linux-kernel - Re: [PATCH 13/16] iomap: make buffered writes work with RWF

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Message-ID: <aeb58f3d-67b2-4df3-abc7-49a2e9bb8270@kernel.dk>
Date: Tue, 12 Nov 2024 10:16:10 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Brian Foster <bfoster@...hat.com>
Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, hannes@...xchg.org,
 clm@...a.com, linux-kernel@...r.kernel.org, willy@...radead.org,
 kirill@...temov.name, linux-btrfs@...r.kernel.org,
 linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH 13/16] iomap: make buffered writes work with RWF_UNCACHED

On 11/12/24 9:37 AM, Brian Foster wrote:
> On Mon, Nov 11, 2024 at 04:37:40PM -0700, Jens Axboe wrote:
>> Add iomap buffered write support for RWF_UNCACHED. If RWF_UNCACHED is
>> set for a write, mark the folios being written with drop_writeback. Then
> 
> s/drop_writeback/uncached/ ?

Ah indeed, guess that never got changed. Thanks, will fix that in the
commit message.

> BTW, this might be getting into wonky "don't care that much" territory,
> but something else to be aware of is that certain writes can potentially
> change pagecache state as a side effect outside of the actual buffered
> write itself.
> 
> For example, xfs calls iomap_zero_range() on write extension (i.e. pos >
> isize), which uses buffered writes and thus could populate a pagecache
> folio without setting it uncached, even if done on behalf of an uncached
> write.
> 
> I've only made a first pass and could be missing some details, but IIUC
> I _think_ this means something like writing out a stream of small,
> sparse and file extending uncached writes could actually end up behaving
> more like sync I/O. Again, not saying that's something we really care
> about, just raising it in case it's worth considering or documenting..

No that's useful info, I'm not really surprised that there would still
be cases where UNCACHED goes unnoticed. In other words, I'd be surprised
if the current patches for eg xfs/ext4 cover all the cases where new
folios are created and should be marked as UNCACHED of IOCB_UNCACHED is
set in the iocb.

I think those can be sorted out or documented as we move forward.
UNCACHED is really just a hint - the kernel should do its best to not
have permanent folios for this IO, but there are certainly cases where
it won't be honored if you're racing with regular buffered IO or mmap.
For the case above, sounds like we could cover that, however, and
probably should.

-- 
Jens Axboe