linux-kernel - Re: Major btrfs fiemap slowdown on file with many extents once in cache (RCU stalls?) (Was: [PATCH 1/3] filemap: Correct the conditions for marking a folio as accessed)

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Message-ID: <Yr21RoL+ztf1W5Od@mit.edu>
Date:   Thu, 30 Jun 2022 10:37:58 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Dominique MARTINET <dominique.martinet@...ark-techno.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-btrfs@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yu Kuai <yukuai3@...wei.com>,
        Kent Overstreet <kent.overstreet@...il.com>
Subject: Re: Major btrfs fiemap slowdown on file with many extents once in
 cache (RCU stalls?) (Was: [PATCH 1/3] filemap: Correct the conditions for
 marking a folio as accessed)

On Thu, Jun 30, 2022 at 04:29:05PM +0900, Dominique MARTINET wrote:
> Hi Willy, linux-btrfs@...r,
> 
> Matthew Wilcox (Oracle) wrote on Sun, Jun 19, 2022 at 04:11:41PM +0100:
> > We had an off-by-one error which meant that we never marked the first page
> > in a read as accessed.  This was visible as a slowdown when re-reading
> > a file as pages were being evicted from cache too soon.  In reviewing
> > this code, we noticed a second bug where a multi-page folio would be
> > marked as accessed multiple times when doing reads that were less than
> > the size of the folio.
> 
> when debugging an unrelated issue (short reads on btrfs with io_uring
> and O_DIRECT[1]), I noticed that my horrible big file copy speeds fell
> down from ~2GB/s (there's compression and lots of zeroes) to ~100MB/s
> the second time I was copying it with cp.
> 
> I've taken a moment to bisect this and came down to this patch.

I think you may have forgotten to include the commit-id that was the
results of your bisect.... ?

					- Ted