Re: [PATCH] iomap: don't lose folio dropbehind state for overwrites

[Jens Axboe <axboe@kernel.dk>]

On 6/2/25 11:46 AM, Ritesh Harjani (IBM) wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
>> DONTCACHE I/O must have the completion punted to a workqueue, just like
>> what is done for unwritten extents, as the completion needs task context
>> to perform the invalidation of the folio(s). However, if writeback is
>> started off filemap_fdatawrite_range() off generic_sync() and it's an
>> overwrite, then the DONTCACHE marking gets lost as iomap_add_to_ioend()
>> don't look at the folio being added and no further state is passed down
>> to help it know that this is a dropbehind/DONTCACHE write.
>>
>> Check if the folio being added is marked as dropbehind, and set
>> IOMAP_IOEND_DONTCACHE if that is the case. Then XFS can factor this into
>> the decision making of completion context in xfs_submit_ioend().
>> Additionally include this ioend flag in the NOMERGE flags, to avoid
>> mixing it with unrelated IO.
>>
>> This fixes extra page cache being instantiated when the write performed
>> is an overwrite, rather than newly instantiated blocks.
>>
>> Fixes: b2cd5ae693a3 ("iomap: make buffered writes work with RWF_DONTCACHE")
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> Found this one while testing the unrelated issue of invalidation being a
>> bit broken before 6.15 release. We need this to ensure that overwrites
>> also prune correctly, just like unwritten extents currently do.
> 
> I guess I did report this to you a while ago when I was adding support
> for uncahed buffered-io to xfs_io. But I never heard back from you :( 
> 
> https://lore.kernel.org/all/87h649trof.fsf@gmail.com/
> 
> No worries, good that we finally have this fixed.

Sorry not sure how I missed that, to be fair there were a lot of emails
in various threads on this topic back at that time. Not trying to make
excuses... Yes, at least the "would otherwise have been !task context"
issue is resolved.

-- 
Jens Axboe