Re: [PATCH v7 16/19] iomap: Add done_before argument to iomap_dio_rw

[Andreas Gruenbacher]

On Fri, Aug 27, 2021 at 8:30 PM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
> On Fri, Aug 27, 2021 at 06:49:23PM +0200, Andreas Gruenbacher wrote:
> > Add a done_before argument to iomap_dio_rw that indicates how much of
> > the request has already been transferred.  When the request succeeds, we
> > report that done_before additional bytes were tranferred.  This is
> > useful for finishing a request asynchronously when part of the request
> > has already been completed synchronously.
> >
> > We'll use that to allow iomap_dio_rw to be used with page faults
> > disabled: when a page fault occurs while submitting a request, we
> > synchronously complete the part of the request that has already been
> > submitted.  The caller can then take care of the page fault and call
> > iomap_dio_rw again for the rest of the request, passing in the number of
> > bytes already tranferred.
> >
> > Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
> > ---
> >  fs/btrfs/file.c       |  5 +++--
> >  fs/ext4/file.c        |  5 +++--
> >  fs/gfs2/file.c        |  4 ++--
> >  fs/iomap/direct-io.c  | 11 ++++++++---
> >  fs/xfs/xfs_file.c     |  6 +++---
> >  fs/zonefs/super.c     |  4 ++--
> >  include/linux/iomap.h |  4 ++--
> >  7 files changed, 23 insertions(+), 16 deletions(-)
> >
>
> <snip to the interesting parts>
>
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index ba88fe51b77a..dcf9a2b4381f 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -31,6 +31,7 @@ struct iomap_dio {
> >       atomic_t                ref;
> >       unsigned                flags;
> >       int                     error;
> > +     size_t                  done_before;
> >       bool                    wait_for_completion;
> >
> >       union {
> > @@ -126,6 +127,9 @@ ssize_t iomap_dio_complete(struct iomap_dio *dio)
> >       if (ret > 0 && (dio->flags & IOMAP_DIO_NEED_SYNC))
> >               ret = generic_write_sync(iocb, ret);
> >
> > +     if (ret > 0)
> > +             ret += dio->done_before;
>
> Pardon my ignorance since this is the first time I've had a crack at
> this patchset, but why is it necessary to carry the "bytes copied"
> count from the /previous/ iomap_dio_rw call all the way through to dio
> completion of the current call?

Consider the following situation:

 * A user submits an asynchronous read request.

 * The first page of the buffer is in memory, but the following
   pages are not. This isn't uncommon for consecutive reads
   into freshly allocated memory.

 * iomap_dio_rw writes into the first page. Then it
   hits the next page which is missing, so it returns a partial
   result, synchronously.

 * We then fault in the remaining pages and call iomap_dio_rw
   for the rest of the request.

 * The rest of the request completes asynchronously.

Does that answer your question?

Thanks,
Andreas