infrastructure-for-handling-splitting-pmds.patch
arm-thp-remove-infrastructure-for-handling-splitting-pmds.patch
mips-thp-remove-infrastructure-for-handling-splitting-pmds.patch
powerpc-thp-remove-infrastructure-for-handling-splitting-pmds.patch
s390-thp-remove-infrastructure-for-handling-splitting-pmds.patch
sparc-thp-remove-infrastructure-for-handling-splitting-pmds.patch
tile-thp-remove-infrastructure-for-handling-splitting-pmds.patch
x86-thp-remove-infrastructure-for-handling-splitting-pmds.patch
mm-thp-remove-infrastructure-for-handling-splitting-pmds.patch
mm-rework-mapcount-accounting-to-enable-4k-mapping-of-thps.patch
mm-differentiate-page_mapped-from-page_mapcount-for-compound-pages.patch
mm-numa-skip-pte-mapped-thp-on-numa-fault.patch
thp-implement-split_huge_pmd.patch
thp-add-option-to-setup-migration-entries-during-pmd-split.patch
thp-mm-split_huge_page-caller-need-to-lock-page.patch
thp-reintroduce-split_huge_page.patch
migrate_pages-try-to-split-pages-on-qeueuing.patch
thp-introduce-deferred_split_huge_page.patch
mm-re-enable-thp.patch
thp-update-documentation.patch
thp-allow-mlocked-thp-again.patch
mm-support-madvisemadv_free-fix-3.patch
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