On Thu, Aug 07, 2025 at 12:25:44PM -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-23908 [010] ..... 10374.902333: nfsd_write_opened: xid=0x7fc5923b fh_hash=0x857ca4fc offset=0 len=47008
> nfsd-23908 [010] ..... 10374.902335: nfsd_analyze_write_dio: xid=0x7fc5923b fh_hash=0x857ca4fc offset=0 len=47008 start=0+0 middle=0+46592 end=46592+416
> nfsd-23908 [010] ..... 10374.902343: xfs_file_direct_write: dev 259:2 ino 0xc00116 disize 0x0 pos 0x0 bytecount 0xb600
> nfsd-23908 [010] ..... 10374.902697: nfsd_write_io_done: xid=0x7fc5923b fh_hash=0x857ca4fc offset=0 len=47008
>
> nfsd-23908 [010] ..... 10374.902925: nfsd_write_opened: xid=0x80c5923b fh_hash=0x857ca4fc offset=47008 len=47008
> nfsd-23908 [010] ..... 10374.902926: nfsd_analyze_write_dio: xid=0x80c5923b fh_hash=0x857ca4fc offset=47008 len=47008 start=47008+96 middle=47104+46592 end=93696+320
> nfsd-23908 [010] ..... 10374.903010: xfs_file_direct_write: dev 259:2 ino 0xc00116 disize 0xb800 pos 0xb800 bytecount 0xb600
> nfsd-23908 [010] ..... 10374.903239: nfsd_write_io_done: xid=0x80c5923b fh_hash=0x857ca4fc offset=47008 len=47008
>
> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
> ---
> fs/nfsd/vfs.c | 183 ++++++++++++++++++++++++++++++++++++++++++++++----
> 1 file changed, 170 insertions(+), 13 deletions(-)
>
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index be083a8812717..1b5aa3e6e6623 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1315,6 +1315,167 @@ 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 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;
> +
> + 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))
> + return false;
> +
> + memset(write_dio, 0, sizeof(*write_dio));
> +
> + 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 true;
> +}
> +
> +/*
> + * Setup as many as 3 iov_iter based on extents described by @write_dio.
> + * @iterp: pointer to pointer to onstack array of 3 iov_iter structs from caller.
> + * @iter_is_dio_aligned: pointer to onstack array of 3 bools 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_dio_iters(struct iov_iter **iterp, bool *iter_is_dio_aligned,
> + 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) {
> + iter_is_dio_aligned[n_iters] = false;
> + iov_iter_bvec(&iters[n_iters], ITER_SOURCE, rq_bvec, nvecs, cnt);
> + iters[n_iters].count = write_dio->start_len;
> + n_iters++;
> + }
> +
> + /* Setup DIO-aligned middle */
> + iter_is_dio_aligned[n_iters] = true;
> + 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) {
> + iter_is_dio_aligned[n_iters] = false;
> + 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;
> +}
> +
> +static int
> +nfsd_issue_write_buffered(struct svc_rqst *rqstp, struct file *file,
> + unsigned int nvecs, unsigned long *cnt,
> + struct kiocb *kiocb)
> +{
> + struct iov_iter iter;
> + int host_err;
> +
> + iov_iter_bvec(&iter, ITER_SOURCE, rqstp->rq_bvec, nvecs, *cnt);
> + host_err = vfs_iocb_iter_write(file, kiocb, &iter);
> + if (host_err < 0)
> + return host_err;
> + *cnt = host_err;
> +
> + return 0;
> +}
> +
> +static noinline int
> +nfsd_issue_write_dio(struct svc_rqst *rqstp, struct svc_fh *fhp,
> + struct nfsd_file *nf, loff_t offset,
> + unsigned int nvecs, unsigned long *cnt,
> + struct kiocb *kiocb)
> +{
> + struct nfsd_write_dio write_dio;
> + struct file *file = nf->nf_file;
> +
> + if (!nfsd_analyze_write_dio(rqstp, fhp, nf, offset,
> + *cnt, &write_dio)) {
> + return nfsd_issue_write_buffered(rqstp, file,
> + nvecs, cnt, kiocb);
> + } else {
> + bool iter_is_dio_aligned[3];
> + struct iov_iter iter_stack[3];
> + struct iov_iter *iter = iter_stack;
> + unsigned int n_iters = 0;
> + int host_err;
> +
> + n_iters = nfsd_setup_write_dio_iters(&iter,
> + iter_is_dio_aligned, rqstp->rq_bvec,
> + nvecs, *cnt, &write_dio);
> + *cnt = 0;
> + for (int i = 0; i < n_iters; i++) {
> + if (iter_is_dio_aligned[i])
> + 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)
> + return host_err;
> + *cnt += host_err;
> + }
> + }
> +
> + return 0;
> +}
> +
> /**
> * nfsd_vfs_write - write data to an already-open file
> * @rqstp: RPC execution context
> @@ -1342,7 +1503,6 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
> struct super_block *sb = file_inode(file)->i_sb;
> struct kiocb kiocb;
> struct svc_export *exp;
> - struct iov_iter iter;
> errseq_t since;
> __be32 nfserr;
> int host_err;
> @@ -1379,31 +1539,28 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
> kiocb.ki_flags |= IOCB_DSYNC;
>
> nvecs = xdr_buf_to_bvec(rqstp->rq_bvec, rqstp->rq_maxpages, payload);
> - iov_iter_bvec(&iter, ITER_SOURCE, rqstp->rq_bvec, nvecs, *cnt);
> +
> + since = READ_ONCE(file->f_wb_err);
> + if (verf)
> + nfsd_copy_write_verifier(verf, nn);
>
> 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))
> - kiocb.ki_flags |= IOCB_DIRECT;
> + host_err = nfsd_issue_write_dio(rqstp, fhp, nf, offset,
> + nvecs, cnt, &kiocb);
> break;
> case NFSD_IO_DONTCACHE:
> kiocb.ki_flags |= IOCB_DONTCACHE;
> - break;
> + fallthrough;
> case NFSD_IO_BUFFERED:
> + host_err = nfsd_issue_write_buffered(rqstp, file,
> + nvecs, cnt, &kiocb);
> break;
> }
> -
> - since = READ_ONCE(file->f_wb_err);
> - if (verf)
> - nfsd_copy_write_verifier(verf, nn);
> - host_err = vfs_iocb_iter_write(file, &kiocb, &iter);
> if (host_err < 0) {
> commit_reset_write_verifier(nn, rqstp, host_err);
> goto out_nfserr;
> }
> - *cnt = host_err;
> nfsd_stats_io_write_add(nn, exp, *cnt);
> fsnotify_modify(file);
> host_err = filemap_check_wb_err(file->f_mapping, since);
> --
> 2.44.0
>
>
Embarrassingly, turns out I only tested the NFSD_IO_DIRECT case prior
to submitting this v5, if the 'nfsd_io_cache_write' debugfs file is
never written it defaults to NFSD_IO_UNSPECIFIED. But even if that's
the case we need to treat NFSD_IO_UNSPECIFIED like NFSD_IO_BUFFERED.
(that is how nfsd_vfs_read behaves, but I missed this in
nfsd_vfs_write when I refactored the code for v5).
This incremental patch fixes this oversight, Chuck should I submit a
v8 or you're OK with folding this fixup (if the rest of the code is
OK)?
Thanks,
Mike
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 1b5aa3e6e6623..b529754a20bd5 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1505,7 +1505,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
struct svc_export *exp;
errseq_t since;
__be32 nfserr;
- int host_err;
+ int host_err = 0;
unsigned long exp_op_flags = 0;
unsigned int pflags = current->flags;
bool restore_flags = false;
@@ -1552,6 +1552,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
case NFSD_IO_DONTCACHE:
kiocb.ki_flags |= IOCB_DONTCACHE;
fallthrough;
+ case NFSD_IO_UNSPECIFIED:
case NFSD_IO_BUFFERED:
host_err = nfsd_issue_write_buffered(rqstp, file,
nvecs, cnt, &kiocb);
