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Date:   Thu, 30 Sep 2021 10:14:26 +0800
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Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page
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To:     Song Liu <song@kernel.org>, Matthew Wilcox <willy@infradead.org>
Cc:     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>
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From:   Rongwei Wang <rongwei.wang@linux.alibaba.com>
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On 9/30/21 8:41 AM, Song Liu wrote:
> On Wed, Sep 29, 2021 at 5:02 PM Matthew Wilcox <willy@infradead.org> wrote:
>>
>> 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?
> 
> The following fixes the crash in my test. But I am not sure whether this is the
> best fix.
> 
> Rongwei, could you please run more tests on it?
Yes, I'd like to.
> 
> Thanks,
> Song
> 
> 
> diff --git i/fs/open.c w/fs/open.c
> index daa324606a41f..d13c4668b2e53 100644
> --- i/fs/open.c
> +++ w/fs/open.c
> @@ -856,8 +856,11 @@ static int do_dentry_open(struct file *f,
>                   * of THPs into the page cache will fail.
>                   */
>                  smp_mb();
> -               if (filemap_nr_thps(inode->i_mapping))
> +               if (filemap_nr_thps(inode->i_mapping)) {
> +                       filemap_invalidate_lock(inode->i_mapping);
Learned something, Thanks!

But, the race between collapse_file and truncate_pagecache, I am not 
sure whether it exists or not. If exists, whether this patch only can 
fix truncate_pagecache concurrent?

Anyway, I will run more tests on it at first.

Thanks!
>                          truncate_pagecache(inode, 0);
> +                       filemap_invalidate_unlock(inode->i_mapping);
> +               }
>          }
> 
>          return 0;
>