Re: [PATCH v2 3/4] NFSD: issue WRITEs using O_DIRECT even if IO is misaligned

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2025-07-31 at 15:44 -0400, Mike Snitzer wrote:
> 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. Buffered IO is used for the
> misaligned extents and O_DIRECT is used for the middle DIO-aligned
> extent.
> 
> The nfsd_analyze_write_dio trace event shows how NFSD splits a given
> misaligned WRITE into a mix of misaligned extent(s) and a DIO-aligned
> extent.
> 
> This combination of trace events is useful:
> 
>   echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_opened/enable
>   echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_analyze_write_dio/enable
>   echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_io_done/enable
>   echo 1 > /sys/kernel/tracing/events/xfs/xfs_file_direct_write/enable
> 
> Which for this dd command:
> 
>   dd if=/dev/zero of=/mnt/share1/test bs=47008 count=2 oflag=direct
> 
> Results in:
> 
>   nfsd-55714   [043] ..... 79976.260851: nfsd_write_opened: xid=0x966c5d2d fh_hash=0x4d34e6c1 offset=0 len=47008
>   nfsd-55714   [043] ..... 79976.260852: nfsd_analyze_write_dio: xid=0x966c5d2d fh_hash=0x4d34e6c1 offset=0 len=47008 start=0+0 middle=0+45056 end=45056+1952
>   nfsd-55714   [043] ..... 79976.260857: xfs_file_direct_write: dev 259:12 ino 0x3e00008f disize 0x0 pos 0x0 bytecount 0xb000
>   nfsd-55714   [043] ..... 79976.260965: nfsd_write_io_done: xid=0x966c5d2d fh_hash=0x4d34e6c1 offset=0 len=47008
> 
>   nfsd-55714   [043] ..... 79976.307762: nfsd_write_opened: xid=0x67e5ce6f fh_hash=0x4d34e6c1 offset=47008 len=47008
>   nfsd-55714   [043] ..... 79976.307762: nfsd_analyze_write_dio: xid=0x67e5ce6f fh_hash=0x4d34e6c1 offset=47008 len=47008 start=47008+2144 middle=49152+40960 end=90112+3904
>   nfsd-55714   [043] ..... 79976.307797: xfs_file_direct_write: dev 259:12 ino 0x3e00008f disize 0xc000 pos 0xc000 bytecount 0xa000
>   nfsd-55714   [043] ..... 79976.307866: nfsd_write_io_done: xid=0x67e5ce6f fh_hash=0x4d34e6c1 offset=47008 len=47008
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  fs/nfsd/vfs.c | 135 ++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 124 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index e4855c32dad12..23360825455a2 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1314,6 +1314,113 @@ static int wait_for_concurrent_writes(struct file *file)
>  	return err;
>  }
>  
> +struct nfsd_write_dio
> +{
> +	loff_t middle_offset;	/* Offset for start of DIO-aligned middle */
> +	loff_t end_offset;	/* Offset for start of DIO-aligned end */
> +	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 void init_nfsd_write_dio(struct nfsd_write_dio *write_dio)
> +{
> +	memset(write_dio, 0, sizeof(*write_dio));
> +}
> +
> +static bool nfsd_analyze_write_dio(struct svc_rqst *rqstp, struct svc_fh *fhp,
> +				   struct nfsd_file *nf, loff_t offset,
> +				   unsigned long len, struct nfsd_write_dio *write_dio)
> +{
> +	const u32 dio_blocksize = nf->nf_dio_offset_align;
> +	loff_t orig_end, middle_end, start_end, start_offset = offset;
> +	ssize_t start_len = len;
> +	bool aligned = true;
> +
> +	if (WARN_ONCE(!nf->nf_dio_mem_align || !dio_blocksize,
> +		      "%s: underlying filesystem has not provided DIO alignment info\n",
> +		      __func__))
> +		return false;
> +
> +	if (WARN_ONCE(dio_blocksize > PAGE_SIZE,
> +		      "%s: underlying storage's dio_blocksize=%u > PAGE_SIZE=%lu\n",
> +		      __func__, dio_blocksize, PAGE_SIZE))
> +		return false;
> +
> +	if (unlikely(len < dio_blocksize)) {
> +		aligned = false;
> +		goto out;
> +	}
> +
> +	if (((offset | len) & (dio_blocksize-1)) == 0) {
> +		/* already DIO-aligned, no misaligned head or tail */
> +		write_dio->middle_offset = offset;
> +		write_dio->middle_len = len;
> +		/* clear these for the benefit of trace_nfsd_analyze_write_dio */
> +		start_offset = 0;
> +		start_len = 0;
> +		goto out;
> +	}
> +
> +	start_end = round_up(offset, dio_blocksize);
> +	start_len = start_end - offset;
> +	orig_end = offset + len;
> +	middle_end = round_down(orig_end, dio_blocksize);
> +
> +	write_dio->start_len = start_len;
> +	write_dio->middle_offset = start_end;
> +	write_dio->middle_len = middle_end - start_end;
> +	write_dio->end_offset = middle_end;
> +	write_dio->end_len = orig_end - middle_end;
> +out:
> +	trace_nfsd_analyze_write_dio(rqstp, fhp, offset, len, start_offset, start_len,
> +				     write_dio->middle_offset, write_dio->middle_len,
> +				     write_dio->end_offset, write_dio->end_len);
> +	return aligned;
> +}
> +
> +/*
> + * Setup as many as 3 iov_iter based on extents possibly described by @write_dio.
> + * @iterp: pointer to pointer to onstack array of 3 iov_iter structs from caller.
> + * @rq_bvec: backing bio_vec used to setup all 3 iov_iter permutations.
> + * @nvecs: number of segments in @rq_bvec
> + * @cnt: size of the request in bytes
> + * @write_dio: nfsd_write_dio struct that describes start, middle and end extents.
> + *
> + * Returns the number of iov_iter that were setup.
> + */
> +static int nfsd_setup_write_iters(struct iov_iter **iterp, struct bio_vec *rq_bvec,
> +				  unsigned int nvecs, unsigned long cnt,
> +				  struct nfsd_write_dio *write_dio)
> +{
> +	int n_iters = 0;
> +	struct iov_iter *iters = *iterp;
> +
> +	/* Setup misaligned start? */
> +	if (write_dio->start_len) {
> +		iov_iter_bvec(&iters[n_iters], ITER_SOURCE, rq_bvec, nvecs, cnt);
> +		iters[n_iters].count = write_dio->start_len;
> +		n_iters++;
> +	}
> +
> +	/* Setup possibly DIO-aligned middle */
> +	iov_iter_bvec(&iters[n_iters], ITER_SOURCE, rq_bvec, nvecs, cnt);
> +	if (write_dio->start_len)
> +		iov_iter_advance(&iters[n_iters], write_dio->start_len);
> +	iters[n_iters].count -= write_dio->end_len;
> +	n_iters++;
> +
> +	/* Setup misaligned end? */
> +	if (write_dio->end_len) {
> +		iov_iter_bvec(&iters[n_iters], ITER_SOURCE, rq_bvec, nvecs, cnt);
> +		iov_iter_advance(&iters[n_iters],
> +				 write_dio->start_len + write_dio->middle_len);
> +		n_iters++;
> +	}
> +
> +	return n_iters;
> +}
> +
>  /**
>   * nfsd_vfs_write - write data to an already-open file
>   * @rqstp: RPC execution context
> @@ -1348,9 +1455,11 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	unsigned int		pflags = current->flags;
>  	bool			restore_flags = false;
>  	unsigned int		nvecs;
> -	struct iov_iter		iter_stack[1];
> +	struct iov_iter		iter_stack[3];
>  	struct iov_iter		*iter = iter_stack;
>  	unsigned int		n_iters = 0;
> +	bool                    dio_aligned = false;
> +	struct nfsd_write_dio	write_dio;
>  
>  	trace_nfsd_write_opened(rqstp, fhp, offset, *cnt);
>  
> @@ -1379,18 +1488,12 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	if (stable && !fhp->fh_use_wgather)
>  		kiocb.ki_flags |= IOCB_DSYNC;
>  
> -	nvecs = xdr_buf_to_bvec(rqstp->rq_bvec, rqstp->rq_maxpages, payload);
> -	iov_iter_bvec(&iter[0], ITER_SOURCE, rqstp->rq_bvec, nvecs, *cnt);
> -	n_iters++;
> -
> +	init_nfsd_write_dio(&write_dio);
>  	switch (nfsd_io_cache_write) {
>  	case NFSD_IO_DIRECT:
> -		/* direct I/O must be aligned to device logical sector size */
> -		if (nf->nf_dio_mem_align && nf->nf_dio_offset_align &&
> -		    (((offset | *cnt) & (nf->nf_dio_offset_align-1)) == 0) &&
> -		    iov_iter_is_aligned(&iter[0], nf->nf_dio_mem_align - 1,
> -					nf->nf_dio_offset_align - 1))
> -			kiocb.ki_flags = IOCB_DIRECT;
> +		if (nfsd_analyze_write_dio(rqstp, fhp, nf, offset,
> +					   *cnt, &write_dio))
> +			dio_aligned = true;
>  		break;
>  	case NFSD_IO_DONTCACHE:
>  		kiocb.ki_flags = IOCB_DONTCACHE;
> @@ -1399,12 +1502,22 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  		break;
>  	}
>  
> +	nvecs = xdr_buf_to_bvec(rqstp->rq_bvec, rqstp->rq_maxpages, payload);
> +	n_iters = nfsd_setup_write_iters(&iter, rqstp->rq_bvec, nvecs, *cnt, &write_dio);
> +
>  	*cnt = 0;
>  	for (int i = 0; i < n_iters; i++) {
>  		since = READ_ONCE(file->f_wb_err);
>  		if (verf)
>  			nfsd_copy_write_verifier(verf, nn);
>  
> +		if (dio_aligned) {
> +			if (iov_iter_is_aligned(&iter[i], nf->nf_dio_mem_align - 1,
> +						nf->nf_dio_offset_align - 1))
> +				kiocb.ki_flags |= IOCB_DIRECT;
> +			else
> +				kiocb.ki_flags &= ~IOCB_DIRECT;
> +		}
>  		host_err = vfs_iocb_iter_write(file, &kiocb, &iter[i]);
>  		if (host_err < 0) {
>  			commit_reset_write_verifier(nn, rqstp, host_err);

Reviewed-by: Jeff Layton <jlayton@kernel.org>