Re: [PATCH v6 4/5] NFSD: Implement NFSD_IO_DIRECT for NFS WRITE

[Chuck Lever <cel@kernel.org>]

On 10/22/25 4:58 PM, Mike Snitzer wrote:
> On Wed, Oct 22, 2025 at 03:22:07PM -0400, Chuck Lever wrote:
>> From: Mike Snitzer <snitzer@kernel.org>
>>
>> If NFSD_IO_DIRECT is used, split any misaligned WRITE into a start,
>> middle and end as needed. The large middle extent is DIO-aligned and
>> the start and/or end are misaligned. An O_SYNC buffered write (with
>> preference towards using DONTCACHE) is used for the misaligned extents
>> and O_DIRECT is used for the middle DIO-aligned extent.
>>
>> nfsd_issue_write_dio() promotes @stable_how to NFS_FILE_SYNC, which
>> allows the client to drop its dirty data and avoid needing an extra
>> COMMIT operation.
>>
>> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
>> Reviewed-by: Jeff Layton <jlayton@kernel.org>
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>>  fs/nfsd/debugfs.c |   1 +
>>  fs/nfsd/trace.h   |   1 +
>>  fs/nfsd/vfs.c     | 181 ++++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 183 insertions(+)
>>
>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
>> index 41cd2b53d803..29c29a5111f8 100644
>> --- a/fs/nfsd/vfs.c
>> +++ b/fs/nfsd/vfs.c
>> @@ -1254,6 +1254,105 @@ static int wait_for_concurrent_writes(struct file *file)
>>  	return err;
>>  }
>>  
>> +struct nfsd_write_dio {
>> +	ssize_t	start_len;	/* Length for misaligned first extent */
>> +	ssize_t	middle_len;	/* Length for DIO-aligned middle extent */
>> +	ssize_t	end_len;	/* Length for misaligned last extent */
>> +};
>> +
>> +static bool
>> +nfsd_is_write_dio_possible(loff_t offset, unsigned long len,
>> +			   struct nfsd_file *nf,
>> +			   struct nfsd_write_dio *write_dio)
>> +{
>> +	const u32 dio_blocksize = nf->nf_dio_offset_align;
>> +	loff_t start_end, orig_end, middle_end;
>> +
> 
> I see you removed this check:
> 
> -       if (unlikely(!nf->nf_dio_mem_align || !dio_blocksize))
> -               return false;
> 
> Curious why. Seems unsafe because they can be 0.

Hm. I might have removed the wrong check. Will fix that up for the next
round.


> 
>> +	if (unlikely(dio_blocksize > PAGE_SIZE || len < dio_blocksize))
>> +		return false;
>> +
>> +	start_end = round_up(offset, dio_blocksize);
>> +	orig_end = offset + len;
>> +	middle_end = round_down(orig_end, dio_blocksize);
>> +
>> +	write_dio->start_len = start_end - offset;
>> +	write_dio->middle_len = middle_end - start_end;
>> +	write_dio->end_len = orig_end - middle_end;
>> +
>> +	return true;
>> +}
> 
> Otherwise, your other changes all seem fine.
> 
> Thanks,
> Mike


-- 
Chuck Lever