From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792429831; b=NJRk5XziPLuVquMRRjb0o3gbvl0ZHPnWb5Cv6b1FJ/fRfeQqQvH9J/wqwcdEGQZNVcIX nT273R+m0TFMvKtiivATuZKC4GaiR5Aekh5aiSpm5C3BIpJeHg5NpzH5UiipBPAcgyfiw F9Svk2U35l377hjTDiD8/GJwtR8Jq/J3NI9qgbpKHDTBc3oVxZqtL0Lgxq5bU77MsWxfw FKhxIQpwlvK8CoK05i3etorJntLWAHjYPyFXT+Ax/KECxjhdJVhw6QghU7SyQFcPLxPa9 jYEuNIUtseF/7xRJOb/uJAgKyPH/N7BvLtWIMlTPNuutQyFrwhCQIatW1qX3JqPz8gA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792429831; h=Received:DKIM-Signature:Message-ID:Subject:From:To:Cc:Date: In-Reply-To:References:Autocrypt:Content-Type: Content-Transfer-Encoding:User-Agent:MIME-Version; bh=CdxKoYX6MBGmuglBv9ThU/eVqYxOtC/3iRjWSvQFnlA=; b=GH7cRtraM2C1aIm5FG9nRdHvjwXCRW+Z3o1A1On4/jqGmadix5QLrgwoUajIxb58mXtV 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header.b=n6ApIA12 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 16FAD1F000C4; Mon, 19 Oct 2026 17:10:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792429831; bh=CdxKoYX6MBGmuglBv9ThU/eVqYxOtC/3iRjWSvQFnlA=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=n6ApIA12i6Uvx+NaUHuj2gBiTsPc7yTGwXaQ7Z5jpCqER3PjAF/NrnGzMj9UAh5xz hcKj/5KBkjgAAt9jm8ALKIn5t3OuY2A7st/sIHuqt89H7QTyeRQPuBYMVj0A4yOqL5 6SBuPewEhUts0i7HAcuYDNxGg+9c12FZK3uvEwG4pWLBRj9b2VZMqUouKAvY9f9NFM UDVrs7k7s5R1K50BQ25Lvtfn2ut2vCq57lgof6lBaUXGVThvmorRMtmSvjdOeVjk7+ Nwd+0RnpLBcPKzlIRJGOb/JLUe8a5w3qy4lWz+wUoerwb4FnXP70aZKw+Q5oTAebK0 uNnxaZqFAx1yQ== Message-ID: <084de93aac1adeb011e8ff05f1d7625ee75372df.camel@kernel.org> Subject: Re: [PATCH v5 1/6] NFSD: add tracing for how direct-mode READ and WRITE are serviced From: Jeff Layton To: Mike Snitzer , Chuck Lever Cc: linux-nfs@vger.kernel.org, NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Christoph Hellwig , Jens Axboe , Matthew Wilcox , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org Date: Mon, 19 Oct 2026 13:10:28 -0400 In-Reply-To: <20261019150725.1507-2-snitzer@kernel.org> References: <20261019150725.1507-1-snitzer@kernel.org> <20261019150725.1507-2-snitzer@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ cDEsvv5IMmeMe1/qLZ6NaVkNuL3WOXvxaVT9USW1+/SGipO2IpKJjeDZfehlB/kpfF24+RrK+seQf 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Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-2.fc44) X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Mon, 2026-10-19 at 11:07 -0400, Mike Snitzer wrote: > When io_cache_read or io_cache_write selects a direct I/O mode, a > request may still be serviced with DONTCACHE buffered or normal > buffered I/O, and a WRITE may be split into up to three segments that > each take a different path depending on the request's offset/length > alignment, the payload's memory alignment, the filesystem's > FOP_DONTCACHE support, direct_misaligned_min_bytes and > direct_misaligned_dontcache. Until now the only visibility was > nfsd_{read,write}_direct for O_DIRECT and nfsd_{read,write}_vector for > everything else, so the DONTCACHE and cached buffered cases were > indistinguishable and the reason a WRITE was not issued as direct I/O > was not recorded at all. >=20 > Add: >=20 > - nfsd_write_dio_split, emitted once per direct-mode WRITE before any > segment is issued. It records the advertised offset and memory > alignments, the payload's offset within its first page, the start, > middle and end segment sizes, a disposition naming the path taken > (direct, no_alignment, too_small, no_middle, mem_misaligned), and > whether buffered segments are issued IOCB_DONTCACHE. >=20 > - nfsd_write_dontcache and nfsd_read_dontcache, emitted for WRITE > segments and READs serviced with IOCB_DONTCACHE. nfsd_write_vector > and nfsd_read_vector now fire only for normal buffered I/O. >=20 > The disposition enum lives in vfs.h, which trace.h already includes. >=20 > Document the new events in nfsd-io-modes.rst, together with iomap's > iomap_dio_invalidate_fail event, which reports an O_DIRECT segment that > the filesystem serviced with buffered I/O instead. >=20 > Assisted-by: LLM > Signed-off-by: Mike Snitzer > --- > .../filesystems/nfs/nfsd-io-modes.rst | 38 ++++++++- > fs/nfsd/trace.h | 77 +++++++++++++++++++ > fs/nfsd/vfs.c | 47 ++++++++--- > fs/nfsd/vfs.h | 9 +++ > 4 files changed, 156 insertions(+), 15 deletions(-) >=20 > diff --git a/Documentation/filesystems/nfs/nfsd-io-modes.rst b/Documentat= ion/filesystems/nfs/nfsd-io-modes.rst > index 0a5f4ef14a8a..11da2cee76fc 100644 > --- a/Documentation/filesystems/nfs/nfsd-io-modes.rst > +++ b/Documentation/filesystems/nfs/nfsd-io-modes.rst > @@ -171,21 +171,53 @@ Misaligned WRITE: > Tracing: > The nfsd_read_direct trace event shows how NFSD expands any > misaligned READ to the next DIO-aligned block (on either end of the > - original READ, as needed). > + original READ, as needed). A READ that is serviced with buffered IO > + instead emits nfsd_read_dontcache (DONTCACHE buffered IO) or > + nfsd_read_vector (normal buffered IO). > =20 > This combination of trace events is useful for READs:: > =20 > echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_read_vector/enable > + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_read_dontcache/enabl= e > echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_read_direct/enable > echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_read_io_done/enable > echo 1 > /sys/kernel/tracing/events/xfs/xfs_file_direct_read/enabl= e > =20 > - The nfsd_write_direct trace event shows how NFSD splits a given > - misaligned WRITE into a DIO-aligned middle segment. > + The nfsd_write_dio_split trace event is emitted once per WRITE in a > + DIRECT IO mode, before any IO is issued. It records the alignments > + the filesystem advertised, the payload's offset in its first page, > + the sizes of the start, middle and end segments, and a > + disposition:: > + > + direct the aligned middle segment uses O_DIRECT > + mem_misaligned payload memory is misaligned; not split > + no_alignment the filesystem advertises no DIO alignment; not sp= lit > + too_small smaller than the larger of the offset and memory > + alignments; not split > + no_middle no aligned middle, or a misaligned start or end > + with a middle smaller than > + direct_misaligned_min_bytes bytes; not split > + > + dontcache=3D1 means any buffered segment is issued DONTCACHE: the st= art > + and end segments of a split WRITE, or the whole of a WRITE that is > + not split. It is 0 when direct_misaligned_dontcache is N or the > + filesystem lacks FOP_DONTCACHE, and is reported even for a WRITE > + that has no buffered segment. > + > + Each segment then emits nfsd_write_direct (O_DIRECT), > + nfsd_write_dontcache (DONTCACHE buffered IO) or nfsd_write_vector > + (normal buffered IO). > =20 > This combination of trace events is useful for WRITEs:: > =20 > echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_opened/enable > + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_dio_split/enab= le > echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_direct/enable > + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_dontcache/enab= le > + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_vector/enable > echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_io_done/enable > echo 1 > /sys/kernel/tracing/events/xfs/xfs_file_direct_write/enab= le > + echo 1 > /sys/kernel/tracing/events/iomap/iomap_dio_invalidate_fai= l/enable > + > + iomap_dio_invalidate_fail reports an O_DIRECT middle segment that > + the filesystem serviced with buffered IO instead. > diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h > index e0908cb21c4e..e2615cfdf6b7 100644 > --- a/fs/nfsd/trace.h > +++ b/fs/nfsd/trace.h > @@ -501,6 +501,7 @@ DEFINE_EVENT(nfsd_io_class, nfsd_##name, \ > DEFINE_NFSD_IO_EVENT(read_start); > DEFINE_NFSD_IO_EVENT(read_splice); > DEFINE_NFSD_IO_EVENT(read_vector); > +DEFINE_NFSD_IO_EVENT(read_dontcache); > DEFINE_NFSD_IO_EVENT(read_direct); > DEFINE_NFSD_IO_EVENT(read_io_done); > DEFINE_NFSD_IO_EVENT(read_done); > @@ -508,11 +509,87 @@ DEFINE_NFSD_IO_EVENT(write_start); > DEFINE_NFSD_IO_EVENT(write_opened); > DEFINE_NFSD_IO_EVENT(write_direct); > DEFINE_NFSD_IO_EVENT(write_vector); > +DEFINE_NFSD_IO_EVENT(write_dontcache); > DEFINE_NFSD_IO_EVENT(write_io_done); > DEFINE_NFSD_IO_EVENT(write_done); > DEFINE_NFSD_IO_EVENT(commit_start); > DEFINE_NFSD_IO_EVENT(commit_done); > =20 > +TRACE_DEFINE_ENUM(NFSD_WRITE_DIO_DIRECT); > +TRACE_DEFINE_ENUM(NFSD_WRITE_DIO_MEM_MISALIGNED); > +TRACE_DEFINE_ENUM(NFSD_WRITE_DIO_NO_ALIGNMENT); > +TRACE_DEFINE_ENUM(NFSD_WRITE_DIO_TOO_SMALL); > +TRACE_DEFINE_ENUM(NFSD_WRITE_DIO_NO_MIDDLE); > + > +#define show_nfsd_write_dio_disposition(x) \ > + __print_symbolic(x, \ > + { NFSD_WRITE_DIO_DIRECT, "direct" }, \ > + { NFSD_WRITE_DIO_MEM_MISALIGNED, "mem_misaligned" }, \ > + { NFSD_WRITE_DIO_NO_ALIGNMENT, "no_alignment" }, \ > + { NFSD_WRITE_DIO_TOO_SMALL, "too_small" }, \ > + { NFSD_WRITE_DIO_NO_MIDDLE, "no_middle" }) > + > +/* > + * Emitted once per direct-mode WRITE, before any segment is issued. > + * prefix, middle and suffix are zero when not computed. mem_offset is > + * the offset of the payload within its first page. > + */ > +TRACE_EVENT(nfsd_write_dio_split, > + TP_PROTO(struct svc_rqst *rqstp, > + struct svc_fh *fhp, > + u64 offset, > + u32 len, > + u32 offset_align, > + u32 mem_align, > + u32 mem_offset, > + u32 prefix, > + u32 middle, > + u32 suffix, > + enum nfsd_write_dio_disposition disposition, > + bool dontcache), > + TP_ARGS(rqstp, fhp, offset, len, offset_align, mem_align, mem_offset, > + prefix, middle, suffix, disposition, dontcache), > + TP_STRUCT__entry( > + __field(u32, xid) > + __field(u32, fh_hash) > + __field(u64, offset) > + __field(u32, len) > + __field(u32, offset_align) > + __field(u32, mem_align) > + __field(u32, mem_offset) > + __field(u32, prefix) > + __field(u32, middle) > + __field(u32, suffix) > + __field(unsigned int, disposition) > + __field(bool, dontcache) > + ), > + TP_fast_assign( > + __entry->xid =3D be32_to_cpu(rqstp->rq_xid); > + __entry->fh_hash =3D knfsd_fh_hash(&fhp->fh_handle); > + __entry->offset =3D offset; > + __entry->len =3D len; > + __entry->offset_align =3D offset_align; > + __entry->mem_align =3D mem_align; > + __entry->mem_offset =3D mem_offset; > + __entry->prefix =3D prefix; > + __entry->middle =3D middle; > + __entry->suffix =3D suffix; > + __entry->disposition =3D disposition; > + __entry->dontcache =3D dontcache; > + ), > + TP_printk("xid=3D0x%08x fh_hash=3D0x%08x offset=3D%llu len=3D%u " > + "offset_align=3D%u mem_align=3D%u mem_offset=3D%u " > + "prefix=3D%u middle=3D%u suffix=3D%u disposition=3D%s " > + "dontcache=3D%u", > + __entry->xid, __entry->fh_hash, > + __entry->offset, __entry->len, > + __entry->offset_align, __entry->mem_align, > + __entry->mem_offset, > + __entry->prefix, __entry->middle, __entry->suffix, > + show_nfsd_write_dio_disposition(__entry->disposition), > + __entry->dontcache) > +); > + > DECLARE_EVENT_CLASS(nfsd_err_class, > TP_PROTO(struct svc_rqst *rqstp, > struct svc_fh *fhp, > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c > index 32136cd32af9..0e937d116f39 100644 > --- a/fs/nfsd/vfs.c > +++ b/fs/nfsd/vfs.c > @@ -1323,7 +1323,10 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, stru= ct svc_fh *fhp, > } > rqstp->rq_next_page =3D page; > =20 > - trace_nfsd_read_vector(rqstp, fhp, offset, *count - total); > + if (kiocb.ki_flags & IOCB_DONTCACHE) > + trace_nfsd_read_dontcache(rqstp, fhp, offset, *count - total); > + else > + trace_nfsd_read_vector(rqstp, fhp, offset, *count - total); It would be nicer if we could embed the above conditional in something that is only enabled when the tracepoints are enabled. What would you think about making those conditional tracepoints, a'la TRACE_EVENT_CONDITION()? Then the ki_flags tests are only done when the tracepoints are enabled. Alternately you could turn all of those into a single trace_nfsd_read tracepoint that also records ki_flags. > iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, *count - total); > host_err =3D vfs_iocb_iter_read(file, &kiocb, &iter); > return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err)= ; > @@ -1391,7 +1394,8 @@ nfsd_write_dio_seg_init(struct nfsd_write_dio_seg *= segment, > } > =20 > static unsigned int > -nfsd_write_dio_iters_init(struct nfsd_file *nf, struct bio_vec *bvec, > +nfsd_write_dio_iters_init(struct svc_rqst *rqstp, struct svc_fh *fhp, > + struct nfsd_file *nf, struct bio_vec *bvec, > unsigned int nvecs, struct kiocb *iocb, > unsigned long total, > struct nfsd_write_dio_seg segments[3]) > @@ -1399,7 +1403,9 @@ nfsd_write_dio_iters_init(struct nfsd_file *nf, str= uct bio_vec *bvec, > u32 offset_align =3D nf->nf_dio_offset_align; > loff_t prefix_end, orig_end, middle_end; > u32 mem_align =3D nf->nf_dio_mem_align; > - size_t prefix, middle, suffix; > + size_t prefix =3D 0, middle =3D 0, suffix =3D 0; > + enum nfsd_write_dio_disposition disposition; > + u32 mem_offset =3D nvecs ? bvec->bv_offset : 0; > loff_t offset =3D iocb->ki_pos; > unsigned int dontcache_flags =3D 0; > unsigned int buffered_flags; > @@ -1415,9 +1421,14 @@ nfsd_write_dio_iters_init(struct nfsd_file *nf, st= ruct bio_vec *bvec, > * If alignments are not available, the write is too small, > * or no alignment can be found, fall back to buffered I/O. > */ > - if (unlikely(!mem_align || !offset_align) || > - unlikely(total < max(offset_align, mem_align))) > + if (unlikely(!mem_align || !offset_align)) { > + disposition =3D NFSD_WRITE_DIO_NO_ALIGNMENT; > goto no_dio; > + } > + if (unlikely(total < max(offset_align, mem_align))) { > + disposition =3D NFSD_WRITE_DIO_TOO_SMALL; > + goto no_dio; > + } > =20 > orig_end =3D offset + total; > =20 > @@ -1443,8 +1454,10 @@ nfsd_write_dio_iters_init(struct nfsd_file *nf, st= ruct bio_vec *bvec, > =20 > if (!middle || > ((prefix || suffix) && > - middle < nfsd_direct_misaligned_min_bytes)) > + middle < nfsd_direct_misaligned_min_bytes)) { > + disposition =3D NFSD_WRITE_DIO_NO_MIDDLE; > goto no_dio; > + } > =20 > if (prefix) { > nfsd_write_dio_seg_init(&segments[nsegs], bvec, > @@ -1462,28 +1475,35 @@ nfsd_write_dio_iters_init(struct nfsd_file *nf, s= truct bio_vec *bvec, > * the first bvec, all subsequent bvecs start at bv_offset zero > * (page-aligned). Therefore, only the first bvec is checked. > */ > - if (iov_iter_bvec_offset(&segments[nsegs].iter) & (mem_align - 1)) > + if (iov_iter_bvec_offset(&segments[nsegs].iter) & (mem_align - 1)) { > + disposition =3D NFSD_WRITE_DIO_MEM_MISALIGNED; > goto no_dio; > + } > /* > * IOCB_DONTCACHE matters only if the file system falls back to > * buffered I/O (-ENOTBLK). > */ > segments[nsegs++].flags |=3D IOCB_DIRECT | dontcache_flags; > + disposition =3D NFSD_WRITE_DIO_DIRECT; > =20 > if (suffix) { > nfsd_write_dio_seg_init(&segments[nsegs], bvec, nvecs, total, > prefix + middle, suffix, iocb); > segments[nsegs++].flags |=3D buffered_flags; > } > - > - return nsegs; > + goto out; > =20 > no_dio: > /* Issue the whole WRITE as a single buffered segment. */ > nfsd_write_dio_seg_init(&segments[0], bvec, nvecs, total, 0, > total, iocb); > segments[0].flags |=3D buffered_flags; > - return 1; > + nsegs =3D 1; > +out: > + trace_nfsd_write_dio_split(rqstp, fhp, offset, total, offset_align, > + mem_align, mem_offset, prefix, middle, > + suffix, disposition, !!buffered_flags); > + return nsegs; > } > =20 > static noinline_for_stack int > @@ -1499,8 +1519,8 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct sv= c_fh *fhp, > ssize_t host_err; > size_t expected; > =20 > - nsegs =3D nfsd_write_dio_iters_init(nf, rqstp->rq_bvec, nvecs, > - kiocb, *cnt, segments); > + nsegs =3D nfsd_write_dio_iters_init(rqstp, fhp, nf, rqstp->rq_bvec, > + nvecs, kiocb, *cnt, segments); > =20 > /* > * Persist a split synchronous WRITE once, after all of its > @@ -1517,6 +1537,9 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct sv= c_fh *fhp, > if (kiocb->ki_flags & IOCB_DIRECT) > trace_nfsd_write_direct(rqstp, fhp, kiocb->ki_pos, > segments[i].iter.count); > + else if (kiocb->ki_flags & IOCB_DONTCACHE) > + trace_nfsd_write_dontcache(rqstp, fhp, kiocb->ki_pos, > + segments[i].iter.count); > else > trace_nfsd_write_vector(rqstp, fhp, kiocb->ki_pos, > segments[i].iter.count); > diff --git a/fs/nfsd/vfs.h b/fs/nfsd/vfs.h > index 1f05c83a3ea6..e2a7c1c44949 100644 > --- a/fs/nfsd/vfs.h > +++ b/fs/nfsd/vfs.h > @@ -230,4 +230,13 @@ __be32 nfsd_permission(struct svc_cred *cred, struc= t svc_export *exp, > =20 > void nfsd_filp_close(struct file *fp); > =20 > +/* The path nfsd_write_dio_iters_init() took for a direct-mode WRITE. */ > +enum nfsd_write_dio_disposition { > + NFSD_WRITE_DIO_DIRECT, > + NFSD_WRITE_DIO_MEM_MISALIGNED, > + NFSD_WRITE_DIO_NO_ALIGNMENT, > + NFSD_WRITE_DIO_TOO_SMALL, > + NFSD_WRITE_DIO_NO_MIDDLE, > +}; > + > #endif /* LINUX_NFSD_VFS_H */ --=20 Jeff Layton