Re: [PATCH] iomap: Fix direct I/O write consistency check

[Andreas Gruenbacher]

On Thu, Sep 3, 2020 at 11:12 PM Eric Sandeen <sandeen@xxxxxxxxxxx> wrote:
> On 9/3/20 11:56 AM, Andreas Gruenbacher wrote:
> > When a direct I/O write falls back to buffered I/O entirely, dio->size
> > will be 0 in iomap_dio_complete.  Function invalidate_inode_pages2_range
> > will try to invalidate the rest of the address space.
>
> (Because if ->size == 0 and offset == 0, then invalidating up to (0+0-1) will
> invalidate the entire range.)
>
>
>                 err = invalidate_inode_pages2_range(inode->i_mapping,
>                                 offset >> PAGE_SHIFT,
>                                 (offset + dio->size - 1) >> PAGE_SHIFT);
>
> so I guess this behavior is unique to writing to a hole at offset 0?
>
> FWIW, this same test yields the same results on ext3 when it falls back to
> buffered.

That's interesting. An ext3 formatted filesystem will invoke
dio_warn_stale_pagecache and thus log the error message, but the error
isn't immediately reported by the "pwrite 0 4k". It takes adding '-c
"fsync"' to the xfs_io command or similar to make it fail.

An ext4 formatted filesystem doesn't show any of these problems.

Thanks,
Andreas

> -Eric
>
> > If there are any
> > dirty pages in that range, the write will fail and a "Page cache
> > invalidation failure on direct I/O" error will be logged.
> >
> > On gfs2, this can be reproduced as follows:
> >
> >   xfs_io \
> >     -c "open -ft foo" -c "pwrite 4k 4k" -c "close" \
> >     -c "open -d foo" -c "pwrite 0 4k"
> >
> > Fix this by recognizing 0-length writes.
> >
> > Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
> > ---
> >  fs/iomap/direct-io.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index c1aafb2ab990..c9d6b4eecdb7 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -108,7 +108,7 @@ static ssize_t iomap_dio_complete(struct iomap_dio *dio)
> >        * ->end_io() when necessary, otherwise a racing buffer read would cache
> >        * zeros from unwritten extents.
> >        */
> > -     if (!dio->error &&
> > +     if (!dio->error && dio->size &&
> >           (dio->flags & IOMAP_DIO_WRITE) && inode->i_mapping->nrpages) {
> >               int err;
> >               err = invalidate_inode_pages2_range(inode->i_mapping,
> >
>