On Wed, Jul 13, 2022 at 4:45 PM Chuck Lever III <chuck.lever@xxxxxxxxxx> wrote:
>
>
>
> > On Jul 13, 2022, at 3:08 PM, Anna Schumaker <anna@xxxxxxxxxx> wrote:
> >
> > From: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
> >
> > Rather than relying on the underlying filesystem to tell us where hole
> > and data segments are through vfs_llseek(), let's instead do the hole
> > compression ourselves. This has a few advantages over the old
> > implementation:
> >
> > 1) A single call to the underlying filesystem through nfsd_readv() means
> > the file can't change from underneath us in the middle of encoding.
> > 2) A single call to the underlying filestem also means that the
> > underlying filesystem only needs to synchronize cached and on-disk
> > data one time instead of potentially many speeding up the reply.
> > 3) Hole support for filesystems that don't support SEEK_HOLE and SEEK_DATA
>
> I'm not sure I understand why this last one is a good idea.
> Wouldn't that cause holes to appear in the file cached on
> the client where there are no holes in the stored file on
> the server?
It doesn't because the client expands the holes into zero-filled pages
during READ_PLUS decoding. Note that this expansion happens on the
server-side for the classic READ operation.
>
> Is there any encryption-related impact such as the issues
> that David brought up during LSF/MM ?
Not that I'm aware of.
>
>
> > I also included an optimization where we can cut down on the amount of
> > memory being shifed around by doing the compression as (hole, data)
> > pairs.
> >
> > This patch not only fixes xfstests generic/091 and generic/263
> > for me but the "-g quick" group tests also finish about a minute faster.
> >
> > Signed-off-by: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
> > ---
> > fs/nfsd/nfs4xdr.c | 202 +++++++++++++++++++++++-----------------------
> > 1 file changed, 102 insertions(+), 100 deletions(-)
> >
> > diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> > index 61b2aae81abb..0e1e7a37d4e0 100644
> > --- a/fs/nfsd/nfs4xdr.c
> > +++ b/fs/nfsd/nfs4xdr.c
> > @@ -4731,81 +4731,121 @@ nfsd4_encode_offload_status(struct nfsd4_compoundres *resp, __be32 nfserr,
> > return nfserr;
> > }
> >
> > +struct read_plus_segment {
> > + enum data_content4 type;
> > + unsigned long offset;
> > + unsigned long length;
> > + unsigned int page_pos;
> > +};
>
> "unsigned long" is not always 64 bits wide, and note that
> rd_offset is declared as a u64. Thus ::offset and ::length
> need to have explicit bit-width types. How about u64 for both?
Sure, I'll change that.
>
> The same type needs to be used wherever you do an
>
> unsigned long offset = read->rd_offset;
>
> Nit: can this struct declaration use tab-formatting with the
> usual naming convention for the fields, like "rp_type" and
> "rp_offset"? That makes it easier to grep for places these
> fields are used, since the current names are pretty generic.
Yeah, no problem.
>
>
> > +
> > static __be32
> > -nfsd4_encode_read_plus_data(struct nfsd4_compoundres *resp,
> > - struct nfsd4_read *read,
> > - unsigned long *maxcount, u32 *eof,
> > - loff_t *pos)
> > +nfsd4_read_plus_readv(struct nfsd4_compoundres *resp, struct nfsd4_read *read,
> > + unsigned long *maxcount, u32 *eof)
> > {
> > struct xdr_stream *xdr = resp->xdr;
> > - struct file *file = read->rd_nf->nf_file;
> > - int starting_len = xdr->buf->len;
> > - loff_t hole_pos;
> > - __be32 nfserr;
> > - __be32 *p, tmp;
> > - __be64 tmp64;
> > -
> > - hole_pos = pos ? *pos : vfs_llseek(file, read->rd_offset, SEEK_HOLE);
> > - if (hole_pos > read->rd_offset)
> > - *maxcount = min_t(unsigned long, *maxcount, hole_pos - read->rd_offset);
> > - *maxcount = min_t(unsigned long, *maxcount, (xdr->buf->buflen - xdr->buf->len));
> > -
> > - /* Content type, offset, byte count */
> > - p = xdr_reserve_space(xdr, 4 + 8 + 4);
> > - if (!p)
> > - return nfserr_resource;
> > + unsigned int starting_len = xdr->buf->len;
> > + __be32 nfserr, zero = xdr_zero;
> > + int pad;
>
> unsigned int pad;
Ok
>
>
> >
> > + /* xdr_reserve_space_vec() switches us to the xdr->pages */
>
> IIUC this is reserving a maximum estimated size piece of the xdr_stream
> to be used for encoding the READ_PLUS, and then the mechanics of
> encoding the result can trim the message length down a bit. The missing
> xdr_reserve_space calls are a little confusing, as are the operations on
> the stream's xdr_buf rather than using xdr_stream operations, so it would
> help to explain what's going on, perhaps as part of this comment.
Sure, I'll expand on the comment. I'm not sure the best way of
switching xdr_encode_word() and xdr_encode_double() to use an xdr
stream here, since the xdr_stream_encode_*() family of functions all
call xdr_reserve_space() internally but I'm writing to space that has
already been reserved.
>
> Or, move all of this blathering to a kerneldoc comment in front of
> nfsd4_encode_read_plus_segments()
>
>
> > read->rd_vlen = xdr_reserve_space_vec(xdr, resp->rqstp->rq_vec, *maxcount);
> > if (read->rd_vlen < 0)
> > return nfserr_resource;
> >
> > - nfserr = nfsd_readv(resp->rqstp, read->rd_fhp, file, read->rd_offset,
> > - resp->rqstp->rq_vec, read->rd_vlen, maxcount, eof);
> > + nfserr = nfsd_readv(resp->rqstp, read->rd_fhp, read->rd_nf->nf_file,
> > + read->rd_offset, resp->rqstp->rq_vec, read->rd_vlen,
> > + maxcount, eof);
> > if (nfserr)
> > return nfserr;
> > - xdr_truncate_encode(xdr, starting_len + 16 + xdr_align_size(*maxcount));
> > + xdr_truncate_encode(xdr, starting_len + xdr_align_size(*maxcount));
> >
> > - tmp = htonl(NFS4_CONTENT_DATA);
> > - write_bytes_to_xdr_buf(xdr->buf, starting_len, &tmp, 4);
> > - tmp64 = cpu_to_be64(read->rd_offset);
> > - write_bytes_to_xdr_buf(xdr->buf, starting_len + 4, &tmp64, 8);
> > - tmp = htonl(*maxcount);
> > - write_bytes_to_xdr_buf(xdr->buf, starting_len + 12, &tmp, 4);
> > -
> > - tmp = xdr_zero;
> > - write_bytes_to_xdr_buf(xdr->buf, starting_len + 16 + *maxcount, &tmp,
> > - xdr_pad_size(*maxcount));
> > + pad = (*maxcount&3) ? 4 - (*maxcount&3) : 0;
>
> Would xdr_pad_size() be appropriate here?
Probably. It still passes my testing, so I've changed it.
>
>
> > + write_bytes_to_xdr_buf(xdr->buf, starting_len + *maxcount, &zero, pad);
> > return nfs_ok;
> > }
> >
> > +static void
> > +nfsd4_encode_read_plus_segment(struct xdr_stream *xdr,
> > + struct read_plus_segment *segment,
> > + unsigned int *bufpos, unsigned int *segments)
> > +{
> > + struct xdr_buf *buf = xdr->buf;
> > +
> > + xdr_encode_word(buf, *bufpos, segment->type);
> > + xdr_encode_double(buf, *bufpos + 4, segment->offset);
> > +
> > + if (segment->type == NFS4_CONTENT_HOLE) {
> > + xdr_encode_double(buf, *bufpos + 12, segment->length);
> > + *bufpos += 4 + 8 + 8;
>
> Throughout, can you use multiples of XDR_UNIT instead of naked integers?
Sure.
>
>
> > + } else {
> > + size_t align = xdr_align_size(segment->length);
> > + xdr_encode_word(buf, *bufpos + 12, segment->length);
> > + if (*segments == 0)
> > + xdr_buf_trim_head(buf, 4);
> > +
> > + xdr_stream_move_subsegment(xdr,
> > + buf->head[0].iov_len + segment->page_pos,
> > + *bufpos + 16, align);
> > + *bufpos += 4 + 8 + 4 + align;
> > + }
> > +
> > + *segments += 1;
> > +}
> > +
> > static __be32
> > -nfsd4_encode_read_plus_hole(struct nfsd4_compoundres *resp,
> > - struct nfsd4_read *read,
> > - unsigned long *maxcount, u32 *eof)
> > +nfsd4_encode_read_plus_segments(struct nfsd4_compoundres *resp,
> > + struct nfsd4_read *read,
> > + unsigned int *segments, u32 *eof)
> > {
> > - struct file *file = read->rd_nf->nf_file;
> > - loff_t data_pos = vfs_llseek(file, read->rd_offset, SEEK_DATA);
> > - loff_t f_size = i_size_read(file_inode(file));
> > - unsigned long count;
> > - __be32 *p;
> > + enum data_content4 pagetype;
> > + struct read_plus_segment segment;
> > + struct xdr_stream *xdr = resp->xdr;
> > + unsigned long offset = read->rd_offset;
> > + unsigned int bufpos = xdr->buf->len;
> > + unsigned long maxcount;
> > + unsigned int pagelen, i = 0;
> > + char *vpage, *p;
> > + __be32 nfserr;
>
> Nit: try to use reverse christmas tree style where possible.
Will do.
>
>
> > - if (data_pos == -ENXIO)
> > - data_pos = f_size;
> > - else if (data_pos <= read->rd_offset || (data_pos < f_size && data_pos % PAGE_SIZE))
> > - return nfsd4_encode_read_plus_data(resp, read, maxcount, eof, &f_size);
> > - count = data_pos - read->rd_offset;
> > -
> > - /* Content type, offset, byte count */
> > - p = xdr_reserve_space(resp->xdr, 4 + 8 + 8);
> > - if (!p)
> > + /* enough space for a HOLE segment before we switch to the pages */
> > + if (!xdr_reserve_space(xdr, 4 + 8 + 8))
> > return nfserr_resource;
> > + xdr_commit_encode(xdr);
> >
> > - *p++ = htonl(NFS4_CONTENT_HOLE);
> > - p = xdr_encode_hyper(p, read->rd_offset);
> > - p = xdr_encode_hyper(p, count);
> > + maxcount = min_t(unsigned long, read->rd_length,
> > + (xdr->buf->buflen - xdr->buf->len));
> >
> > - *eof = (read->rd_offset + count) >= f_size;
> > - *maxcount = min_t(unsigned long, count, *maxcount);
> > + nfserr = nfsd4_read_plus_readv(resp, read, &maxcount, eof);
> > + if (nfserr)
> > + return nfserr;
> > +
> > + while (maxcount > 0) {
> > + vpage = xdr_buf_nth_page_address(xdr->buf, i, &pagelen);
> > + pagelen = min_t(unsigned int, pagelen, maxcount);
> > + if (!vpage || pagelen == 0)
> > + break;
> > + p = memchr_inv(vpage, 0, pagelen);
>
> So you have to walk every page in the payload, byte-by-byte, to
> sort out how to encode the READ_PLUS result? That's... unfortunate.
> The whole idea of making the READ payload "opaque" is that XDR
> doesn't have to touch those bytes; and then the payload is passed
> to the network layer as pointers to pages for the same reason.
>
> It might be helpful to get this reviewed by fsdevel and linux-mm
> in case there's a better approach. Hugh was attempting to point
> all zero pages at ZERO_PAGE at one point, for example, and that
> would make it very quick to detect a range of zero bytes.
I agree something like that would be better. I brought up tracking
zero pages to Matthew Wilcox during LSF, so it's at least on his
radar.
>
> Another thought is to use a POSIX byte-range lock to prevent
> changes to the range of the file you're encoding, while leaving
> the rest of the file available for other operations. That way
> you could continue to use llseek when that's supported.
Wouldn't that still deadlock when vfs_llseek() also locks the file? Or
am I misunderstanding something about byte-range locking?
>
>
> > + pagetype = (p == NULL) ? NFS4_CONTENT_HOLE : NFS4_CONTENT_DATA;
> > +
> > + if (pagetype != segment.type || i == 0) {
> > + if (likely(i > 0)) {
> > + nfsd4_encode_read_plus_segment(xdr, &segment,
> > + &bufpos, segments);
> > + offset += segment.length;
> > + }
> > + segment.type = pagetype;
> > + segment.offset = offset;
> > + segment.length = pagelen;
> > + segment.page_pos = i * PAGE_SIZE;
> > + } else
> > + segment.length += pagelen;
> > +
> > + maxcount -= pagelen;
> > + i++;
> > + }
> > +
> > + nfsd4_encode_read_plus_segment(xdr, &segment, &bufpos, segments);
> > + xdr_truncate_encode(xdr, bufpos);
> > return nfs_ok;
> > }
> >
> > @@ -4813,69 +4853,31 @@ static __be32
> > nfsd4_encode_read_plus(struct nfsd4_compoundres *resp, __be32 nfserr,
> > struct nfsd4_read *read)
> > {
> > - unsigned long maxcount, count;
> > struct xdr_stream *xdr = resp->xdr;
> > - struct file *file;
> > int starting_len = xdr->buf->len;
> > - int last_segment = xdr->buf->len;
> > - int segments = 0;
> > - __be32 *p, tmp;
> > - bool is_data;
> > - loff_t pos;
> > + unsigned int segments = 0;
> > u32 eof;
> >
> > if (nfserr)
> > return nfserr;
> > - file = read->rd_nf->nf_file;
> >
> > /* eof flag, segment count */
> > - p = xdr_reserve_space(xdr, 4 + 4);
> > - if (!p)
> > + if (!xdr_reserve_space(xdr, 4 + 4))
> > return nfserr_resource;
> > xdr_commit_encode(xdr);
> >
> > - maxcount = min_t(unsigned long, read->rd_length,
> > - (xdr->buf->buflen - xdr->buf->len));
> > - count = maxcount;
> > -
> > - eof = read->rd_offset >= i_size_read(file_inode(file));
> > + eof = read->rd_offset >= i_size_read(file_inode(read->rd_nf->nf_file));
> > if (eof)
> > goto out;
> >
> > - pos = vfs_llseek(file, read->rd_offset, SEEK_HOLE);
> > - is_data = pos > read->rd_offset;
> > -
> > - while (count > 0 && !eof) {
> > - maxcount = count;
> > - if (is_data)
> > - nfserr = nfsd4_encode_read_plus_data(resp, read, &maxcount, &eof,
> > - segments == 0 ? &pos : NULL);
> > - else
> > - nfserr = nfsd4_encode_read_plus_hole(resp, read, &maxcount, &eof);
> > - if (nfserr)
> > - goto out;
> > - count -= maxcount;
> > - read->rd_offset += maxcount;
> > - is_data = !is_data;
> > - last_segment = xdr->buf->len;
> > - segments++;
> > - }
> > -
> > + nfserr = nfsd4_encode_read_plus_segments(resp, read, &segments, &eof);
> > out:
> > - if (nfserr && segments == 0)
> > + if (nfserr)
> > xdr_truncate_encode(xdr, starting_len);
> > else {
> > - if (nfserr) {
> > - xdr_truncate_encode(xdr, last_segment);
> > - nfserr = nfs_ok;
> > - eof = 0;
> > - }
> > - tmp = htonl(eof);
> > - write_bytes_to_xdr_buf(xdr->buf, starting_len, &tmp, 4);
> > - tmp = htonl(segments);
> > - write_bytes_to_xdr_buf(xdr->buf, starting_len + 4, &tmp, 4);
> > + xdr_encode_word(xdr->buf, starting_len, eof);
> > + xdr_encode_word(xdr->buf, starting_len + 4, segments);
> > }
> > -
> > return nfserr;
> > }
>
> The clean-ups in nfsd4_encode_read_plus() LGTM.
Thanks for the review!
Anna
>
>
> --
> Chuck Lever
>
>
>
