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

[Long Li <leo.lilong@huawei.com>]

On Mon, Dec 09, 2024 at 09:06:14AM -0500, Brian Foster wrote:
> 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?
> 

Agreed with you.

> 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

Yes, I think we can handle the cleanup through a separate patch later?                                                                            

Thanks,
Long Li