From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
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/<53440991.9090001@xxxxxxxxxx>
[2] https://lkml.kernel.org/g/<5310C56C.60709@xxxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Bob Liu <lliubbo@xxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Dave Jones <davej@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.13+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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;
_
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