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SubjectRe: [PATCH v7] iomap: make inline data support more flexible
On Mon, Jul 26, 2021 at 12:16:39AM +0200, Andreas Gruenbacher wrote:
> Here's a fixed and cleaned up version that passes fstests on gfs2.
> I see no reason why the combination of tail packing + writing should
> cause any issues, so in my opinion, the check that disables that
> combination in iomap_write_begin_inline should still be removed.

Since there is no such fs for tail-packing write, I just do a wild
guess, for example,
1) the tail-end block was not inlined, so iomap_write_end() dirtied
the whole page (or buffer) for the page writeback;
2) then it was truncated into a tail-packing inline block so the last
extent(page) became INLINE but dirty instead;
3) during the late page writeback for dirty pages,
if (WARN_ON_ONCE(wpc->iomap.type == IOMAP_INLINE))
would be triggered in iomap_writepage_map() for such dirty page.

As Matthew pointed out before,
currently tail-packing inline won't interact with page writeback, but
I'm afraid a supported tail-packing write fs needs to reconsider the
whole stuff how page, inode writeback works and what the pattern is
with the tail-packing.

> It turns out that returning the number of bytes copied from
> iomap_read_inline_data is a bit irritating: the function is really used
> for filling the page, but that's not always the "progress" we're looking
> for. In the iomap_readpage case, we actually need to advance by an
> antire page, but in the iomap_file_buffered_write case, we need to
> advance by the length parameter of iomap_write_actor or less. So I've
> changed that back.
> I've also renamed iomap_inline_buf to iomap_inline_data and I've turned
> iomap_inline_data_size_valid into iomap_within_inline_data, which seems
> more useful to me.
> Thanks,
> Andreas
> --
> Subject: [PATCH] iomap: Support tail packing
> The existing inline data support only works for cases where the entire
> file is stored as inline data. For larger files, EROFS stores the
> initial blocks separately and then can pack a small tail adjacent to the
> inode. Generalise inline data to allow for tail packing. Tails may not
> cross a page boundary in memory.
> We currently have no filesystems that support tail packing and writing,
> so that case is currently disabled (see iomap_write_begin_inline). I'm
> not aware of any reason why this code path shouldn't work, however.
> Cc: Christoph Hellwig <>
> Cc: Darrick J. Wong <>
> Cc: Matthew Wilcox <>
> Cc: Andreas Gruenbacher <>
> Tested-by: Huang Jianan <> # erofs
> Signed-off-by: Gao Xiang <>
> ---
> fs/iomap/buffered-io.c | 34 +++++++++++++++++++++++-----------
> fs/iomap/direct-io.c | 11 ++++++-----
> include/linux/iomap.h | 22 +++++++++++++++++++++-
> 3 files changed, 50 insertions(+), 17 deletions(-)
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 87ccb3438bec..334bf98fdd4a 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -205,25 +205,29 @@ struct iomap_readpage_ctx {
> struct readahead_control *rac;
> };
> -static void
> -iomap_read_inline_data(struct inode *inode, struct page *page,
> +static int iomap_read_inline_data(struct inode *inode, struct page *page,
> struct iomap *iomap)
> {
> - size_t size = i_size_read(inode);
> + size_t size = i_size_read(inode) - iomap->offset;

I wonder why you use i_size / iomap->offset here, and why you completely
ignoring iomap->length field returning by fs.

Using i_size here instead of iomap->length seems coupling to me in the
beginning (even currently in practice there is some limitation.)

Gao Xiang

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