Re: [PATCH v6 1/3] iomap: pass byte granular end position to iomap_add_to_ioend

[Brian Foster <bfoster@redhat.com>]

On Mon, Dec 09, 2024 at 07:42:39PM +0800, Long Li wrote:
> This is a preparatory patch for fixing zero padding issues in concurrent
> append write scenarios. In the following patches, we need to obtain
> byte-granular writeback end position for io_size trimming after EOF
> handling.
> 
> Due to concurrent writeback and truncate operations, inode size may
> shrink. Resampling inode size would force writeback code to handle the
> newly appeared post-EOF blocks, which is undesirable. As Dave
> explained in [1]:
> 
> "Really, the issue is that writeback mappings have to be able to
> handle the range being mapped suddenly appear to be beyond EOF.
> This behaviour is a longstanding writeback constraint, and is what
> iomap_writepage_handle_eof() is attempting to handle.
> 
> We handle this by only sampling i_size_read() whilst we have the
> folio locked and can determine the action we should take with that
> folio (i.e. nothing, partial zeroing, or skip altogether). Once
> we've made the decision that the folio is within EOF and taken
> action on it (i.e. moved the folio to writeback state), we cannot
> then resample the inode size because a truncate may have started
> and changed the inode size."
> 
> To avoid resampling inode size after EOF handling, we convert end_pos
> to byte-granular writeback position and return it from EOF handling
> function.
> 
> Since iomap_set_range_dirty() can handle unaligned lengths, this
> conversion has no impact on it. However, iomap_find_dirty_range()
> requires aligned start and end range to find dirty blocks within the
> given range, so the end position needs to be rounded up when passed
> to it.
> 
> LINK [1]: https://lore.kernel.org/linux-xfs/Z1Gg0pAa54MoeYME@localhost.localdomain/
> Signed-off-by: Long Li <leo.lilong@huawei.com>
> ---
>  fs/iomap/buffered-io.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 955f19e27e47..bcc7831d03af 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
...
> @@ -1914,6 +1915,7 @@ static int iomap_writepage_map(struct iomap_writepage_ctx *wpc,
>  	struct inode *inode = folio->mapping->host;
>  	u64 pos = folio_pos(folio);
>  	u64 end_pos = pos + folio_size(folio);
> +	u64 end_aligned = 0;
>  	unsigned count = 0;
>  	int error = 0;
>  	u32 rlen;
> @@ -1955,9 +1957,10 @@ static int iomap_writepage_map(struct iomap_writepage_ctx *wpc,
>  	/*
>  	 * Walk through the folio to find dirty areas to write back.
>  	 */
> -	while ((rlen = iomap_find_dirty_range(folio, &pos, end_pos))) {
> +	end_aligned = round_up(end_pos, i_blocksize(inode));

So do I follow correctly that the set_range_dirty() path doesn't need
the alignment because it uses inclusive first_blk/last_blk logic,
whereas this find_dirty_range() path does the opposite and thus does
require the round_up? If so, presumably that means if we fixed up the
find path we wouldn't need end_aligned at all anymore?

If I follow the reasoning correctly, then this looks Ok to me:

Reviewed-by: Brian Foster <bfoster@redhat.com>

... but as a followup exercise it might be nice to clean up the
iomap_find_dirty_range() path to either do the rounding itself or be
more consistent with set_range_dirty().

Brian

> +	while ((rlen = iomap_find_dirty_range(folio, &pos, end_aligned))) {
>  		error = iomap_writepage_map_blocks(wpc, wbc, folio, inode,
> -				pos, rlen, &count);
> +				pos, end_pos, rlen, &count);
>  		if (error)
>  			break;
>  		pos += rlen;
> -- 
> 2.39.2
> 
>