Re: [PATCH 7/7] nfs: don't reuse partially completed requests in nfs_lock_and_join_requests

[Christoph Hellwig <hch@lst.de>]

On Tue, Jul 02, 2024 at 11:07:13AM +0300, Sagi Grimberg wrote:
> On 01/07/2024 8:26, Christoph Hellwig wrote:
>> When NFS requests are split into sub-requests, nfs_inode_remove_request
>> calls nfs_page_group_sync_on_bit to set PG_REMOVE on this sub-request and
>> only completes the head requests once PG_REMOVE is set on all requests.
>> This means that when nfs_lock_and_join_requests sees a PG_REMOVE bit, I/O
>> on the request is in progress and has partially completed.   If such a
>> request is returned to nfs_try_to_update_request, it could be extended
>> with the newly dirtied region and I/O for the combined range will be
>> re-scheduled, leading to extra I/O.
>
> Probably worth noting in the change log that large folios makes this 
> potentially much
> worse?

That assumes large folios actually create more subrequest.  One big
reason to create subrequests is flexfiles mirroring, which of course
doesn't change with large folio.  The other is that if ->pg_test
doesn't allow the nfs_page to cover everything, which is roughly
bound by a page array allocation and for PNFS the layout segment
size, and the chance for that to fail could very slightly increase.