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Date:   Mon, 21 Oct 2019 14:03:14 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        open list <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "matthew.wilcox@oracle.com" <matthew.wilcox@oracle.com>,
        Kernel Team <Kernel-team@fb.com>,
        "william.kucharski@oracle.com" <william.kucharski@oracle.com>,
        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP
Message-ID: <20191021180314.GA185593@cmpxchg.org>
References: <20191018180345.4188310-1-songliubraving@fb.com>
 <20191018181712.91dd9e9f9941642300e1b8d9@linux-foundation.org>
 <9DC29F5B-1DF5-408F-BEDF-FD1FBAAB1361@fb.com>
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On Sat, Oct 19, 2019 at 05:24:00AM +0000, Song Liu 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. Add a warning with the PageDirty() check as it should 
> not happen. This warning is added after page_mapping() check, because 
> if the page is truncated, it might be dirty.
> 
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: William Kucharski <william.kucharski@oracle.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Song Liu <songliubraving@fb.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>