Re: [v6.18-rcX PATCH 5/3] nfs/localio: do not issue misaligned DIO out-of-order

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Oct 29, 2025 at 07:19:30PM -0400, Mike Snitzer wrote:
> From https://lore.kernel.org/linux-nfs/aQHASIumLJyOoZGH@infradead.org/
> 
> On Wed, Oct 29, 2025 at 12:20:40AM -0700, Christoph Hellwig wrote:
> > On Mon, Oct 27, 2025 at 12:18:30PM -0400, Mike Snitzer wrote:
> > > LOCALIO's misaligned DIO will issue head/tail followed by O_DIRECT
> > > middle (via AIO completion of that aligned middle). So out of order
> > > relative to file offset.
> >
> > That's in general a really bad idea.  It will obviously work, but
> > both on SSDs and out of place write file systems it is a sure way
> > to increase your garbage collection overhead a lot down the line.
> 
> Fix this by never issuing misaligned DIO out-of-order. This fix means
> the DIO-aligned segment will only use AIO completion if there is no
> misaligned end segment. Otherwise, all 3 segments of a misaligned DIO
> will be issued without AIO completion to ensure file offset increases
> properly for all partial READ or WRITE situations.
> 
> Fixes: c817248fc831 ("nfs/localio: add proper O_DIRECT support for READ and WRITE")
> Reported-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  fs/nfs/localio.c | 83 +++++++++++++++++-------------------------------
>  1 file changed, 29 insertions(+), 54 deletions(-)
> 
> Anna, apologies for stringing fixes together like this; and that this
> same commit c817248fc831 has so many follow-on Fixes is not lost on
> me.  But the full series of commit c817248fc831 fixes is composed of:
> 
> [v6.18-rcX PATCH 1/3] nfs/localio: remove unecessary ENOTBLK handling in DIO WRITE support
> [v6.18-rcX PATCH 2/3] nfs/localio: add refcounting for each iocb IO associated with NFS pgio header
> [v6.18-rcX PATCH 3/3] nfs/localio: backfill missing partial read support for misaligned DIO
> [v6.18-rcX PATCH 4/3] nfs/localio: Ensure DIO WRITE's IO on stable storage upon completion
> [v6.18-rcX PATCH 5/3] nfs/localio: do not issue misaligned DIO out-of-order
> 
> NOTE: PATCH 4/3's use of IOCBD_DSYNC|IOCB_SYNC _is_ conservative, but I
> will audit and adjust this further (informed by NFSD Direct's ongoing
> evolution for handling this same situaiton) for the v6.19 merge window.

Hi Anna,

Please don't pick up this PATCH 5/3, further testing shows there is
something wrong with it.  I'll circle back once I fix it.  But this
5/3 patch doesn't impact the other 4.

Thanks,
Mike