RE: [PATCH v6.1 6/7] fs/xfs: Handle CoW for fsdax write() path

[ruansy.fnst@xxxxxxxxxxx]

> -----Original Message-----
> From: Darrick J. Wong <djwong@xxxxxxxxxx>
> Subject: Re: [PATCH v6.1 6/7] fs/xfs: Handle CoW for fsdax write() path
> 
> On Mon, Jun 28, 2021 at 02:55:03AM +0000, ruansy.fnst@xxxxxxxxxxx wrote:
> > > -----Original Message-----
> > > Subject: Re: [PATCH v6.1 6/7] fs/xfs: Handle CoW for fsdax write()
> > > path
> > >
> > > On Thu, Jun 24, 2021 at 08:49:17AM +0000, ruansy.fnst@xxxxxxxxxxx wrote:
> > > > Hi Darrick,
> > > >
> > > > Do you have any comment on this?
> > >
> > > Sorry, was on vacation.
> > >
> > > > Thanks,
> > > > Ruan.
> > > >
> > > > > -----Original Message-----
> > > > > From: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
> > > > > Subject: [PATCH v6.1 6/7] fs/xfs: Handle CoW for fsdax write()
> > > > > path
> > > > >
> > > > > Hi Darrick,
> > > > >
> > > > > Since other patches looks good, I post this RFC patch singly to
> > > > > hot-fix the problem in xfs_dax_write_iomap_ops->iomap_end() of
> > > > > v6 that the error code was ingored. I will split this in two
> > > > > patches(changes in iomap and xfs
> > > > > respectively) in next formal version if it looks ok.
> > > > >
> > > > > ====
> > > > >
> > > > > Introduce a new interface called "iomap_post_actor()" in iomap_ops.
> > > > > And call it between ->actor() and ->iomap_end().  It is mean to
> > > > > handle the error code returned from ->actor().  In this
> > > > > patchset, it is used to remap or cancel the CoW extents according to the
> error code.
> > > > >
> > > > > Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
> > > > > ---
> > > > >  fs/dax.c               | 27 ++++++++++++++++++---------
> > > > >  fs/iomap/apply.c       |  4 ++++
> > > > >  fs/xfs/xfs_bmap_util.c |  3 +--
> > > > >  fs/xfs/xfs_file.c      |  5 +++--
> > > > >  fs/xfs/xfs_iomap.c     | 33 ++++++++++++++++++++++++++++++++-
> > > > >  fs/xfs/xfs_iomap.h     | 24 ++++++++++++++++++++++++
> > > > >  fs/xfs/xfs_iops.c      |  7 +++----
> > > > >  fs/xfs/xfs_reflink.c   |  3 +--
> > > > >  include/linux/iomap.h  |  8 ++++++++
> > > > >  9 files changed, 94 insertions(+), 20 deletions(-)
> > > > >
> > > > > diff --git a/fs/dax.c b/fs/dax.c index
> > > > > 93f16210847b..0740c2610b6f 100644
> > > > > --- a/fs/dax.c
> > > > > +++ b/fs/dax.c
> > > > > @@ -1537,7 +1537,7 @@ static vm_fault_t
> > > > > dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> > > > >  	struct iomap iomap = { .type = IOMAP_HOLE };
> > > > >  	struct iomap srcmap = { .type = IOMAP_HOLE };
> > > > >  	unsigned flags = IOMAP_FAULT;
> > > > > -	int error;
> > > > > +	int error, copied = PAGE_SIZE;
> > > > >  	bool write = vmf->flags & FAULT_FLAG_WRITE;
> > > > >  	vm_fault_t ret = 0, major = 0;
> > > > >  	void *entry;
> > > > > @@ -1598,7 +1598,7 @@ static vm_fault_t
> > > > > dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> > > > >  	ret = dax_fault_actor(vmf, pfnp, &xas, &entry, false, flags,
> > > > >  			      &iomap, &srcmap);
> > > > >  	if (ret == VM_FAULT_SIGBUS)
> > > > > -		goto finish_iomap;
> > > > > +		goto finish_iomap_actor;
> > > > >
> > > > >  	/* read/write MAPPED, CoW UNWRITTEN */
> > > > >  	if (iomap.flags & IOMAP_F_NEW) { @@ -1607,10 +1607,16 @@
> > > > > static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf,
> > > > > pfn_t *pfnp,
> > > > >  		major = VM_FAULT_MAJOR;
> > > > >  	}
> > > > >
> > > > > + finish_iomap_actor:
> > > > > +	if (ops->iomap_post_actor) {
> > > > > +		if (ret & VM_FAULT_ERROR)
> > > > > +			copied = 0;
> > > > > +		ops->iomap_post_actor(inode, pos, PMD_SIZE, copied, flags,
> > > > > +				      &iomap, &srcmap);
> > > > > +	}
> > > > > +
> > > > >  finish_iomap:
> > > > >  	if (ops->iomap_end) {
> > > > > -		int copied = PAGE_SIZE;
> > > > > -
> > > > >  		if (ret & VM_FAULT_ERROR)
> > > > >  			copied = 0;
> > > > >  		/*
> > > > > @@ -1677,7 +1683,7 @@ static vm_fault_t
> > > > > dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
> > > > >  	pgoff_t max_pgoff;
> > > > >  	void *entry;
> > > > >  	loff_t pos;
> > > > > -	int error;
> > > > > +	int error, copied = PMD_SIZE;
> > > > >
> > > > >  	/*
> > > > >  	 * Check whether offset isn't beyond end of file now. Caller
> > > > > is @@
> > > > > -1736,12
> > > > > +1742,15 @@ static vm_fault_t dax_iomap_pmd_fault(struct
> > > > > +vm_fault *vmf,
> > > > > pfn_t *pfnp,
> > > > >  	ret = dax_fault_actor(vmf, pfnp, &xas, &entry, true, flags,
> > > > >  			      &iomap, &srcmap);
> > > > >
> > > > > +	if (ret == VM_FAULT_FALLBACK)
> > > > > +		copied = 0;
> > > > > +	if (ops->iomap_post_actor) {
> > > > > +		ops->iomap_post_actor(inode, pos, PMD_SIZE, copied, flags,
> > > > > +				      &iomap, &srcmap);
> > > > > +	}
> > > > > +
> > > > >  finish_iomap:
> > > > >  	if (ops->iomap_end) {
> > > > > -		int copied = PMD_SIZE;
> > > > > -
> > > > > -		if (ret == VM_FAULT_FALLBACK)
> > > > > -			copied = 0;
> > > > >  		/*
> > > > >  		 * The fault is done by now and there's no way back (other
> > > > >  		 * thread may be already happily using PMD we have installed).
> > > > > diff --git a/fs/iomap/apply.c b/fs/iomap/apply.c index
> > > > > 0493da5286ad..26a54ded184f 100644
> > > > > --- a/fs/iomap/apply.c
> > > > > +++ b/fs/iomap/apply.c
> > > > > @@ -84,6 +84,10 @@ iomap_apply(struct inode *inode, loff_t pos,
> > > > > loff_t length, unsigned flags,
> > > > >  	written = actor(inode, pos, length, data, &iomap,
> > > > >  			srcmap.type != IOMAP_HOLE ? &srcmap : &iomap);
> > > > >
> > > > > +	if (ops->iomap_post_actor) {
> > > > > +		written = ops->iomap_post_actor(inode, pos, length, written,
> > > > > +						flags, &iomap, &srcmap);
> > >
> > > How many operations actually need an iomap_post_actor?  It's just
> > > the dax ones, right?  Which is ... iomap_truncate_page,
> > > iomap_zero_range, dax_iomap_fault, and dax_iomap_rw, right?  We
> > > don't need a post_actor for other iomap functionality (like FIEMAP,
> > > SEEK_DATA/SEEK_HOLE, etc.) so adding a new function pointer for all
> operations feels a bit overbroad.
> >
> > Yes.
> >
> > >
> > > I had imagined that you'd create a struct dax_iomap_ops to wrap all
> > > the extra functionality that you need for dax operations:
> > >
> > > struct dax_iomap_ops {
> > > 	struct iomap_ops	iomap_ops;
> > >
> > > 	int			(*end_io)(inode, pos, length...);
> > > };
> > >
> > > And alter the four functions that you need to take the special
> dax_iomap_ops.
> > > I guess the downside is that this makes iomap_truncate_page and
> > > iomap_zero_range more complicated, but maybe it's just time to split
> > > those into DAX-specific versions.  Then we'd be rid of the
> > > cross-links betwee fs/iomap/buffered-io.c and fs/dax.c.
> >
> > This seems to be a better solution.  I'll try in this way.  Thanks for your
> guidance.
> 
> I started writing on Friday a patchset to apply this style cleanup both to the
> directio and dax paths.  The cleanups were pretty straightforward until I
> started reading the dax code paths again and realized that file writes still have
> the weird behavior of mapping extents into a file, zeroing them, then issuing the
> actual write to the extent.  IOWs, a double-write to avoid exposing stale
> contents if crash.

The current code seems not zeroing an unwritten extent when writing in fsdax mode?  Just allocate unwritten extents in filesystem, and then write data in fsdax.

> 
> Apparently the reason for this was that dax (at least 6 years ago) had no
> concept paralleling the page lock, so it was necessary to do that to avoid page
> fault handlers racing to map pfns into the file mapping?
> That would seem to prevent us from doing the more standard behavior of
> allocate unwritten, write data, convert mapping... but is that still the case?  Or
> can we get rid of this bad quirk?

I am not sure about this...


--
Thanks,
Ruan.

> 
> --D
> 
> >
> > >
> > > > > +	}
> > > > >  out:
> > > > >  	/*
> > > > >  	 * Now the data has been copied, commit the range we've copied.
> > > > > This 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..89406ec6741b
> > > > > 100644
> > > > > --- a/fs/xfs/xfs_file.c
> > > > > +++ b/fs/xfs/xfs_file.c
> > > > > @@ -684,11 +684,12 @@ 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);
> > > > > +	ret = dax_iomap_rw(iocb, from, &xfs_dax_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);
> > > > >  	}
> > > > > +
> > > > >  out:
> > > > >  	if (iolock)
> > > > >  		xfs_iunlock(ip, iolock);
> > > > > @@ -1309,7 +1310,7 @@ __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..2f322e2f8544
> > > > > 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 +855,36 @@ const struct iomap_ops
> > > > > xfs_direct_write_iomap_ops = {
> > > > >  	.iomap_begin		= xfs_direct_write_iomap_begin,
> > > > >  };
> > > > >
> > > > > +static int
> > > > > +xfs_dax_write_iomap_post_actor(
> > > > > +	struct inode		*inode,
> > > > > +	loff_t			pos,
> > > > > +	loff_t			length,
> > > > > +	ssize_t			written,
> > > > > +	unsigned int		flags,
> > > > > +	struct iomap		*iomap,
> > > > > +	struct iomap		*srcmap)
> > > > > +{
> > > > > +	int			error = 0;
> > > > > +	struct xfs_inode	*ip = XFS_I(inode);
> > > > > +	bool			cow = xfs_is_cow_inode(ip);
> > > > > +
> > > > > +	if (written <= 0) {
> > > > > +		if (cow)
> > > > > +			xfs_reflink_cancel_cow_range(ip, pos, length, true);
> > > > > +		return written;
> > > > > +	}
> > > > > +
> > > > > +	if (cow)
> > > > > +		error = xfs_reflink_end_cow(ip, pos, written);
> > > > > +	return error ?: written;
> > > > > +}
> > >
> > > This is pretty much the same as what xfs_dio_write_end_io does, right?
> >
> > It just handles the end part of CoW here.
> > xfs_dio_write_end_io() also updates file size, which is only needed in write()
> but not in page fault.  And the update file size work is done in
> xfs_dax_file_write(), it's fine, no need to modify it.
> >
> > >
> > > I had imagined that you'd change the function signatures to drop the
> > > iocb so that you could reuse this code instead of creating a whole new
> callback.
> > >
> > > Ah well.  Can I send you some prep patches to clean up some of the
> > > weird iomap code as a preparation series for this?
> >
> > Sure.  Thanks.
> >
> >
> > --
> > Ruan.
> >
> > >
> > > --D
> > >
> > > > > +
> > > > > +const struct iomap_ops xfs_dax_write_iomap_ops = {
> > > > > +	.iomap_begin		= xfs_direct_write_iomap_begin,
> > > > > +	.iomap_post_actor	= xfs_dax_write_iomap_post_actor,
> > > > > +};
> > > > > +
> > > > >  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);
> > > > >  }
> > > > >
> > > > >  /*
> > > > > diff --git a/include/linux/iomap.h b/include/linux/iomap.h index
> > > > > 95562f863ad0..58f2e1c78018 100644
> > > > > --- a/include/linux/iomap.h
> > > > > +++ b/include/linux/iomap.h
> > > > > @@ -135,6 +135,14 @@ struct iomap_ops {
> > > > >  			unsigned flags, struct iomap *iomap,
> > > > >  			struct iomap *srcmap);
> > > > >
> > > > > +	/*
> > > > > +	 * Handle the error code from actor(). Do the finishing jobs for extra
> > > > > +	 * operations, such as CoW, according to whether written is negative.
> > > > > +	 */
> > > > > +	int (*iomap_post_actor)(struct inode *inode, loff_t pos, loff_t length,
> > > > > +			ssize_t written, unsigned flags, struct iomap *iomap,
> > > > > +			struct iomap *srcmap);
> > > > > +
> > > > >  	/*
> > > > >  	 * Commit and/or unreserve space previous allocated using
> > > iomap_begin.
> > > > >  	 * Written indicates the length of the successful write
> > > > > operation which
> > > > > --
> > > > > 2.31.1
> > > >