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mm/thp: try_to_unmap() use TTU_SYNC for safe splitting

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[ Upstream commit 732ed55823fc3ad998d43b86bf771887bcc5ec67 ]

Stressing huge tmpfs often crashed on unmap_page()'s VM_BUG_ON_PAGE
(!unmap_success): with dump_page() showing mapcount:1, but then its raw
struct page output showing _mapcount ffffffff i.e.  mapcount 0.

And even if that particular VM_BUG_ON_PAGE(!unmap_success) is removed,
it is immediately followed by a VM_BUG_ON_PAGE(compound_mapcount(head)),
and further down an IS_ENABLED(CONFIG_DEBUG_VM) total_mapcount BUG():
all indicative of some mapcount difficulty in development here perhaps.
But the !CONFIG_DEBUG_VM path handles the failures correctly and
silently.

I believe the problem is that once a racing unmap has cleared pte or
pmd, try_to_unmap_one() may skip taking the page table lock, and emerge
from try_to_unmap() before the racing task has reached decrementing
mapcount.

Instead of abandoning the unsafe VM_BUG_ON_PAGE(), and the ones that
follow, use PVMW_SYNC in try_to_unmap_one() in this case: adding
TTU_SYNC to the options, and passing that from unmap_page().

When CONFIG_DEBUG_VM, or for non-debug too? Consensus is to do the same
for both: the slight overhead added should rarely matter, except perhaps
if splitting sparsely-populated multiply-mapped shmem.  Once confident
that bugs are fixed, TTU_SYNC here can be removed, and the race
tolerated.

Link: https://lkml.kernel.org/r/c1e95853-8bcd-d8fd-55fa-e7f2488e78f@google.com
Fixes: fec89c109f3a ("thp: rewrite freeze_page()/unfreeze_page() with generic rmap walkers")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jue Wang <juew@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Wang Yugui <wangyugui@e16-tech.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Note on stable backport: upstream TTU_SYNC 0x10 takes the value which
5.11 commit 013339df116c ("mm/rmap: always do TTU_IGNORE_ACCESS") freed.
It is very tempting to backport that commit (as 5.10 already did) and
make no change here; but on reflection, good as that commit is, I'm
reluctant to include any possible side-effect of it in this series.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Hugh Dickins 2021-06-15 18:23:53 -07:00 committed by Sasha Levin
parent bd092a0f19
commit 4b0a34e222
4 changed files with 30 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/rmap.h
View File

@ -98,7 +98,8 @@ enum ttu_flags {
* do a final flush if necessary */
TTU_RMAP_LOCKED = 0x80, /* do not grab rmap lock:
* caller holds it */
TTU_SPLIT_FREEZE = 0x100, /* freeze pte under splitting thp */
TTU_SPLIT_FREEZE = 0x100, /* freeze pte under splitting thp */
TTU_SYNC = 0x200, /* avoid racy checks with PVMW_SYNC */
};
#ifdef CONFIG_MMU

2
mm/huge_memory.c
View File

@ -2461,7 +2461,7 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
static void unmap_page(struct page *page)
{
enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS |
TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD | TTU_SYNC;
bool unmap_success;
VM_BUG_ON_PAGE(!PageHead(page), page);

11
mm/page_vma_mapped.c
View File

@ -207,6 +207,17 @@ restart:
pvmw->ptl = NULL;
}
} else if (!pmd_present(pmde)) {
/*
* If PVMW_SYNC, take and drop THP pmd lock so that we
* cannot return prematurely, while zap_huge_pmd() has
* cleared *pmd but not decremented compound_mapcount().
*/
if ((pvmw->flags & PVMW_SYNC) &&
PageTransCompound(pvmw->page)) {
spinlock_t *ptl = pmd_lock(mm, pvmw->pmd);
spin_unlock(ptl);
}
return false;
}
if (!map_pte(pvmw))

17
mm/rmap.c
View File

@ -1353,6 +1353,15 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
struct mmu_notifier_range range;
enum ttu_flags flags = (enum ttu_flags)arg;
/*
* When racing against e.g. zap_pte_range() on another cpu,
* in between its ptep_get_and_clear_full() and page_remove_rmap(),
* try_to_unmap() may return false when it is about to become true,
* if page table locking is skipped: use TTU_SYNC to wait for that.
*/
if (flags & TTU_SYNC)
pvmw.flags = PVMW_SYNC;
/* munlock has nothing to gain from examining un-locked vmas */
if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
return true;
@ -1731,7 +1740,13 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
else
rmap_walk(page, &rwc);
return !page_mapcount(page) ? true : false;
/*
* When racing against e.g. zap_pte_range() on another cpu,
* in between its ptep_get_and_clear_full() and page_remove_rmap(),
* try_to_unmap() may return false when it is about to become true,
* if page table locking is skipped: use TTU_SYNC to wait for that.
*/
return !page_mapcount(page);
}
/**