> -----Original Message-----
> From: Darrick J. Wong <djwong@xxxxxxxxxx>
> Subject: Re: [PATCH v6 6/7] fs/xfs: Handle CoW for fsdax write() path
>
> On Wed, May 19, 2021 at 02:00:44PM +0800, Shiyang Ruan wrote:
> > In fsdax mode, WRITE and ZERO on a shared extent need CoW performed.
> > After CoW, new allocated extents needs to be remapped to the file.
> > So, add an iomap_end for dax write ops to do the remapping work.
> >
> > Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
> > ---
> > fs/xfs/xfs_bmap_util.c | 3 +--
> > fs/xfs/xfs_file.c | 9 +++------
> > fs/xfs/xfs_iomap.c | 38 +++++++++++++++++++++++++++++++++++++-
> > fs/xfs/xfs_iomap.h | 24 ++++++++++++++++++++++++
> > fs/xfs/xfs_iops.c | 7 +++----
> > fs/xfs/xfs_reflink.c | 3 +--
> > 6 files changed, 69 insertions(+), 15 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c index
> > a5e9d7d34023..2a36dc93ff27 100644
> > --- a/fs/xfs/xfs_bmap_util.c
> > +++ b/fs/xfs/xfs_bmap_util.c
> > @@ -965,8 +965,7 @@ xfs_free_file_space(
> > return 0;
> > if (offset + len > XFS_ISIZE(ip))
> > len = XFS_ISIZE(ip) - offset;
> > - error = iomap_zero_range(VFS_I(ip), offset, len, NULL,
> > - &xfs_buffered_write_iomap_ops);
> > + error = xfs_iomap_zero_range(ip, offset, len, NULL);
> > if (error)
> > return error;
> >
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c index
> > 396ef36dcd0a..38d8eca05aee 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -684,11 +684,8 @@ xfs_file_dax_write(
> > pos = iocb->ki_pos;
> >
> > trace_xfs_file_dax_write(iocb, from);
> > - ret = dax_iomap_rw(iocb, from, &xfs_direct_write_iomap_ops);
> > - if (ret > 0 && iocb->ki_pos > i_size_read(inode)) {
> > - i_size_write(inode, iocb->ki_pos);
> > - error = xfs_setfilesize(ip, pos, ret);
> > - }
> > + ret = dax_iomap_rw(iocb, from, &xfs_dax_write_iomap_ops);
> > +
> > out:
> > if (iolock)
> > xfs_iunlock(ip, iolock);
> > @@ -1309,7 +1306,7 @@ __xfs_filemap_fault(
> >
> > ret = dax_iomap_fault(vmf, pe_size, &pfn, NULL,
> > (write_fault && !vmf->cow_page) ?
> > - &xfs_direct_write_iomap_ops :
> > + &xfs_dax_write_iomap_ops :
> > &xfs_read_iomap_ops);
> > if (ret & VM_FAULT_NEEDDSYNC)
> > ret = dax_finish_sync_fault(vmf, pe_size, pfn); diff --git
> > a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c index
> > d154f42e2dc6..938723aa137d 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -761,7 +761,8 @@ xfs_direct_write_iomap_begin(
> >
> > /* may drop and re-acquire the ilock */
> > error = xfs_reflink_allocate_cow(ip, &imap, &cmap, &shared,
> > - &lockmode, flags & IOMAP_DIRECT);
> > + &lockmode,
> > + (flags & IOMAP_DIRECT) || IS_DAX(inode));
> > if (error)
> > goto out_unlock;
> > if (shared)
> > @@ -854,6 +855,41 @@ const struct iomap_ops xfs_direct_write_iomap_ops
> = {
> > .iomap_begin = xfs_direct_write_iomap_begin,
> > };
> >
> > +static int
> > +xfs_dax_write_iomap_end(
> > + struct inode *inode,
> > + loff_t pos,
> > + loff_t length,
> > + ssize_t written,
> > + unsigned int flags,
> > + struct iomap *iomap)
> > +{
> > + int error = 0;
> > + struct xfs_inode *ip = XFS_I(inode);
> > + bool cow = xfs_is_cow_inode(ip);
> > +
> > + if (!written)
> > + return 0;
> > +
> > + if (pos + written > i_size_read(inode) && !(flags & IOMAP_FAULT)) {
> > + i_size_write(inode, pos + written);
> > + error = xfs_setfilesize(ip, pos, written);
> > + if (error && cow) {
> > + xfs_reflink_cancel_cow_range(ip, pos, written, true);
> > + return error;
> > + }
> > + }
> > + if (cow)
> > + error = xfs_reflink_end_cow(ip, pos, written);
> > +
> > + return error;
>
> I think this (the ->iomap_end handler) is the wrong place to be performing COW
> remapping, because of this chunk in iomap_apply:
>
> /*
> * Now the data has been copied, commit the range we've copied.
> * This should not fail unless the filesystem has had a fatal
> * error.
> */
> if (ops->iomap_end) {
> ret = ops->iomap_end(inode, pos, length,
> written > 0 ? written : 0,
> flags, &iomap);
> }
>
> return written ? written : ret;
>
> If we managed to write something but the remap fails, we'll eat the error
> message and return the length of the write to the caller.
> Eventually the callers /may/ notice that they can still read old file contents after
> a "successful" write.
Understood.
>
> I think what needs to happen here is that we call out to the filesystem to remap
> the blocks at the end of dax_iomap_actor, similar to how the iomap directio
> code calls xfs_dio_write_end_io after all of the write bios complete. If the
> remap fails, we return that error out of dax_iomap_actor, which will be
> returned to the caller as a short write or an error code if nothing got written.
>
> IOWs, the end of dax_iomap_actor should become:
>
> /* dax_copy_{to,from}_iter calls here */
>
> pos += xfer;
> length -= xfer;
> done += xfer;
>
> if (xfer == 0)
> ret = -EFAULT;
> if (xfer < map_len)
> break;
> }
> dax_read_unlock(id);
>
> if (dops && dops->end_io) {
> unsigned flags = 0;
>
> if (srcmap->addr != iomap->addr)
> flags |= IOMAP_DIO_COW;
>
> ret = dops->end_io(iocb, done, ret, flags);
> }
>
> if (likely(!ret)) {
> ret = done;
> /* check for short read */
> if (offset + ret > i_size_read(inode) && !write)
> ret = i_size_read(inode) - offset;
> iocb->ki_pos += ret;
> }
>
> return ret;
> }
>
> And I think you can even reuse the struct iomap_dio_ops and
> xfs_dio_write_end_io for this purpose.
This is a good idea. And the two dax page fault handlers also need it. So I am thinking, instead of copying this logic three times, how about adding an interface, called ->iomap_post_actor() for example, in the iomap_ops, between ->actor() and ->iomap_end() ? And this looks more common.
In addition, the dops->end_io() requires struct kiocb and its not available in dax page fault handler.
--
Ruan.
>
> > +}
> > +
> > +const struct iomap_ops xfs_dax_write_iomap_ops = {
> > + .iomap_begin = xfs_direct_write_iomap_begin,
> > + .iomap_end = xfs_dax_write_iomap_end,
> > +};
> > +
> > static int
> > xfs_buffered_write_iomap_begin(
> > struct inode *inode,
> > diff --git a/fs/xfs/xfs_iomap.h b/fs/xfs/xfs_iomap.h index
> > 7d3703556d0e..fbacf638ab21 100644
> > --- a/fs/xfs/xfs_iomap.h
> > +++ b/fs/xfs/xfs_iomap.h
> > @@ -42,8 +42,32 @@ xfs_aligned_fsb_count(
> >
> > extern const struct iomap_ops xfs_buffered_write_iomap_ops; extern
> > const struct iomap_ops xfs_direct_write_iomap_ops;
> > +extern const struct iomap_ops xfs_dax_write_iomap_ops;
> > extern const struct iomap_ops xfs_read_iomap_ops; extern const
> > struct iomap_ops xfs_seek_iomap_ops; extern const struct iomap_ops
> > xfs_xattr_iomap_ops;
> >
> > +static inline int
> > +xfs_iomap_zero_range(
> > + struct xfs_inode *ip,
> > + loff_t offset,
> > + loff_t len,
> > + bool *did_zero)
> > +{
> > + return iomap_zero_range(VFS_I(ip), offset, len, did_zero,
> > + IS_DAX(VFS_I(ip)) ? &xfs_dax_write_iomap_ops
> > + : &xfs_buffered_write_iomap_ops); }
> > +
> > +static inline int
> > +xfs_iomap_truncate_page(
> > + struct xfs_inode *ip,
> > + loff_t pos,
> > + bool *did_zero)
> > +{
> > + return iomap_truncate_page(VFS_I(ip), pos, did_zero,
> > + IS_DAX(VFS_I(ip)) ? &xfs_dax_write_iomap_ops
> > + : &xfs_buffered_write_iomap_ops); }
> > +
> > #endif /* __XFS_IOMAP_H__*/
> > diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c index
> > dfe24b7f26e5..6d936c3e1a6e 100644
> > --- a/fs/xfs/xfs_iops.c
> > +++ b/fs/xfs/xfs_iops.c
> > @@ -911,8 +911,8 @@ xfs_setattr_size(
> > */
> > if (newsize > oldsize) {
> > trace_xfs_zero_eof(ip, oldsize, newsize - oldsize);
> > - error = iomap_zero_range(inode, oldsize, newsize - oldsize,
> > - &did_zeroing, &xfs_buffered_write_iomap_ops);
> > + error = xfs_iomap_zero_range(ip, oldsize, newsize - oldsize,
> > + &did_zeroing);
> > } else {
> > /*
> > * iomap won't detect a dirty page over an unwritten block (or a @@
> > -924,8 +924,7 @@ xfs_setattr_size(
> > newsize);
> > if (error)
> > return error;
> > - error = iomap_truncate_page(inode, newsize, &did_zeroing,
> > - &xfs_buffered_write_iomap_ops);
> > + error = xfs_iomap_truncate_page(ip, newsize, &did_zeroing);
> > }
> >
> > if (error)
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c index
> > d25434f93235..9a780948dbd0 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -1266,8 +1266,7 @@ xfs_reflink_zero_posteof(
> > return 0;
> >
> > trace_xfs_zero_eof(ip, isize, pos - isize);
> > - return iomap_zero_range(VFS_I(ip), isize, pos - isize, NULL,
> > - &xfs_buffered_write_iomap_ops);
> > + return xfs_iomap_zero_range(ip, isize, pos - isize, NULL);
> > }
> >
> > /*
> > --
> > 2.31.1
> >
> >
> >
