On 8/7/25 10:30 PM, Mike Snitzer wrote:
> 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)?
Hold off on v6 for now. Let's wait for more review comments.
> 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);
--
Chuck Lever
