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From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Song Liu <songliubraving@fb.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] khugepaged: retract_page_tables() remember to test exit
Date: Mon, 3 Aug 2020 00:44:08 +0300	[thread overview]
Message-ID: <20200802214408.patvlf3sghro3nhi@box> (raw)
In-Reply-To: <alpine.LSU.2.11.2008021215400.27773@eggly.anvils>

On Sun, Aug 02, 2020 at 12:16:53PM -0700, Hugh Dickins wrote:
> Only once have I seen this scenario (and forgot even to notice what
> forced the eventual crash): a sequence of "BUG: Bad page map" alerts
> from vm_normal_page(), from zap_pte_range() servicing exit_mmap();
> pmd:00000000, pte values corresponding to data in physical page 0.
> 
> The pte mappings being zapped in this case were supposed to be from a
> huge page of ext4 text (but could as well have been shmem): my belief
> is that it was racing with collapse_file()'s retract_page_tables(),
> found *pmd pointing to a page table, locked it, but *pmd had become
> 0 by the time start_pte was decided.
> 
> In most cases, that possibility is excluded by holding mmap lock;
> but exit_mmap() proceeds without mmap lock.  Most of what's run by
> khugepaged checks khugepaged_test_exit() after acquiring mmap lock:
> khugepaged_collapse_pte_mapped_thps() and hugepage_vma_revalidate()
> do so, for example.  But retract_page_tables() did not: fix that
> (using an mm variable instead of vma->vm_mm repeatedly).

Hm. I'm not sure I follow. vma->vm_mm has to be valid as long as we hold
i_mmap lock, no? Unlinking a VMA requires it.

-- 
 Kirill A. Shutemov

  reply	other threads:[~2020-08-02 21:44 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-02 19:12 [PATCH] khugepaged: collapse_pte_mapped_thp() flush the right range Hugh Dickins
2020-08-02 19:12 ` Hugh Dickins
2020-08-02 19:15 ` [PATCH] khugepaged: collapse_pte_mapped_thp() protect the pmd lock Hugh Dickins
2020-08-02 19:15   ` Hugh Dickins
2020-08-02 21:23   ` Kirill A. Shutemov
2020-08-02 19:16 ` [PATCH] khugepaged: retract_page_tables() remember to test exit Hugh Dickins
2020-08-02 19:16   ` Hugh Dickins
2020-08-02 21:44   ` Kirill A. Shutemov [this message]
2020-08-03  0:35     ` Hugh Dickins
2020-08-03  0:35       ` Hugh Dickins
2020-08-03  8:59       ` Kirill A. Shutemov
2020-08-02 19:18 ` [PATCH] khugepaged: khugepaged_test_exit() check mmget_still_valid() Hugh Dickins
2020-08-02 19:18   ` Hugh Dickins
2020-08-14 22:13   ` [PATCH] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged_enter() Hugh Dickins
2020-08-14 22:13     ` Hugh Dickins
2020-08-17 20:50     ` Yang Shi
2020-08-17 20:50       ` Yang Shi
2020-08-02 21:07 ` [PATCH] khugepaged: collapse_pte_mapped_thp() flush the right range Kirill A. Shutemov

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