Re: [PATCH v8 14/17] iomap: Add done_before argument to iomap_dio_rw

Andreas Gruenbacher
2021-10-19T19:31:15Z

On Tue, Oct 19, 2021 at 5:51 PM Darrick J. Wong <djwong@kernel.org> wrote:
> On Tue, Oct 19, 2021 at 03:42:01PM +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@redhat.com>
> > ---
> > fs/btrfs/file.c | 5 +++--
> > fs/erofs/data.c | 2 +-
> > 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 ++--
> > 8 files changed, 24 insertions(+), 17 deletions(-)
> >
> > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> > index f37211d3bb69..9d41b28c67ba 100644
> > --- a/fs/btrfs/file.c
> > +++ b/fs/btrfs/file.c
> > @@ -1957,7 +1957,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
> > }
> >
> > dio = __iomap_dio_rw(iocb, from, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
> > - 0);
> > + 0, 0);
> >
> > btrfs_inode_unlock(inode, ilock_flags);
> >
> > @@ -3658,7 +3658,8 @@ static ssize_t btrfs_direct_read(struct kiocb *iocb, struct iov_iter *to)
> > return 0;
> >
> > btrfs_inode_lock(inode, BTRFS_ILOCK_SHARED);
> > - ret = iomap_dio_rw(iocb, to, &btrfs_dio_iomap_ops, &btrfs_dio_ops, 0);
> > + ret = iomap_dio_rw(iocb, to, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
> > + 0, 0);
> > btrfs_inode_unlock(inode, BTRFS_ILOCK_SHARED);
> > return ret;
> > }
> > diff --git a/fs/erofs/data.c b/fs/erofs/data.c
> > index 9db829715652..16a41d0db55a 100644
> > --- a/fs/erofs/data.c
> > +++ b/fs/erofs/data.c
> > @@ -287,7 +287,7 @@ static ssize_t erofs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> >
> > if (!err)
> > return iomap_dio_rw(iocb, to, &erofs_iomap_ops,
> > - NULL, 0);
> > + NULL, 0, 0);
> > if (err < 0)
> > return err;
> > }
> > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > index ac0e11bbb445..b25c1f8f7c4f 100644
> > --- a/fs/ext4/file.c
> > +++ b/fs/ext4/file.c
> > @@ -74,7 +74,7 @@ static ssize_t ext4_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
> > return generic_file_read_iter(iocb, to);
> > }
> >
> > - ret = iomap_dio_rw(iocb, to, &ext4_iomap_ops, NULL, 0);
> > + ret = iomap_dio_rw(iocb, to, &ext4_iomap_ops, NULL, 0, 0);
> > inode_unlock_shared(inode);
> >
> > file_accessed(iocb->ki_filp);
> > @@ -566,7 +566,8 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
> > if (ilock_shared)
> > iomap_ops = &ext4_iomap_overwrite_ops;
> > ret = iomap_dio_rw(iocb, from, iomap_ops, &ext4_dio_write_ops,
> > - (unaligned_io || extend) ? IOMAP_DIO_FORCE_WAIT : 0);
> > + (unaligned_io || extend) ? IOMAP_DIO_FORCE_WAIT : 0,
> > + 0);
> > if (ret == -ENOTBLK)
> > ret = 0;
> >
> > diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
> > index b07b9c2d0446..ae06defcf369 100644
> > --- a/fs/gfs2/file.c
> > +++ b/fs/gfs2/file.c
> > @@ -822,7 +822,7 @@ static ssize_t gfs2_file_direct_read(struct kiocb *iocb, struct iov_iter *to,
> > if (ret)
> > goto out_uninit;
> >
> > - ret = iomap_dio_rw(iocb, to, &gfs2_iomap_ops, NULL, 0);
> > + ret = iomap_dio_rw(iocb, to, &gfs2_iomap_ops, NULL, 0, 0);
> > gfs2_glock_dq(gh);
> > out_uninit:
> > gfs2_holder_uninit(gh);
> > @@ -856,7 +856,7 @@ static ssize_t gfs2_file_direct_write(struct kiocb *iocb, struct iov_iter *from,
> > if (offset + len > i_size_read(&ip->i_inode))
> > goto out;
> >
> > - ret = iomap_dio_rw(iocb, from, &gfs2_iomap_ops, NULL, 0);
> > + ret = iomap_dio_rw(iocb, from, &gfs2_iomap_ops, NULL, 0, 0);
> > if (ret == -ENOTBLK)
> > ret = 0;
> > out:
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index a434fb7887b2..fdf68339bc8b 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;
>
> I have basically the same comment as last time[1]:
>
> "So, now that I actually understand the reason why the count of
> previously transferred bytes has to be passed into the iomap_dio, I
> would like this field to have a comment so that stupid maintainers like
> me don't forget the subtleties again:
>
> /*
> * For asynchronous IO, we have one chance to call the iocb
> * completion method with the results of the directio operation.
> * If this operation is a resubmission after a previous partial
> * completion (e.g. page fault), we need to know about that
> * progress so that we can report both the results of the prior
> * completion and the result of the resubmission to the iocb
> * submitter.
> */
> size_t done_before;
>
> With that added, I think I can live with this enough to:
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Indeed, sorry for missing that. How about the below change instead
though; I think that better sums up what's going on?

--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -454,6 +454,14 @@ static loff_t iomap_dio_iter(const struct iomap_iter *iter,
* may be pure data writes. In that case, we still need to do a full data sync
* completion.
*
+ * When page faults are disabled, __iomap_dio_rw has already prepared some data
+ * to be transferred, @iter references a non-resident page, and @dio_flags
+ * includes IOMAP_DIO_PARTIAL, __iomap_dio_rw will return a partial result. In
+ * that case, the page or pages can be faulted in and the request resumed with
+ * @done_before set to the number of bytes previously transferred. The request
+ * will then complete with the correct total number of bytes transferred; this
+ * is essential for completing partial requests asynchronously.
+ *
* Returns -ENOTBLK In case of a page invalidation invalidation failure for
* writes. The callers needs to fall back to buffered I/O in this case.
*/

Thanks,
Andreas

>
> --D
>
> [1] https://lore.kernel.org/linux-fsdevel/20210903185351.GD9892@magnolia/
>
> > bool wait_for_completion;
> >
> > union {
> > @@ -124,6 +125,9 @@ 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;
> > +
> > kfree(dio);
> >
> > return ret;
> > @@ -456,7 +460,7 @@ static loff_t iomap_dio_iter(const struct iomap_iter *iter,
> > struct iomap_dio *
> > __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> > const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
> > - unsigned int dio_flags)
> > + unsigned int dio_flags, size_t done_before)
> > {
> > struct address_space *mapping = iocb->ki_filp->f_mapping;
> > struct inode *inode = file_inode(iocb->ki_filp);
> > @@ -486,6 +490,7 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> > dio->dops = dops;
> > dio->error = 0;
> > dio->flags = 0;
> > + dio->done_before = done_before;
> >
> > dio->submit.iter = iter;
> > dio->submit.waiter = current;
> > @@ -652,11 +657,11 @@ EXPORT_SYMBOL_GPL(__iomap_dio_rw);
> > ssize_t
> > iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> > const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
> > - unsigned int dio_flags)
> > + unsigned int dio_flags, size_t done_before)
> > {
> > struct iomap_dio *dio;
> >
> > - dio = __iomap_dio_rw(iocb, iter, ops, dops, dio_flags);
> > + dio = __iomap_dio_rw(iocb, iter, ops, dops, dio_flags, done_before);
> > if (IS_ERR_OR_NULL(dio))
> > return PTR_ERR_OR_ZERO(dio);
> > return iomap_dio_complete(dio);
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 7aa943edfc02..240eb932c014 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -259,7 +259,7 @@ xfs_file_dio_read(
> > ret = xfs_ilock_iocb(iocb, XFS_IOLOCK_SHARED);
> > if (ret)
> > return ret;
> > - ret = iomap_dio_rw(iocb, to, &xfs_read_iomap_ops, NULL, 0);
> > + ret = iomap_dio_rw(iocb, to, &xfs_read_iomap_ops, NULL, 0, 0);
> > xfs_iunlock(ip, XFS_IOLOCK_SHARED);
> >
> > return ret;
> > @@ -569,7 +569,7 @@ xfs_file_dio_write_aligned(
> > }
> > trace_xfs_file_direct_write(iocb, from);
> > ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
> > - &xfs_dio_write_ops, 0);
> > + &xfs_dio_write_ops, 0, 0);
> > out_unlock:
> > if (iolock)
> > xfs_iunlock(ip, iolock);
> > @@ -647,7 +647,7 @@ xfs_file_dio_write_unaligned(
> >
> > trace_xfs_file_direct_write(iocb, from);
> > ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
> > - &xfs_dio_write_ops, flags);
> > + &xfs_dio_write_ops, flags, 0);
> >
> > /*
> > * Retry unaligned I/O with exclusive blocking semantics if the DIO
> > diff --git a/fs/zonefs/super.c b/fs/zonefs/super.c
> > index ddc346a9df9b..6122c38ab44d 100644
> > --- a/fs/zonefs/super.c
> > +++ b/fs/zonefs/super.c
> > @@ -852,7 +852,7 @@ static ssize_t zonefs_file_dio_write(struct kiocb *iocb, struct iov_iter *from)
> > ret = zonefs_file_dio_append(iocb, from);
> > else
> > ret = iomap_dio_rw(iocb, from, &zonefs_iomap_ops,
> > - &zonefs_write_dio_ops, 0);
> > + &zonefs_write_dio_ops, 0, 0);
> > if (zi->i_ztype == ZONEFS_ZTYPE_SEQ &&
> > (ret > 0 || ret == -EIOCBQUEUED)) {
> > if (ret > 0)
> > @@ -987,7 +987,7 @@ static ssize_t zonefs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> > }
> > file_accessed(iocb->ki_filp);
> > ret = iomap_dio_rw(iocb, to, &zonefs_iomap_ops,
> > - &zonefs_read_dio_ops, 0);
> > + &zonefs_read_dio_ops, 0, 0);
> > } else {
> > ret = generic_file_read_iter(iocb, to);
> > if (ret == -EIO)
> > diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> > index 2a213b0d1e1f..829f2325ecba 100644
> > --- a/include/linux/iomap.h
> > +++ b/include/linux/iomap.h
> > @@ -339,10 +339,10 @@ struct iomap_dio_ops {
> >
> > ssize_t iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> > const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
> > - unsigned int dio_flags);
> > + unsigned int dio_flags, size_t done_before);
> > struct iomap_dio *__iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> > const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
> > - unsigned int dio_flags);
> > + unsigned int dio_flags, size_t done_before);
> > ssize_t iomap_dio_complete(struct iomap_dio *dio);
> > int iomap_dio_iopoll(struct kiocb *kiocb, bool spin);
> >
> > --
> > 2.26.3
> >
>