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@xxxxxxxxxx> > > --- > > 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@xxxxxxxxxx> > > ... 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
