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Message-ID: <4a9e607e-36d1-4ea7-1754-c443906b3a1c@huaweicloud.com>
Date: Tue, 12 Mar 2024 20:59:15 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: "Darrick J. Wong" <djwong@...nel.org>
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 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.
Thanks,
Yi.
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