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@xxxxxxxxxxxxx>
> > ---
> > 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.
