Re: [PATCH v3 3/7] iomap: optional zero range dirty folio processing

Zhang Yi
2025-07-19T11:07:58Z

On 2025/7/18 21:48, Brian Foster wrote:
> On Fri, Jul 18, 2025 at 07:30:10PM +0800, Zhang Yi wrote:
>> On 2025/7/15 13:22, Darrick J. Wong wrote:
>>> On Mon, Jul 14, 2025 at 04:41:18PM -0400, Brian Foster wrote:
>>>> The only way zero range can currently process unwritten mappings
>>>> with dirty pagecache is to check whether the range is dirty before
>>>> mapping lookup and then flush when at least one underlying mapping
>>>> is unwritten. This ordering is required to prevent iomap lookup from
>>>> racing with folio writeback and reclaim.
>>>>
>>>> Since zero range can skip ranges of unwritten mappings that are
>>>> clean in cache, this operation can be improved by allowing the
>>>> filesystem to provide a set of dirty folios that require zeroing. In
>>>> turn, rather than flush or iterate file offsets, zero range can
>>>> iterate on folios in the batch and advance over clean or uncached
>>>> ranges in between.
>>>>
>>>> Add a folio_batch in struct iomap and provide a helper for fs' to
>>>
>>> /me confused by the single quote; is this supposed to read:
>>>
>>> "...for the fs to populate..."?
>>>
>>> Either way the code changes look like a reasonable thing to do for the
>>> pagecache (try to grab a bunch of dirty folios while XFS holds the
>>> mapping lock) so
>>>
>>> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
>>>
>>> --D
>>>
>>>
>>>> populate the batch at lookup time. Update the folio lookup path to
>>>> return the next folio in the batch, if provided, and advance the
>>>> iter if the folio starts beyond the current offset.
>>>>
>>>> Signed-off-by: Brian Foster <bfoster@redhat.com>
>>>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>>>> ---
>>>> fs/iomap/buffered-io.c | 89 +++++++++++++++++++++++++++++++++++++++---
>>>> fs/iomap/iter.c | 6 +++
>>>> include/linux/iomap.h | 4 ++
>>>> 3 files changed, 94 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
>>>> index 38da2fa6e6b0..194e3cc0857f 100644
>>>> --- a/fs/iomap/buffered-io.c
>>>> +++ b/fs/iomap/buffered-io.c
>> [...]
>>>> @@ -1398,6 +1452,26 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
>>>> return status;
>>>> }
>>>>
>>>> +loff_t
>>>> +iomap_fill_dirty_folios(
>>>> + struct iomap_iter *iter,
>>>> + loff_t offset,
>>>> + loff_t length)
>>>> +{
>>>> + struct address_space *mapping = iter->inode->i_mapping;
>>>> + pgoff_t start = offset >> PAGE_SHIFT;
>>>> + pgoff_t end = (offset + length - 1) >> PAGE_SHIFT;
>>>> +
>>>> + iter->fbatch = kmalloc(sizeof(struct folio_batch), GFP_KERNEL);
>>>> + if (!iter->fbatch)
>>
>> Hi, Brian!
>>
>> I think ext4 needs to be aware of this failure after it converts to use
>> iomap infrastructure. It is because if we fail to add dirty folios to the
>> fbatch, iomap_zero_range() will flush those unwritten and dirty range.
>> This could potentially lead to a deadlock, as most calls to
>> ext4_block_zero_page_range() occur under an active journal handle.
>> Writeback operations under an active journal handle may result in circular
>> waiting within journal transactions. So please return this error code, and
>> then ext4 can interrupt zero operations to prevent deadlock.
>>
>
> Hi Yi,
>
> Thanks for looking at this.
>
> Huh.. so the reason for falling back like this here is just that this
> was considered an optional optimization, with the flush in
> iomap_zero_range() being default fallback behavior. IIUC, what you're
> saying means that the current zero range behavior without this series is
> problematic for ext4-on-iomap..?

Yes.

> If so, have you observed issues you can share details about?

Sure.

Before delving into the specific details of this issue, I would like
to provide some background information on the rule that ext4 cannot
wait for writeback in an active journal handle. If you are aware of
this background, please skip this paragraph. During ext4 writing back
the page cache, it may start a new journal handle to allocate blocks,
update the disksize, and convert unwritten extents after the I/O is
completed. When starting this new journal handle, if the current
running journal transaction is in the process of being submitted or
if the journal space is insufficient, it must wait for the ongoing
transaction to be completed, but the prerequisite for this is that all
currently running handles must be terminated. However, if we flush the
page cache under an active journal handle, we cannot stop it, which
may lead to a deadlock.

Now, the issue I have observed occurs when I attempt to use
iomap_zero_range() within ext4_block_zero_page_range(). My current
implementation are below(based on the latest fs-next).

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 28547663e4fd..1a21667f3f7c 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4147,6 +4147,53 @@ static int ext4_iomap_buffered_da_write_end(struct inode *inode, loff_t offset,
return 0;
}

+static int ext4_iomap_buffered_zero_begin(struct inode *inode, loff_t offset,
+ loff_t length, unsigned int flags, struct iomap *iomap,
+ struct iomap *srcmap)
+{
+ struct iomap_iter *iter = container_of(iomap, struct iomap_iter, iomap);
+ struct ext4_map_blocks map;
+ u8 blkbits = inode->i_blkbits;
+ int ret;
+
+ ret = ext4_emergency_state(inode->i_sb);
+ if (unlikely(ret))
+ return ret;
+
+ if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
+ return -EINVAL;
+
+ /* Calculate the first and last logical blocks respectively. */
+ map.m_lblk = offset >> blkbits;
+ map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
+ EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
+
+ ret = ext4_map_blocks(NULL, inode, &map, 0);
+ if (ret < 0)
+ return ret;
+
+ /*
+ * Look up dirty folios for unwritten mappings within EOF. Providing
+ * this bypasses the flush iomap uses to trigger extent conversion
+ * when unwritten mappings have dirty pagecache in need of zeroing.
+ */
+ if ((map.m_flags & EXT4_MAP_UNWRITTEN) &&
+ map.m_lblk < EXT4_B_TO_LBLK(inode, i_size_read(inode))) {
+ loff_t end;
+
+ end = iomap_fill_dirty_folios(iter, map.m_lblk << blkbits,
+ map.m_len << blkbits);
+ if ((end >> blkbits) < map.m_lblk + map.m_len)
+ map.m_len = (end >> blkbits) - map.m_lblk;
+ }
+
+ ext4_set_iomap(inode, iomap, &map, offset, length, flags);
+ return 0;
+}
+
+const struct iomap_ops ext4_iomap_buffered_zero_ops = {
+ .iomap_begin = ext4_iomap_buffered_zero_begin,
+};

const struct iomap_ops ext4_iomap_buffered_write_ops = {
.iomap_begin = ext4_iomap_buffered_write_begin,
@@ -4611,6 +4658,17 @@ static int __ext4_block_zero_page_range(handle_t *handle,
return err;
}

+static inline int ext4_iomap_zero_range(struct inode *inode, loff_t from,
+ loff_t length)
+{
+ WARN_ON_ONCE(!inode_is_locked(inode) &&
+ !rwsem_is_locked(&inode->i_mapping->invalidate_lock));
+
+ return iomap_zero_range(inode, from, length, NULL,
+ &ext4_iomap_buffered_zero_ops,
+ &ext4_iomap_write_ops, NULL);
+}
+
/*
* ext4_block_zero_page_range() zeros out a mapping of length 'length'
* starting from file offset 'from'. The range to be zero'd must
@@ -4636,6 +4694,8 @@ static int ext4_block_zero_page_range(handle_t *handle,
if (IS_DAX(inode)) {
return dax_zero_range(inode, from, length, NULL,
&ext4_iomap_ops);
+ } else if (ext4_test_inode_state(inode, EXT4_STATE_BUFFERED_IOMAP)) {
+ return ext4_iomap_zero_range(inode, from, length);
}
return __ext4_block_zero_page_range(handle, mapping, from, length);
}

The problem is most calls to ext4_block_zero_page_range() occur under
an active journal handle, so I can reproduce the deadlock issue easily
without this series.

>
> FWIW, I think your suggestion is reasonable, but I'm also curious what
> the error handling would look like in ext4. Do you expect to the fail
> the higher level operation, for example? Cycle locks and retry, etc.?

Originally, I wanted ext4_block_zero_page_range() to return a failure
to the higher level operation. However, unfortunately, after my testing
today, I discovered that even though we implement this, this series still
cannot resolve the issue. The corner case is:

Assume we have a dirty folio covers both hole and unwritten mappings.

|- dirty folio -|
[hhhhhhhhuuuuuuuu] h:hole, u:unwrtten

If we punch the range of the hole, ext4_punch_hole()->
ext4_zero_partial_blocks() will zero out the first half of the dirty folio.
Then, ext4_iomap_buffered_zero_begin() will skip adding this dirty folio
since the target range is a hole. Finally, iomap_zero_range() will still
flush this whole folio and lead to deadlock during writeback the latter
half of the folio.

>
> The reason I ask is because the folio_batch handling has come up through
> discussions on this series. My position so far has been to keep it as a
> separate allocation and to keep things simple since it is currently
> isolated to zero range, but that may change if the usage spills over to
> other operations (which seems expected at this point). I suspect that if
> a filesystem actually depends on this for correct behavior, that is
> another data point worth considering on that topic.
>
> So that has me wondering if it would be better/easier here to perhaps
> embed the batch in iomap_iter, or maybe as an incremental step put it on
> the stack in iomap_zero_range() and initialize the iomap_iter pointer
> there instead of doing the dynamic allocation (then the fill helper
> would set a flag to indicate the fs did pagecache lookup). Thoughts on
> something like that?
>
> Also IIUC ext4-on-iomap is still a WIP and review on this series seems
> to have mostly wound down. Any objection if the fix for that comes along
> as a followup patch rather than a rework of this series?

It seems that we don't need to modify this series, we need to consider
other solutions to resolve this deadlock issue.

In my v1 ext4-on-iomap series [1], I resolved this issue by moving all
instances of ext4_block_zero_page_range() out of the running journal
handle(please see patch 19-21). But I don't think this is a good solution
since it's complex and fragile. Besides, after commit c7fc0366c6562
("ext4: partial zero eof block on unaligned inode size extension"), you
added more invocations of ext4_zero_partial_blocks(), and the situation
has become more complicated (Althrough I think the calls in the three
write_end callbacks can be removed).

Besides, IIUC, it seems that ext4 doesn't need to flush dirty folios
over unwritten mappings before zeroing partial blocks. This is because
ext4 always zeroes the in-memory page cache before zeroing(e.g, in
ext4_setattr() and ext4_punch_hole()), it means if the target range is
still dirty and unwritten when calling ext4_block_zero_page_range(), it
must has already been zeroed. Was I missing something? Therefore, I was
wondering if there are any ways to prevent flushing in
iomap_zero_range()? Any ideas?

[1] https://lore.kernel.org/linux-ext4/20241022111059.2566137-1-yi.zhang@huaweicloud.com/

>
> Brian
>
> P.S., I'm heading on vacation so it will likely be a week or two before
> I follow up from here, JFYI.

Wishing you a wonderful time! :-)

Best regards,
Yi.

>>
>>>> + return offset + length;
>>>> + folio_batch_init(iter->fbatch);
>>>> +
>>>> + filemap_get_folios_dirty(mapping, &start, end, iter->fbatch);
>>>> + return (start << PAGE_SHIFT);
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(iomap_fill_dirty_folios);
>>