Re: [PATCH v3 5/7] xfs: fill dirty folios on zero range of unwritten mappings

[Brian Foster <bfoster@redhat.com>]

On Mon, Jul 14, 2025 at 10:28:11PM -0700, Darrick J. Wong wrote:
> On Mon, Jul 14, 2025 at 04:41:20PM -0400, Brian Foster wrote:
> > Use the iomap folio batch mechanism to select folios to zero on zero
> > range of unwritten mappings. Trim the resulting mapping if the batch
> > is filled (unlikely for current use cases) to distinguish between a
> > range to skip and one that requires another iteration due to a full
> > batch.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/xfs/xfs_iomap.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> > 
> > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > index b5cf5bc6308d..63054f7ead0e 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -1691,6 +1691,8 @@ xfs_buffered_write_iomap_begin(
> >  	struct iomap		*iomap,
> >  	struct iomap		*srcmap)
> >  {
> > +	struct iomap_iter	*iter = container_of(iomap, struct iomap_iter,
> > +						     iomap);
> >  	struct xfs_inode	*ip = XFS_I(inode);
> >  	struct xfs_mount	*mp = ip->i_mount;
> >  	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
> > @@ -1762,6 +1764,7 @@ xfs_buffered_write_iomap_begin(
> >  	 */
> >  	if (flags & IOMAP_ZERO) {
> >  		xfs_fileoff_t eof_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
> > +		u64 end;
> >  
> >  		if (isnullstartblock(imap.br_startblock) &&
> >  		    offset_fsb >= eof_fsb)
> > @@ -1769,6 +1772,26 @@ xfs_buffered_write_iomap_begin(
> >  		if (offset_fsb < eof_fsb && end_fsb > eof_fsb)
> >  			end_fsb = eof_fsb;
> >  
> > +		/*
> > +		 * Look up dirty folios for unwritten mappings within EOF.
> > +		 * Providing this bypasses the flush iomap uses to trigger
> > +		 * extent conversion when unwritten mappings have dirty
> > +		 * pagecache in need of zeroing.
> > +		 *
> > +		 * Trim the mapping to the end pos of the lookup, which in turn
> > +		 * was trimmed to the end of the batch if it became full before
> > +		 * the end of the mapping.
> > +		 */
> > +		if (imap.br_state == XFS_EXT_UNWRITTEN &&
> > +		    offset_fsb < eof_fsb) {
> > +			loff_t len = min(count,
> > +					 XFS_FSB_TO_B(mp, imap.br_blockcount));
> > +
> > +			end = iomap_fill_dirty_folios(iter, offset, len);
> > +			end_fsb = min_t(xfs_fileoff_t, end_fsb,
> > +					XFS_B_TO_FSB(mp, end));
> 
> Hrmm.  XFS_B_TO_FSB and not _FSBT?  Can the rounding up behavior result
> in a missed byte range?  I think the answer is no because @end should be
> aligned to a folio boundary, and folios can't be smaller than an
> fsblock.
> 

Hmm.. not that I'm aware of..? Please elaborate if there's a case you're
suspicious of because I could have certainly got my wires crossed.

My thinking is that end_fsb reflects the first fsb beyond the target
range. I.e., it's calculated and used as such in xfs_iomap_end_fsb() and
the various xfs_trim_extent() calls throughout the rest of the function.

Brian

> If the answer to the second question is indeed "no" then I think this is
> ok and
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> 
> --D
> 
> 
> > +		}
> > +
> >  		xfs_trim_extent(&imap, offset_fsb, end_fsb - offset_fsb);
> >  	}
> >  
> > -- 
> > 2.50.0
> > 
> > 
>