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Date:   Thu, 28 May 2020 11:34:50 -0500
From:   Goldwyn Rodrigues <rgoldwyn@suse.de>
To:     Filipe Manana <fdmanana@gmail.com>
Cc:     Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
        "hch@infradead.org" <hch@infradead.org>,
        "dsterba@suse.cz" <dsterba@suse.cz>
Subject: Re: [PATCH 4/7] btrfs: Switch to iomap_dio_rw() for dio
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On 16:13 28/05, Filipe Manana wrote:
> On Tue, May 26, 2020 at 5:47 PM Goldwyn Rodrigues <rgoldwyn@suse.de> wrote:
> >
> > On 15:03 26/05, Johannes Thumshirn wrote:
> > > Just as a heads up, this one gives me lot's of Page cache invalidation
> > > failure prints from dio_warn_stale_pagecache() on btrfs/004 with
> > > current misc-next:
> > >
> > <snip>
> >
> > > [   23.696400] Page cache invalidation failure on direct I/O.  Possible data corruption due to collision with buffered I/O!
> > > [   23.698115] File: /mnt/scratch/bgnoise/p0/f0 PID: 6562 Comm: fsstress
> > >
> > > I have no idea yet why but I'm investigating.
> >
> > This is caused because we are trying to release a page when the extent
> > has locked the page and release page returns false.
> 
> By "we" you mean what exaclty, a direct IO read, a direct IO write?
> 
> And who locked the extent range before?

This is usually locked by a previous buffered write or read.

> 
> That seems alarming to me, specially if it's a direct IO write failing
> to invalidate the page cache, since a subsequent buffered read could
> get stale data (what's in the page cache), and not what the direct IO
> write wrote.
> 
> Can you elaborate more on all those details?

The origin of the message is when iomap_dio_rw() tries to invalidate the
inode pages, but fails and calls dio_warn_stale_pagecache().

In the vanilla code, generic_file_direct_write() aborts direct writes
and returns 0 so that it may fallback to buffered I/O. Perhaps this
should be changed in iomap_dio_rw() as well. I will write a patch to
accomodate that.

-- 
Goldwyn