Re: [PATCH v3 6/7] iomap: remove old partial eof zeroing optimization

Darrick J. Wong
2025-07-15T05:34:22Z

On Mon, Jul 14, 2025 at 04:41:21PM -0400, Brian Foster wrote:
> iomap_zero_range() optimizes the partial eof block zeroing use case
> by force zeroing if the mapping is dirty. This is to avoid frequent
> flushing on file extending workloads, which hurts performance.
>
> Now that the folio batch mechanism provides a more generic solution
> and is used by the only real zero range user (XFS), this isolated
> optimization is no longer needed. Remove the unnecessary code and
> let callers use the folio batch or fall back to flushing by default.
>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>

Heh, I was staring at this last Friday chasing fuse+iomap bugs in
fallocate zerorange and straining to remember what this does.
Is this chunk still needed if the ->iomap_begin implementation doesn't
(or forgets to) grab the folio batch for iomap?

My bug turned out to be a bug in my fuse+iomap design -- with the way
iomap_zero_range does things, you have to flush+unmap, punch the range
and zero the range. If you punch and realloc the range and *then* try
to zero the range, the new unwritten extents cause iomap to miss dirty
pages that fuse should've unmapped. Ooops.

--D

> ---
> fs/iomap/buffered-io.c | 24 ------------------------
> 1 file changed, 24 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 194e3cc0857f..d2bbed692c06 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1484,33 +1484,9 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
> .private = private,
> };
> struct address_space *mapping = inode->i_mapping;
> - unsigned int blocksize = i_blocksize(inode);
> - unsigned int off = pos & (blocksize - 1);
> - loff_t plen = min_t(loff_t, len, blocksize - off);
> int ret;
> bool range_dirty;
>
> - /*
> - * Zero range can skip mappings that are zero on disk so long as
> - * pagecache is clean. If pagecache was dirty prior to zero range, the
> - * mapping converts on writeback completion and so must be zeroed.
> - *
> - * The simplest way to deal with this across a range is to flush
> - * pagecache and process the updated mappings. To avoid excessive
> - * flushing on partial eof zeroing, special case it to zero the
> - * unaligned start portion if already dirty in pagecache.
> - */
> - if (!iter.fbatch && off &&
> - filemap_range_needs_writeback(mapping, pos, pos + plen - 1)) {
> - iter.len = plen;
> - while ((ret = iomap_iter(&iter, ops)) > 0)
> - iter.status = iomap_zero_iter(&iter, did_zero);
> -
> - iter.len = len - (iter.pos - pos);
> - if (ret || !iter.len)
> - return ret;
> - }
> -
> /*
> * To avoid an unconditional flush, check pagecache state and only flush
> * if dirty and the fs returns a mapping that might convert on
> --
> 2.50.0
>
>