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Re: [PATCH 09/10] huge pagecache: mmap_sem is unlocked when truncation splits pmd

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On Tue, Apr 05, 2016 at 01:55:23PM -0700, Hugh Dickins wrote:
> zap_pmd_range()'s CONFIG_DEBUG_VM !rwsem_is_locked(&mmap_sem) BUG()
> will be invalid with huge pagecache, in whatever way it is implemented:
> truncation of a hugely-mapped file to an unhugely-aligned size would
> easily hit it.
> 
> (Although anon THP could in principle apply khugepaged to private file
> mappings, which are not excluded by the MADV_HUGEPAGE restrictions, in
> practice there's a vm_ops check which excludes them, so it never hits
> this BUG() - there's no interface to "truncate" an anonymous mapping.)
> 
> We could complicate the test, to check i_mmap_rwsem also when there's a
> vm_file; but my inclination was to make zap_pmd_range() more readable by
> simply deleting this check.  A search has shown no report of the issue in
> the years since commit e0897d75f0b2 ("mm, thp: print useful information
> when mmap_sem is unlocked in zap_pmd_range") expanded it from VM_BUG_ON()
> - though I cannot point to what commit I would say then fixed the issue.
> 
> But there are a couple of other patches now floating around, neither
> yet in the tree: let's agree to retain the check as a VM_BUG_ON_VMA(),
> as Matthew Wilcox has done; but subject to a vma_is_anonymous() check,
> as Kirill Shutemov has done.  And let's get this in, without waiting
> for any particular huge pagecache implementation to reach the tree.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov

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[PATCH 00/10] mm: easy preliminaries to THPagecache Hugh Dickins <hughd@google.com> - 2016-04-05 22:40 +0200
  [PATCH 01/10] mm: update_lru_size warn and reset bad lru_size Hugh Dickins <hughd@google.com> - 2016-04-05 22:50 +0200
    Re: [PATCH 01/10] mm: update_lru_size warn and reset bad lru_size Vlastimil Babka <vbabka@suse.cz> - 2016-04-14 14:00 +0200
  [PATCH 06/10] mm: /proc/sys/vm/stat_refresh to force vmstat update Hugh Dickins <hughd@google.com> - 2016-04-05 22:50 +0200
  [PATCH 03/10] mm: use __SetPageSwapBacked and dont  ClearPageSwapBacked Hugh Dickins <hughd@google.com> - 2016-04-05 22:50 +0200
    Re: [PATCH 03/10] mm: use __SetPageSwapBacked and dont  ClearPageSwapBacked Mel Gorman <mgorman@techsingularity.net> - 2016-04-06 12:00 +0200
  [PATCH 02/10] mm: update_lru_size do the __mod_zone_page_state Hugh Dickins <hughd@google.com> - 2016-04-05 22:50 +0200
  [PATCH 04/10] tmpfs: preliminary minor tidyups Hugh Dickins <hughd@google.com> - 2016-04-05 22:50 +0200
  [PATCH 05/10] tmpfs: mem_cgroup charge fault to vm_mm not current  mm Hugh Dickins <hughd@google.com> - 2016-04-05 22:50 +0200
  [PATCH 09/10] huge pagecache: mmap_sem is unlocked when truncation  splits pmd Hugh Dickins <hughd@google.com> - 2016-04-05 23:00 +0200
    Re: [PATCH 09/10] huge pagecache: mmap_sem is unlocked when  truncation splits pmd "Kirill A. Shutemov" <kirill@shutemov.name> - 2016-04-11 12:40 +0200
    Re: [PATCH 09/10] huge pagecache: mmap_sem is unlocked when  truncation splits pmd Matthew Wilcox <willy@linux.intel.com> - 2016-04-14 19:40 +0200
  [PATCH 07/10] huge mm: move_huge_pmd does not need new_vma Hugh Dickins <hughd@google.com> - 2016-04-05 23:00 +0200
    Re: [PATCH 07/10] huge mm: move_huge_pmd does not need new_vma "Kirill A. Shutemov" <kirill@shutemov.name> - 2016-04-11 12:30 +0200
  [PATCH 08/10] huge pagecache: extend mremap pmd rmap lockout to  files Hugh Dickins <hughd@google.com> - 2016-04-05 23:00 +0200
    Re: [PATCH 08/10] huge pagecache: extend mremap pmd rmap lockout to  files "Kirill A. Shutemov" <kirill@shutemov.name> - 2016-04-11 12:40 +0200
  [PATCH 10/10] arch: fix has_transparent_hugepage() Hugh Dickins <hughd@google.com> - 2016-04-05 23:10 +0200
    Re: [PATCH 10/10] arch: fix has_transparent_hugepage() David Miller <davem@davemloft.net> - 2016-04-06 01:30 +0200
    Re: [PATCH 10/10] arch: fix has_transparent_hugepage() Ingo Molnar <mingo@kernel.org> - 2016-04-06 09:00 +0200
      Re: [PATCH 10/10] arch: fix has_transparent_hugepage() Ingo Molnar <mingo@kernel.org> - 2016-04-07 00:00 +0200
    Re: [PATCH 10/10] arch: fix has_transparent_hugepage() Gerald Schaefer <gerald.schaefer@de.ibm.com> - 2016-04-06 14:00 +0200

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