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From: Andrew Morton <akpm@linux-foundation.org>
To: Song Liu <songliubraving@fb.com>
Cc: <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
	<matthew.wilcox@oracle.com>, <kernel-team@fb.com>,
	<william.kucharski@oracle.com>, <kirill.shutemov@linux.intel.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v5 1/2] mm,thp: recheck each page before collapsing file THP
Date: Thu, 7 Nov 2019 19:53:28 -0800	[thread overview]
Message-ID: <20191107195328.600f302bbde69cf9c1089500@linux-foundation.org> (raw)
In-Reply-To: <20191106060930.2571389-2-songliubraving@fb.com>

On Tue, 5 Nov 2019 22:09:29 -0800 Song Liu <songliubraving@fb.com> wrote:

> In collapse_file(), for !is_shmem case, current check cannot guarantee
> the locked page is up-to-date.  Specifically, xas_unlock_irq() should
> not be called before lock_page() and get_page(); and it is necessary to
> recheck PageUptodate() after locking the page.
> 
> With this bug and CONFIG_READ_ONLY_THP_FOR_FS=y, madvise(HUGE)'ed .text
> may contain corrupted data.  This is because khugepaged mistakenly
> collapses some not up-to-date sub pages into a huge page, and assumes
> the huge page is up-to-date.  This will NOT corrupt data in the disk,
> because the page is read-only and never written back.  Fix this by
> properly checking PageUptodate() after locking the page.  This check
> replaces "VM_BUG_ON_PAGE(!PageUptodate(page), page);".
> 
> Also, move PageDirty() check after locking the page.  Current
> khugepaged should not try to collapse dirty file THP, because it is
> limited to read-only .text. The only case we hit a dirty page here is
> when the page hasn't been written since write. Bail out and retry when
> this happens.

Incorrect data is pretty serious.  Should we backport this into -stable
kernels?

(I suspect I already asked this in response to earier versions, sorry ;))


  reply	other threads:[~2019-11-08  3:53 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-06  6:09 [PATCH v5 0/2] fix and improvement for file THP Song Liu
2019-11-06  6:09 ` [PATCH v5 1/2] mm,thp: recheck each page before collapsing " Song Liu
2019-11-08  3:53   ` Andrew Morton [this message]
2019-11-08  3:58     ` Song Liu
2019-11-13 23:47   ` Song Liu
2019-11-15  0:10     ` Andrew Morton
2019-11-06  6:09 ` [PATCH v5 2/2] mm/thp: flush file for !is_shmem PageDirty() case in collapse_file() Song Liu

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