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Message-ID: <20240312162407.GC1927156@frogsfrogsfrogs>
Date: Tue, 12 Mar 2024 09:24:07 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, hch@...radead.org, brauner@...nel.org,
david@...morbit.com, tytso@....edu, jack@...e.cz,
yi.zhang@...wei.com, chengzhihao1@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH 3/4] iomap: don't increase i_size if it's not a write
operation
On Tue, Mar 12, 2024 at 08:59:15PM +0800, Zhang Yi wrote:
> On 2024/3/11 23:48, Darrick J. Wong wrote:
> > On Mon, Mar 11, 2024 at 08:22:54PM +0800, Zhang Yi wrote:
> >> From: Zhang Yi <yi.zhang@...wei.com>
> >>
> >> Increase i_size in iomap_zero_range() and iomap_unshare_iter() is not
> >> needed, the caller should handle it. Especially, when truncate partial
> >> block, we could not increase i_size beyond the new EOF here. It doesn't
> >> affect xfs and gfs2 now because they set the new file size after zero
> >> out, it doesn't matter that a transient increase in i_size, but it will
> >> affect ext4 because it set file size before truncate.
> >
> >> At the same time,
> >> iomap_write_failed() is also not needed for above two cases too, so
> >> factor them out and move them to iomap_write_iter() and
> >> iomap_zero_iter().
> >
> > This change should be a separate patch with its own justification.
> > Which is, AFAICT, something along the lines of:
> >
> > "Unsharing and zeroing can only happen within EOF, so there is never a
> > need to perform posteof pagecache truncation if write begin fails."
>
> Sure.
>
> >
> >> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> >
> > Doesn't this patch fix a bug in ext4?
>
> Yeah, the same as Christoph answered.
>
> >
> >> ---
> >> fs/iomap/buffered-io.c | 59 +++++++++++++++++++++---------------------
> >> 1 file changed, 30 insertions(+), 29 deletions(-)
> >>
> >> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> >> index 093c4515b22a..19f91324c690 100644
> >> --- a/fs/iomap/buffered-io.c
> >> +++ b/fs/iomap/buffered-io.c
> >> @@ -786,7 +786,6 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
> >>
> >> out_unlock:
> >> __iomap_put_folio(iter, pos, 0, folio);
> >> - iomap_write_failed(iter->inode, pos, len);
> >>
> >> return status;
> >> }
> >> @@ -838,34 +837,13 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
> >> size_t copied, struct folio *folio)
> >> {
> >> const struct iomap *srcmap = iomap_iter_srcmap(iter);
> >> - loff_t old_size = iter->inode->i_size;
> >> - size_t ret;
> >> -
> >> - if (srcmap->type == IOMAP_INLINE) {
> >> - ret = iomap_write_end_inline(iter, folio, pos, copied);
> >> - } else if (srcmap->flags & IOMAP_F_BUFFER_HEAD) {
> >> - ret = block_write_end(NULL, iter->inode->i_mapping, pos, len,
> >> - copied, &folio->page, NULL);
> >> - } else {
> >> - ret = __iomap_write_end(iter->inode, pos, len, copied, folio);
> >> - }
> >>
> >> - /*
> >> - * Update the in-memory inode size after copying the data into the page
> >> - * cache. It's up to the file system to write the updated size to disk,
> >> - * preferably after I/O completion so that no stale data is exposed.
> >> - */
> >> - if (pos + ret > old_size) {
> >> - i_size_write(iter->inode, pos + ret);
> >> - iter->iomap.flags |= IOMAP_F_SIZE_CHANGED;
> >> - }
> >> - __iomap_put_folio(iter, pos, ret, folio);
> >> -
> >> - if (old_size < pos)
> >> - pagecache_isize_extended(iter->inode, old_size, pos);
> >> - if (ret < len)
> >> - iomap_write_failed(iter->inode, pos + ret, len - ret);
> >> - return ret;
> >> + if (srcmap->type == IOMAP_INLINE)
> >> + return iomap_write_end_inline(iter, folio, pos, copied);
> >> + if (srcmap->flags & IOMAP_F_BUFFER_HEAD)
> >> + return block_write_end(NULL, iter->inode->i_mapping, pos, len,
> >> + copied, &folio->page, NULL);
> >> + return __iomap_write_end(iter->inode, pos, len, copied, folio);
> >> }
> >>
> >> static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >> @@ -880,6 +858,7 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >>
> >> do {
> >> struct folio *folio;
> >> + loff_t old_size;
> >> size_t offset; /* Offset into folio */
> >> size_t bytes; /* Bytes to write to folio */
> >> size_t copied; /* Bytes copied from user */
> >> @@ -912,8 +891,10 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >> }
> >>
> >> status = iomap_write_begin(iter, pos, bytes, &folio);
> >> - if (unlikely(status))
> >> + if (unlikely(status)) {
> >> + iomap_write_failed(iter->inode, pos, bytes);
> >> break;
> >> + }
> >> if (iter->iomap.flags & IOMAP_F_STALE)
> >> break;
> >>
> >> @@ -927,6 +908,24 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >> copied = copy_folio_from_iter_atomic(folio, offset, bytes, i);
> >> status = iomap_write_end(iter, pos, bytes, copied, folio);
> >>
> >> + /*
> >> + * Update the in-memory inode size after copying the data into
> >> + * the page cache. It's up to the file system to write the
> >> + * updated size to disk, preferably after I/O completion so that
> >> + * no stale data is exposed.
> >> + */
> >> + old_size = iter->inode->i_size;
> >> + if (pos + status > old_size) {
> >> + i_size_write(iter->inode, pos + status);
> >> + iter->iomap.flags |= IOMAP_F_SIZE_CHANGED;
> >> + }
> >> + __iomap_put_folio(iter, pos, status, folio);
> >
> > Why is it necessary to hoist the __iomap_put_folio calls from
> > iomap_write_end into iomap_write_iter, iomap_unshare_iter, and
> > iomap_zero_iter? None of those functions seem to use it, and it makes
> > more sense to me that iomap_write_end releases the folio that
> > iomap_write_begin returned.
> >
>
> Because we have to update i_size before __iomap_put_folio() in
> iomap_write_iter(). If not, once we unlock folio, it could be raced
> by the backgroud write back which could start writing back and call
> folio_zero_segment() (please see iomap_writepage_handle_eof()) to
> zero out the valid data beyond the not updated i_size. So we
> have to move out __iomap_put_folio() out together with the i_size
> updating.
Ahah. Please make a note of that in the comment for dunces like me.
/*
* Update the in-memory inode size after copying the data into
* the page cache. It's up to the file system to write the
* updated size to disk, preferably after I/O completion so that
* no stale data is exposed. Only once that's done can we
* unlock and release the folio.
*/
--D
> Thanks,
> Yi.
>
>
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