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? 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 > + 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 > >
