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Date:   Thu, 30 Sep 2021 01:00:41 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Song Liu <song@kernel.org>
Cc:     Rongwei Wang <rongwei.wang@linux.alibaba.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        William Kucharski <william.kucharski@oracle.com>,
        Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page
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On Wed, Sep 29, 2021 at 04:41:48PM -0700, Song Liu wrote:
> The issue is NOT caused by concurrent khugepaged:collapse_file() and
> truncate_pagecache(inode, 0). With some printks, we can see a clear
> time gap (>2 second )  between collapse_file() finishes, and
> truncate_pagecache() (which crashes soon). Therefore, my earlier
> suggestion that adds deny_write_access() to collapse_file() does NOT
> work.
> 
> The crash is actually caused by concurrent truncate_pagecache(inode, 0).
> If I change the number of write thread in stress_madvise_dso.c to one,
> (IOW, one thread_read and one thread_write), I cannot reproduce the
> crash anymore.
> 
> I think this means we cannot fix this issue in collapse_file(), because it
> finishes long before the crash.

Ah!  So are we missing one or more of these locks:

	inode_lock(inode);
	filemap_invalidate_lock(mapping);

in the open path?