linux-kernel - Re: [PATCH v5 1/2] mm,thp: recheck each page before collapsing file THP

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Date:   Thu, 14 Nov 2019 16:10:58 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Song Liu <songliubraving@...com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        Kernel Team <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "Hugh Dickins" <hughd@...gle.com>
Subject: Re: [PATCH v5 1/2] mm,thp: recheck each page before collapsing file
 THP

On Wed, 13 Nov 2019 23:47:06 +0000 Song Liu <songliubraving@...com> wrote:

> 
> > On Nov 5, 2019, at 10:09 PM, Song Liu <songliubraving@...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.
> > 
> > syzbot reported bug on previous version of this patch.
> > 
> > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> > Reported-by: syzbot+efb9e48b9fbdc49bb34a@...kaller.appspotmail.com
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > Cc: Hugh Dickins <hughd@...gle.com>
> > Cc: William Kucharski <william.kucharski@...cle.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Song Liu <songliubraving@...com>
> 
> I think we need this in 5.4 official, but I haven't seen it in Linus' 
> master branch. 
> 
> Hi Andrew,
> 
> Could you please send patch/pull-request for it? 

Yep, today or tomorrow...