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Date:   Thu, 17 Dec 2020 12:12:46 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 18/25] btrfs: Use readahead_batch_length
Message-ID: <20201217121246.GD15600@casper.infradead.org>
References: <20201216182335.27227-1-willy@infradead.org>
 <20201216182335.27227-19-willy@infradead.org>
 <a5b979d7-1086-fe6c-6e82-f20ecb56d24c@nvidia.com>
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On Thu, Dec 17, 2020 at 01:15:10AM -0800, John Hubbard wrote:
> On 12/16/20 10:23 AM, Matthew Wilcox (Oracle) wrote:
> > Implement readahead_batch_length() to determine the number of bytes in
> > the current batch of readahead pages and use it in btrfs.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > ---
> >   fs/btrfs/extent_io.c    | 6 ++----
> >   include/linux/pagemap.h | 9 +++++++++
> >   2 files changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > index 6e3b72e63e42..42936a83a91b 100644
> > --- a/fs/btrfs/extent_io.c
> > +++ b/fs/btrfs/extent_io.c
> > @@ -4436,10 +4436,8 @@ void extent_readahead(struct readahead_control *rac)
> >   	int nr;
> >   	while ((nr = readahead_page_batch(rac, pagepool))) {
> > -		u64 contig_start = page_offset(pagepool[0]);
> > -		u64 contig_end = page_offset(pagepool[nr - 1]) + PAGE_SIZE - 1;
> > -
> > -		ASSERT(contig_start + nr * PAGE_SIZE - 1 == contig_end);
> > +		u64 contig_start = readahead_pos(rac);
> > +		u64 contig_end = contig_start + readahead_batch_length(rac);
> 
> Something in this tiny change is breaking btrfs: it hangs my Fedora 33 test
> system (which changed over to btrfs) on boot. I haven't quite figured out
> what's really wrong, but git bisect lands here, *and* turning the whole
> extent_readahead() function into a no-op (on top of the whole series)
> allows everything to work once again.
> 
> Sorry for not actually solving the root cause, but I figured you'd be able
> to jump straight to the answer, with the above information, so I'm sending
> it out early.

ehh ... probably an off-by-one.  Does subtracting 1 from contig_end fix it?
I'll spool up a test VM shortly and try it out.