Subject: [merged] thp-close-race-between-split-and-zap-huge-pages.patch removed
from -mm tree
To:
[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected]
From: [email protected]
Date: Mon, 21 Apr 2014 11:11:15 -0700
The patch titled
Subject: thp: close race between split and zap huge pages
has been removed from the -mm tree. Its filename was
thp-close-race-between-split-and-zap-huge-pages.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: "Kirill A. Shutemov" <[email protected]>
Subject: thp: close race between split and zap huge pages
Sasha Levin has reported two THP BUGs[1][2]. I believe both of them have
the same root cause. Let's look to them one by one.
The first bug[1] is "kernel BUG at mm/huge_memory.c:1829!". It's
BUG_ON(mapcount != page_mapcount(page)) in __split_huge_page(). From my
testing I see that page_mapcount() is higher than mapcount here.
I think it happens due to race between zap_huge_pmd() and
page_check_address_pmd(). page_check_address_pmd() misses PMD which is
under zap:
CPU0 CPU1
zap_huge_pmd()
pmdp_get_and_clear()
__split_huge_page()
anon_vma_interval_tree_foreach()
__split_huge_page_splitting()
page_check_address_pmd()
mm_find_pmd()
/*
* We check if PMD present without taking ptl: no
* serialization against zap_huge_pmd(). We miss this PMD,
* it's not accounted to 'mapcount' in __split_huge_page().
*/
pmd_present(pmd) == 0
BUG_ON(mapcount != page_mapcount(page)) // CRASH!!!
page_remove_rmap(page)
atomic_add_negative(-1,
&page->_mapcount)
The second bug[2] is "kernel BUG at mm/huge_memory.c:1371!".
It's VM_BUG_ON_PAGE(!PageHead(page), page) in zap_huge_pmd().
This happens in similar way:
CPU0 CPU1
zap_huge_pmd()
pmdp_get_and_clear()
page_remove_rmap(page)
atomic_add_negative(-1,
&page->_mapcount)
__split_huge_page()
anon_vma_interval_tree_foreach()
__split_huge_page_splitting()
page_check_address_pmd()
mm_find_pmd()
pmd_present(pmd) == 0 /* The same comment as above */
/*
* No crash this time since we already decremented page->_mapcount in
* zap_huge_pmd().
*/
BUG_ON(mapcount != page_mapcount(page))
/*
* We split the compound page here into small pages without
* serialization against zap_huge_pmd()
*/
__split_huge_page_refcount()
VM_BUG_ON_PAGE(!PageHead(page),
page); // CRASH!!!
So my understanding the problem is pmd_present() check in mm_find_pmd()
without taking page table lock.
The bug was introduced by me commit with commit 117b0791ac42. Sorry for
that. :(
Let's open code mm_find_pmd() in page_check_address_pmd() and do the
check under page table lock.
Note that __page_check_address() does the same for PTE entires
if sync != 0.
I've stress tested split and zap code paths for 36+ hours by now and
don't see crashes with the patch applied. Before it took <20 min to
trigger the first bug and few hours for second one (if we ignore
first).
[1] https://lkml.kernel.org/g/<[email protected]>
[2] https://lkml.kernel.org/g/<[email protected]>
Signed-off-by: Kirill A. Shutemov <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Tested-by: Sasha Levin <[email protected]>
Cc: Bob Liu <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Dave Jones <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: <[email protected]> [3.13+]
Signed-off-by: Andrew Morton <[email protected]>
---
mm/huge_memory.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff -puN mm/huge_memory.c~thp-close-race-between-split-and-zap-huge-pages
mm/huge_memory.c
--- a/mm/huge_memory.c~thp-close-race-between-split-and-zap-huge-pages
+++ a/mm/huge_memory.c
@@ -1536,16 +1536,23 @@ pmd_t *page_check_address_pmd(struct pag
enum page_check_address_pmd_flag flag,
spinlock_t **ptl)
{
+ pgd_t *pgd;
+ pud_t *pud;
pmd_t *pmd;
if (address & ~HPAGE_PMD_MASK)
return NULL;
- pmd = mm_find_pmd(mm, address);
- if (!pmd)
+ pgd = pgd_offset(mm, address);
+ if (!pgd_present(*pgd))
return NULL;
+ pud = pud_offset(pgd, address);
+ if (!pud_present(*pud))
+ return NULL;
+ pmd = pmd_offset(pud, address);
+
*ptl = pmd_lock(mm, pmd);
- if (pmd_none(*pmd))
+ if (!pmd_present(*pmd))
goto unlock;
if (pmd_page(*pmd) != page)
goto unlock;
_
Patches currently in -mm which might be from [email protected] are
pagewalk-update-page-table-walker-core.patch
pagewalk-add-walk_page_vma.patch
smaps-redefine-callback-functions-for-page-table-walker.patch
clear_refs-redefine-callback-functions-for-page-table-walker.patch
pagemap-redefine-callback-functions-for-page-table-walker.patch
numa_maps-redefine-callback-functions-for-page-table-walker.patch
memcg-redefine-callback-functions-for-page-table-walker.patch
arch-powerpc-mm-subpage-protc-use-walk_page_vma-instead-of-walk_page_range.patch
pagewalk-remove-argument-hmask-from-hugetlb_entry.patch
mempolicy-apply-page-table-walker-on-queue_pages_range.patch
mm-introduce-do_shared_fault-and-drop-do_fault-fix-fix.patch
thp-consolidate-assert-checks-in-__split_huge_page.patch
mm-huge_memoryc-complete-conversion-to-pr_foo.patch
mm-pass-vm_bug_on-reason-to-dump_page.patch
mm-pass-vm_bug_on-reason-to-dump_page-fix.patch
hugetlb-prep_compound_gigantic_page-drop-__init-marker.patch
hugetlb-add-hstate_is_gigantic.patch
hugetlb-update_and_free_page-dont-clear-pg_reserved-bit.patch
hugetlb-move-helpers-up-in-the-file.patch
hugetlb-add-support-for-gigantic-page-allocation-at-runtime.patch
mm-move-get_user_pages-related-code-to-separate-file.patch
mm-extract-in_gate_area-case-from-__get_user_pages.patch
mm-cleanup-follow_page_mask.patch
mm-extract-code-to-fault-in-a-page-from-__get_user_pages.patch
mm-cleanup-__get_user_pages.patch
mm-gupc-tweaks.patch
do_shared_fault-check-that-mmap_sem-is-held.patch
--
To unsubscribe from this list: send the line "unsubscribe stable" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
