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From:   Benjamin Maynard <benmaynard@google.com>
Date:   Mon, 14 Nov 2022 16:03:18 +0000
Message-ID: <CA+QRt4s4+pF+C3Bk-wgoPjGNZwDbBGFPAQ=-Fbrgt6ejrfqsTA@mail.gmail.com>
Subject: Re: [PATCH v9 3/5] NFS: Convert buffered read paths to use netfs when
 fscache is enabled
To:     David Wysochanski <dwysocha@redhat.com>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        Trond Myklebust <trondmy@kernel.org>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        David Howells <dhowells@redhat.com>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "linux-cachefs@redhat.com" <linux-cachefs@redhat.com>,
        Daire Byrne <daire.byrne@gmail.com>,
        Brennan Doyle <brennandoyle@google.com>,
        Jeff Layton <jlayton@poochiereds.net>
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Hi Dave,

I've added responses to your questions inline below.

I also tried adding the noatime option to the mount on the source
filer as Jeff suggested, but this has not made any difference and the
issue is still persisting for me.

I created the following diagram that explains my setup, and the exact
tests I am performing:
https://drive.google.com/file/d/12Xf-9yHCKM4eMr2YGqdSAVfGcximW4OG/view?usp=
=3Dsharing.

Hopefully this is clearer than my explanations below (let me know if
you'd prefer me to share an alternative way).

In order to remove the re-exporting layer of complexity, I also
performed the tests without the re-export server (architecture:
https://drive.google.com/file/d/1DQKhqo_UnQ8ul-z5Iram5LpisDmkKziQ/view?usp=
=3Dshare_link):

Source NFS Server <-- Client (with FS-Cache)

The same is happening, I cannot get FS-Cache to serve from cache.
Heavy writes, but no reads, even when the same file is copied many
times.

Hopefully something I am doing wrong on my end, but I can't figure out what=
.

Kind Regards
Benjamin Maynard


On Mon, 14 Nov 2022 at 13:47, David Wysochanski <dwysocha@redhat.com> wrote=
:
>
> I apologize I did not read carefully enough and I missed some details
> in your original post.
> More below.
>
> On Sat, Nov 12, 2022 at 7:47 AM Benjamin Maynard <benmaynard@google.com> =
wrote:
> >
> > Hi all,
> >
> > I've been doing some more testing with these patches, I applied all of
> > the patches (v10 from
> > https://patchwork.kernel.org/project/linux-nfs/list/?series=3D691729)
> > apart from Patch 6 (the RFC patch) to version 6.0.8 of the kernel.
> >
> > I have the following setup:
> >
> > Source NFS Server <-- Re-Export Server (with FS-Cache) <-- NFS Client.
> >
> > I have a 500Gb file on the Source NFS Server, which I am then copying
> > to the NFS Client via the Re-Export Server.
> >
> > On the first copy, I see heavy writes to /var/cache/fscache on the
> > re-export server, and once the file copy completes I see that
> > /var/cache/fscache is approximately 500Gb in size. All good so far.
> >
> > I then deleted that file from the NFS Client, and dropped the caches
> > just to be safe (echo 3 > /proc/sys/vm/drop_caches on the NFS Client).
> >
> If you delete the file from the NFS client, how does that not delete the
> file from the original NFS server?

Sorry - to be clear, I never deleted the file from the NFS mount
(which I know would in turn delete it from the re-export server and
the source filer).

In order to perform the performance test, I copied the file from the
NFS mount on the NFS Client, to a local directory (cp
/mnt/nfs/500gb.img /tmp).

When I said "I then deleted that file from the NFS Client", I meant I
deleted the local copy of that file. Not the file on the mount (rm
/tmp/500gb.img).

Just to also stress, I have never dropped the caches on the Re-Export
Server (the one with FS-Cache) at any point in any of these tests, so
I don't think this is the problem. I have only ever dropped the caches
on the NFS client that is mounting the Re-Export Server.

> > I then performed another copy of the 500Gb file on the NFS Client,
> > again via the Re-Export Server. What I expected would happen is that I
> > would see heavy reads from the /var/cache/fscache volume as the file
> > should be served from FS-Cache.
> >
> I don't understand this.  When you say you "performed another copy"
> of what file?  Wasn't the file deleted in the above step?

As above, only the local copy was deleted.

> > However what I actually saw was no reads whatsoever, FS-Cache seems to
> > be ignored and the file is pulled from the Source NFS Filer again. I
> > also see heavy writes to /var/cache/fscache, so it appears that
> > FS-Cache is overwriting its existing cache, and never using it.
>
> That would happen if the file was changed or re-created.
>
> > I only have 104Gb of memory on the Re-Export Server (with FS-Cache) so
> > it is not possible that the file is being served from the page cache.
> >
> > We saw this behaviour before on an older set of the patches when our
> > mount between the Re-Export Server and the Source NFS Filer was using
> > the "sync" option, but we are now using the "async" option and the
> > same is happening.
> >
> > Mount options:
> >
> > Source NFS Server <-- Re-Export Server (with FS-Cache):
> >
> > 10.0.0.49:/files /srv/nfs/files nfs
> > rw,noatime,vers=3D3,rsize=3D1048576,wsize=3D1048576,namlen=3D255,acregm=
in=3D600,acregmax=3D600,acdirmin=3D600,acdirmax=3D600,hard,nocto,proto=3Dtc=
p,nconnect=3D16,timeo=3D600,retrans=3D2,sec=3Dsys,mountaddr=3D10.0.0.49,mou=
ntvers=3D3,mountport=3D37485,mountproto=3Dtcp,fsc,local_lock=3Dnone,addr=3D=
10.0.0.49
> >
> > Re-Export Server (with FS-Cache) <-- NFS Client:
> >
> > 10.0.0.3:/files /mnt/nfs nfs
> > rw,relatime,vers=3D3,rsize=3D1048576,wsize=3D1048576,namlen=3D255,hard,=
proto=3Dtcp,timeo=3D600,retrans=3D2,sec=3Dsys,mountaddr=3D10.0.0.3,mountver=
s=3D3,mountport=3D20048,mountproto=3Dtcp,local_lock=3Dnone,addr=3D10.0.0.3
> >
> > It is also worth noting this behaviour is not unique to the re-export
> > use case. I see FS-Cache not being used with the following setup:
> >
> > Source NFS Server <-- Client (with FS-Cache).
> >
>
> This points at something more fundamental like something missed
> in the test or maybe a mount option.  Can you explain what test
> you're doing here when you say "this behavior is not unique"?

I've created the following diagram which explains the test I am
performing. I think it is a little easier to follow than explaining in
text. This should be viewable without any authentication:
https://drive.google.com/file/d/12Xf-9yHCKM4eMr2YGqdSAVfGcximW4OG/view?usp=
=3Dsharing.

By "this behaviour is not unique to the re-export use case" I mean
that the same happens if I remove the re-export server completely, and
just have the following setup:

Source NFS Server <-- Client (with FS-Cache).

> Can you show the mount options for both:
> - fscache filesystem on the re-export server (/var/cache/fscache)

root@reexport:~$ mount | grep /var/cache/fscache
/dev/md127 on /var/cache/fscache type ext4
(rw,relatime,discard,nobarrier,stripe=3D1024)

> - exported filesystem on the NFS server (filesystem in /etc/exports)

I have tried both:

root@source:~$ mount | grep files
/dev/sdb1 on /files type ext4 (rw)

root@source:~$ cat /etc/exports
/files 10.0.0.0/8(rw,sync,wdelay,no_root_squash,no_all_squash,no_subtree_ch=
eck,sec=3Dsys,secure,nohide)

and (at Jeff's suggestion):

root@source:~$ mount | grep files
/dev/sdb1 on /files type ext4 (rw,noatime)

root@source:~$ cat /etc/exports
/files 10.0.0.0/8(rw,sync,wdelay,no_root_squash,no_all_squash,no_subtree_ch=
eck,sec=3Dsys,secure,nohide)


> Unfortunately the problem with drop_caches makes it more difficult
> to know when fscache is truly working.  But some other unit test
> I have shows fscache does work with this patchset so I'm puzzled why
> you're not seeing it work at all.
>
> I pinged dhowells on the drop_caches issue so maybe we can get
> that one sorted out soon but I'm not sure since it's part of a series
> and proposes changes in mm.

Just to be clear, I have never used drop_caches on the re-export
server in any of these tests. I have only ever done this on the NFS
Client.

>
> > Thanks,
> > Ben
> >
> >
> > Kind Regards
> >
> > Benjamin Maynard
> >
> > Customer Engineer
> >
> > benmaynard@google.com
> >
> > Google, Inc.
> >
> >
> >
> >
> > On Mon, 31 Oct 2022 at 22:22, Trond Myklebust <trondmy@hammerspace.com>=
 wrote:
> > >
> > >
> > >
> > > > On Oct 30, 2022, at 19:25, David Wysochanski <dwysocha@redhat.com> =
wrote:
> > > >
> > > > On Sat, Oct 29, 2022 at 12:46 PM David Wysochanski <dwysocha@redhat=
.com> wrote:
> > > >>
> > > >> On Fri, Oct 28, 2022 at 12:59 PM Trond Myklebust <trondmy@kernel.o=
rg> wrote:
> > > >>>
> > > >>> On Fri, 2022-10-28 at 07:50 -0400, David Wysochanski wrote:
> > > >>>> On Thu, Oct 27, 2022 at 3:16 PM Trond Myklebust <trondmy@kernel.=
org>
> > > >>>> wrote:
> > > >>>>>
> > > >>>>> On Mon, 2022-10-17 at 06:52 -0400, Dave Wysochanski wrote:
> > > >>>>>> Convert the NFS buffered read code paths to corresponding netf=
s
> > > >>>>>> APIs,
> > > >>>>>> but only when fscache is configured and enabled.
> > > >>>>>>
> > > >>>>>> The netfs API defines struct netfs_request_ops which must be
> > > >>>>>> filled
> > > >>>>>> in by the network filesystem.  For NFS, we only need to define=
 5
> > > >>>>>> of
> > > >>>>>> the functions, the main one being the issue_read() function.
> > > >>>>>> The issue_read() function is called by the netfs layer when a
> > > >>>>>> read
> > > >>>>>> cannot be fulfilled locally, and must be sent to the server
> > > >>>>>> (either
> > > >>>>>> the cache is not active, or it is active but the data is not
> > > >>>>>> available).
> > > >>>>>> Once the read from the server is complete, netfs requires a ca=
ll
> > > >>>>>> to
> > > >>>>>> netfs_subreq_terminated() which conveys either how many bytes
> > > >>>>>> were
> > > >>>>>> read
> > > >>>>>> successfully, or an error.  Note that issue_read() is called w=
ith
> > > >>>>>> a
> > > >>>>>> structure, netfs_io_subrequest, which defines the IO requested=
,
> > > >>>>>> and
> > > >>>>>> contains a start and a length (both in bytes), and assumes the
> > > >>>>>> underlying
> > > >>>>>> netfs will return a either an error on the whole region, or th=
e
> > > >>>>>> number
> > > >>>>>> of bytes successfully read.
> > > >>>>>>
> > > >>>>>> The NFS IO path is page based and the main APIs are the pgio A=
PIs
> > > >>>>>> defined
> > > >>>>>> in pagelist.c.  For the pgio APIs, there is no way for the cal=
ler
> > > >>>>>> to
> > > >>>>>> know how many RPCs will be sent and how the pages will be brok=
en
> > > >>>>>> up
> > > >>>>>> into underlying RPCs, each of which will have their own
> > > >>>>>> completion
> > > >>>>>> and
> > > >>>>>> return code.  In contrast, netfs is subrequest based, a single
> > > >>>>>> subrequest may contain multiple pages, and a single subrequest=
 is
> > > >>>>>> initiated with issue_read() and terminated with
> > > >>>>>> netfs_subreq_terminated().
> > > >>>>>> Thus, to utilze the netfs APIs, NFS needs some way to accommod=
ate
> > > >>>>>> the netfs API requirement on the single response to the whole
> > > >>>>>> subrequest, while also minimizing disruptive changes to the NF=
S
> > > >>>>>> pgio layer.
> > > >>>>>>
> > > >>>>>> The approach taken with this patch is to allocate a small
> > > >>>>>> structure
> > > >>>>>> for each nfs_netfs_issue_read() call, store the final error an=
d
> > > >>>>>> number
> > > >>>>>> of bytes successfully transferred in the structure, and update
> > > >>>>>> these
> > > >>>>>> values
> > > >>>>>> as each RPC completes.  The refcount on the structure is used =
as
> > > >>>>>> a
> > > >>>>>> marker
> > > >>>>>> for the last RPC completion, is incremented in
> > > >>>>>> nfs_netfs_read_initiate(),
> > > >>>>>> and decremented inside nfs_netfs_read_completion(), when a
> > > >>>>>> nfs_pgio_header
> > > >>>>>> contains a valid pointer to the data.  On the final put (which
> > > >>>>>> signals
> > > >>>>>> the final outstanding RPC is complete) in
> > > >>>>>> nfs_netfs_read_completion(),
> > > >>>>>> call netfs_subreq_terminated() with either the final error val=
ue
> > > >>>>>> (if
> > > >>>>>> one or more READs complete with an error) or the number of byt=
es
> > > >>>>>> successfully transferred (if all RPCs complete successfully).
> > > >>>>>> Note
> > > >>>>>> that when all RPCs complete successfully, the number of bytes
> > > >>>>>> transferred
> > > >>>>>> is capped to the length of the subrequest.  Capping the
> > > >>>>>> transferred
> > > >>>>>> length
> > > >>>>>> to the subrequest length prevents "Subreq overread" warnings f=
rom
> > > >>>>>> netfs.
> > > >>>>>> This is due to the "aligned_len" in nfs_pageio_add_page(), and
> > > >>>>>> the
> > > >>>>>> corner case where NFS requests a full page at the end of the
> > > >>>>>> file,
> > > >>>>>> even when i_size reflects only a partial page (NFS overread).
> > > >>>>>>
> > > >>>>>> Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
> > > >>>>>> Reviewed-by: Jeff Layton <jlayton@kernel.org>
> > > >>>>>
> > > >>>>>
> > > >>>>> This is not doing what I asked for, which was to separate out t=
he
> > > >>>>> fscache functionality, so that we can call that if and when it =
is
> > > >>>>> available.
> > > >>>>>
> > > >>>> I must have misunderstood then.
> > > >>>>
> > > >>>> The last feedback I have from you was that you wanted it to be
> > > >>>> an opt-in feature, and it was a comment on a previous patch
> > > >>>> to Kconfig.  I was proceeding the best I knew how, but
> > > >>>> let me try to get back on track.
> > > >>>>
> > > >>>>> Instead, it is just wrapping the NFS requests inside netfs
> > > >>>>> requests. As
> > > >>>>> it stands, that means it is just duplicating information, and
> > > >>>>> adding
> > > >>>>> unnecessary overhead to the standard I/O path (extra allocation=
s,
> > > >>>>> extra
> > > >>>>> indirect calls, and extra bloat to the inode).
> > > >>>>>
> > > >>>> I think I understand what you're saying but I'm not sure.  Let m=
e
> > > >>>> ask some clarifying questions.
> > > >>>>
> > > >>>> Are you objecting to the code when CONFIG_NFS_FSCACHE is
> > > >>>> configured?  Or when it is not?  Or both?  I think you're object=
ing
> > > >>>> when it's configured, but not enabled (we mount without 'fsc').
> > > >>>> Am I right?
> > > >>>>
> > > >>>> Also, are you objecting to the design that to use fcache we now
> > > >>>> have to use netfs, specifically:
> > > >>>> - call into netfs via either netfs_read_folio or netfs_readahead
> > > >>>> - if fscache is enabled, then the IO can be satisfied from fscac=
he
> > > >>>> - if fscache is not enabled, or some of the IO cannot be satisfi=
ed
> > > >>>> from the cache, then NFS is called back via netfs_issue_read
> > > >>>> and we use the normal NFS read pageio interface.  This requires
> > > >>>> we call netfs_subreq_terminated() when all the RPCs complete,
> > > >>>> which is the reason for the small changes to pagelist.c
> > > >>>
> > > >>> I'm objecting to any middle layer "solution" that adds overhead t=
o the
> > > >>> NFS I/O paths.
> > > >>>
> > > >> Got it.
> > > >>
> > > >>> I'm willing to consider solutions that are specific only to the f=
scache
> > > >>> use case (i.e. when the 'fsc' mount option is specified). However=
 when
> > > >>> I perform a normal NFS mount, and do I/O, then I don't want to se=
e
> > > >>> extra memory allocations, extra indirect calls and larger inode
> > > >>> footprints.
> > > >>>
> > > >>> IOW: I want the code to optimise for the case of standard NFS, no=
t for
> > > >>> the case of 'NFS with cachefs additions'.
> > > >>>
> > > >> I agree completely.  Are you seeing extra memory allocations
> > > >> happen on mounts without 'fsc' or is it more a concern or how
> > > >> some of the patches look?  We should not be calling any netfs or
> > > >> fscache code if 'fsc' is not on the mount and I don't see any in m=
y
> > > >> testing. So either there's a misunderstanding here, or there's a
> > > >> bug I'm missing.
> > > >>
> > > >> If fscache is not configured, then nfs_netfs_read_folio() and
> > > >> nfs_netfs_readahead() is a wrapper that returns -ENOBUFS.
> > > >> If it's configured but not enabled, then the checks for
> > > >> netfs_inode(inode)->cache should skip over any netfs code.
> > > >> But maybe there's a non-obvious bug you're seeing and
> > > >> somehow netfs is still getting called?  Because I cannot
> > > >> see netfs getting called if 'fsc' is not on the mount in my
> > > >> tests.
> > > >>
> > > >> int nfs_netfs_read_folio(struct file *file, struct folio *folio)
> > > >> {
> > > >>       if (!netfs_inode(folio_inode(folio))->cache)
> > > >>               return -ENOBUFS;
> > > >>
> > > >>       return netfs_read_folio(file, folio);
> > > >> }
> > > >>
> > > >> int nfs_netfs_readahead(struct readahead_control *ractl)
> > > >> {
> > > >>       struct inode *inode =3D ractl->mapping->host;
> > > >>
> > > >>       if (!netfs_inode(inode)->cache)
> > > >>               return -ENOBUFS;
> > > >>
> > > >>       netfs_readahead(ractl);
> > > >>       return 0;
> > > >> }
> > > >>
> > > >>
> > > >>>>
> > > >>>> Can you be more specific as to the portions of the patch you don=
't
> > > >>>> like
> > > >>>> so I can move it in the right direction?
> > > >>>>
> > > >>>> This is from patch #2 which you didn't comment on.  I'm not sure
> > > >>>> you're
> > > >>>> ok with it though, since you mention "extra bloat to the inode".
> > > >>>> Do you object to this even though it's wrapped in an
> > > >>>> #ifdef CONFIG_NFS_FSCACHE?  If so, do you require no
> > > >>>> extra size be added to nfs_inode?
> > > >>>>
> > > >>>> @@ -204,9 +208,11 @@ struct nfs_inode {
> > > >>>>       __u64 write_io;
> > > >>>>       __u64 read_io;
> > > >>>> #ifdef CONFIG_NFS_FSCACHE
> > > >>>> -       struct fscache_cookie   *fscache;
> > > >>>> -#endif
> > > >>>> +       struct netfs_inode      netfs; /* netfs context and VFS =
inode
> > > >>>> */
> > > >>>> +#else
> > > >>>>       struct inode            vfs_inode;
> > > >>>> +#endif
> > > >>>> +
> > > >>>
> > > >>> Ideally, I'd prefer no extra size. I can live with it up to a cer=
tain
> > > >>> point, however for now NFS is not unconditionally opting into the=
 netfs
> > > >>> project. If we're to ever do that, then I want to see streamlined=
 code
> > > >>> for the standard I/O case.
> > > >>>
> > > >> Ok and understood about standard I/O case.
> > > >>
> > > >> I was thinking how we might not increase the size, but I don't thi=
nk
> > > >> I can make it work.
> > > >>
> > > >> I thought we could change to something like the below, without an
> > > >> embedded struct inode:
> > > >>
> > > >> @@ -204,9 +208,11 @@ struct nfs_inode {
> > > >>       __u64 write_io;
> > > >>       __u64 read_io;
> > > >> #ifdef CONFIG_NFS_FSCACHE
> > > >> -       struct fscache_cookie   *fscache;
> > > >> -#endif
> > > >> +       struct netfs_inode      *netfs; /* netfs context and VFS i=
node */
> > > >> +#else
> > > >>       struct inode            vfs_inode;
> > > >> +#endif
> > > >> +
> > > >>
> > > >> Then I would need to alloc/free a netfs_inode at the time of
> > > >> nfs_inode initiation.  Unfortunately this has the issue that the N=
FS_I()
> > > >> macro cannot work, because it requires an embedded "struct inode"
> > > >> due to "container_of" use:
> > > >>
> > > >> +#ifdef CONFIG_NFS_FSCACHE
> > > >> +static inline struct inode *VFS_I(struct nfs_inode *nfsi)
> > > >> +{
> > > >> +       return &nfsi->netfs.inode;
> > > >> +}
> > > >> +static inline struct nfs_inode *NFS_I(const struct inode *inode)
> > > >> +{
> > > >> +       return container_of(inode, struct nfs_inode, netfs.inode);
> > > >> +}
> > > >> +#else
> > > >> +static inline struct inode *VFS_I(struct nfs_inode *nfsi)
> > > >> +{
> > > >> +       return &nfsi->vfs_inode;
> > > >> +}
> > > >> static inline struct nfs_inode *NFS_I(const struct inode *inode)
> > > >> {
> > > >>       return container_of(inode, struct nfs_inode, vfs_inode);
> > > >> }
> > > >> +#endif
> > > >>
> > > >>
> > > >
> > > > Actually Trond maybe we can achieve a "0 length increase" of
> > > > nfs_inode if dhowells would take a patch to modify the definition
> > > > of struct netfs_inode and netfs_inode_init(), something like the WI=
P
> > > > patch below.  What do you think?
> > >
> > > That works for me.
> > >
> > > >
> > > > I think maybe this could be a follow-on patch and if you/dhowells
> > > > think it's an ok idea I can try to work out what is needed across
> > > > the tree.  I thought about it more and I kinda agree that in the
> > > > case for NFS where fscache is "configured but not enabled",
> > > > then even though we're only adding 24 bytes to the nfs_inode
> > > > each time, it will add up so it is worth at least a discussion.
> > > >
> > > > diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> > > > index f2402ddeafbf..195714f1c355 100644
> > > > --- a/include/linux/netfs.h
> > > > +++ b/include/linux/netfs.h
> > > > @@ -118,11 +118,7 @@ enum netfs_io_source {
> > > > typedef void (*netfs_io_terminated_t)(void *priv, ssize_t transferr=
ed_or_error,
> > > >                                     bool was_async);
> > > >
> > > > -/*
> > > > - * Per-inode context.  This wraps the VFS inode.
> > > > - */
> > > > -struct netfs_inode {
> > > > -       struct inode            inode;          /* The VFS inode */
> > > > +struct netfs_info {
> > > >       const struct netfs_request_ops *ops;
> > > > #if IS_ENABLED(CONFIG_FSCACHE)
> > > >       struct fscache_cookie   *cache;
> > > > @@ -130,6 +126,14 @@ struct netfs_inode {
> > > >       loff_t                  remote_i_size;  /* Size of the remote=
 file */
> > > > };
> > > >
> > > > +/*
> > > > + * Per-inode context.  This wraps the VFS inode.
> > > > + */
> > > > +struct netfs_inode {
> > > > +       struct inode            inode;          /* The VFS inode */
> > > > +       struct netfs_info       *netfs;         /* Rest of netfs da=
ta */
> > > > +};
> > > > +
> > > > /*
> > > > * Resources required to do operations on a cache.
> > > > */
> > > > @@ -312,10 +316,12 @@ static inline struct netfs_inode
> > > > *netfs_inode(struct inode *inode)
> > > > static inline void netfs_inode_init(struct netfs_inode *ctx,
> > > >                                   const struct netfs_request_ops *o=
ps)
> > > > {
> > > > -       ctx->ops =3D ops;
> > > > -       ctx->remote_i_size =3D i_size_read(&ctx->inode);
> > > > +       ctx->netfs =3D kzalloc(sizeof(struct netfs_info)), GFP_KERN=
EL);
> > > > +       /* FIXME: Check for NULL */
> > > > +       ctx->netfs->ops =3D ops;
> > > > +       ctx->netfs->remote_i_size =3D i_size_read(&ctx->inode);
> > > > #if IS_ENABLED(CONFIG_FSCACHE)
> > > > -       ctx->cache =3D NULL;
> > > > +       ctx->netfs->cache =3D NULL;
> > > > #endif
> > > > }
> > > >
> > > >
> > > >
> > > >>
> > > >>>>
> > > >>>>
> > > >>>> Are you ok with the stub functions which are placed in fscache.h=
, and
> > > >>>> when CONFIG_NFS_FSCACHE is not set, become either a no-op
> > > >>>> or a 1-liner (nfs_netfs_readpage_release)?
> > > >>>>
> > > >>>> #else /* CONFIG_NFS_FSCACHE */
> > > >>>> +static inline void nfs_netfs_inode_init(struct nfs_inode *nfsi)=
 {}
> > > >>>> +static inline void nfs_netfs_initiate_read(struct nfs_pgio_head=
er
> > > >>>> *hdr) {}
> > > >>>> +static inline void nfs_netfs_read_completion(struct nfs_pgio_he=
ader
> > > >>>> *hdr) {}
> > > >>>> +static inline void nfs_netfs_readpage_release(struct nfs_page *=
req)
> > > >>>> +{
> > > >>>> +       unlock_page(req->wb_page);
> > > >>>> +}
> > > >>>> static inline void nfs_fscache_release_super_cookie(struct
> > > >>>> super_block *sb) {}
> > > >>>> static inline void nfs_fscache_init_inode(struct inode *inode) {=
}
> > > >>>>
> > > >>>>
> > > >>>> Do you object to the below?  If so, then do you want
> > > >>>> #ifdef CONFIG_NFS_FSCACHE here?
> > > >>>>
> > > >>>> -- a/fs/nfs/inode.c
> > > >>>> +++ b/fs/nfs/inode.c
> > > >>>> @@ -2249,6 +2249,8 @@ struct inode *nfs_alloc_inode(struct
> > > >>>> super_block *sb)
> > > >>>> #ifdef CONFIG_NFS_V4_2
> > > >>>>       nfsi->xattr_cache =3D NULL;
> > > >>>> #endif
> > > >>>> +       nfs_netfs_inode_init(nfsi);
> > > >>>> +
> > > >>>>       return VFS_I(nfsi);
> > > >>>> }
> > > >>>> EXPORT_SYMBOL_GPL(nfs_alloc_i
> > > >>>> node);
> > > >>>>
> > > >>>>
> > > >>>> Do you object to the changes in fs/nfs/read.c?  Specifically,
> > > >>>> how about the below calls to netfs from nfs_read_folio and
> > > >>>> nfs_readahead into equivalent netfs calls?  So when
> > > >>>> NFS_CONFIG_FSCACHE is set, but fscache is not enabled
> > > >>>> ('fsc' not on mount), these netfs functions do immediately call
> > > >>>> netfs_alloc_request().  But I wonder if we could simply add a
> > > >>>> check to see if fscache is enabled on the mount, and skip
> > > >>>> over to satisfy what you want.  Am I understanding what you
> > > >>>> want?
> > > >>>
> > > >>> Quite frankly, I'd prefer that we just split out the functionalit=
y that
> > > >>> is needed from the netfs code so that it can be optimised. Howeve=
r I'm
> > > >>> not interested enough in the cachefs functionality to work on tha=
t
> > > >>> myself. ...and as I indicated above, I might be OK with opting in=
to the
> > > >>> netfs project, once the overhead can be made to disappear.
> > > >>>
> > > >> Understood.
> > > >>
> > > >> If you think it makes more sense, I can move some of the nfs_netfs=
_*
> > > >> functions into a netfs.c file as a starting point.  Or that can ma=
ybe
> > > >> be done in a future patchset?
> > > >>
> > > >> For now I was equating netfs and fscache together so we can
> > > >> move on from the much older and single-page limiting fscache
> > > >> interface that is likely to go away soon.
> > > >>
> > > >>>>
> > > >>>> @@ -355,6 +343,10 @@ int nfs_read_folio(struct file *file, struc=
t
> > > >>>> folio *folio)
> > > >>>>       if (NFS_STALE(inode))
> > > >>>>               goto out_unlock;
> > > >>>>
> > > >>>> +       ret =3D nfs_netfs_read_folio(file, folio);
> > > >>>> +       if (!ret)
> > > >>>> +               goto out;
> > > >>>> +
> > > >>>>
> > > >>>> @@ -405,6 +399,10 @@ void nfs_readahead(struct readahead_control
> > > >>>> *ractl)
> > > >>>>       if (NFS_STALE(inode))
> > > >>>>               goto out;
> > > >>>>
> > > >>>> +       ret =3D nfs_netfs_readahead(ractl);
> > > >>>> +       if (!ret)
> > > >>>> +               goto out;
> > > >>>> +
> > > >>>>
> > > >> The above wrappers should prevent any additional overhead when fsc=
ache
> > > >> is not enabled.  As far as I know these work to avoid calling netf=
s
> > > >> when 'fsc' is not on the mount.
> > > >>
> > > >>>>
> > > >>>> And how about these calls from different points in the read
> > > >>>> path to the earlier mentioned stub functions?
> > > >>>>
> > > >>>> @@ -110,20 +110,13 @@ EXPORT_SYMBOL_GPL(nfs_pageio_reset_read_md=
s);
> > > >>>>
> > > >>>> static void nfs_readpage_release(struct nfs_page *req, int error=
)
> > > >>>> {
> > > >>>> -       struct inode *inode =3D d_inode(nfs_req_openctx(req)->de=
ntry);
> > > >>>>       struct page *page =3D req->wb_page;
> > > >>>>
> > > >>>> -       dprintk("NFS: read done (%s/%llu %d@%lld)\n", inode->i_s=
b-
> > > >>>>> s_id,
> > > >>>> -               (unsigned long long)NFS_FILEID(inode), req->wb_b=
ytes,
> > > >>>> -               (long long)req_offset(req));
> > > >>>> -
> > > >>>>       if (nfs_error_is_fatal_on_server(error) && error !=3D -
> > > >>>> ETIMEDOUT)
> > > >>>>               SetPageError(page);
> > > >>>> -       if (nfs_page_group_sync_on_bit(req, PG_UNLOCKPAGE)) {
> > > >>>> -               if (PageUptodate(page))
> > > >>>> -                       nfs_fscache_write_page(inode, page);
> > > >>>> -               unlock_page(page);
> > > >>>> -       }
> > > >>>> +       if (nfs_page_group_sync_on_bit(req, PG_UNLOCKPAGE))
> > > >>>> +               nfs_netfs_readpage_release(req);
> > > >>>> +
> > > >>>
> > > >>> I'm not seeing the value of wrapping unlock_page(), no... That co=
de is
> > > >>> going to need to change when we move it to use folios natively an=
yway.
> > > >>>
> > > >> Ok, how about I make it conditional on whether fscache is configur=
ed
> > > >> and enabled then, similar to the nfs_netfs_read_folio() and
> > > >> nfs_netfs_readahead()?  Below is what that would look like.
> > > >> I could inline the code in nfs_netfs_readpage_release() if you
> > > >> think it would be clearer.
> > > >>
> > > >> static void nfs_readpage_release(struct nfs_page *req, int error)
> > > >> {
> > > >>       struct page *page =3D req->wb_page;
> > > >>
> > > >>       if (nfs_error_is_fatal_on_server(error) && error !=3D -ETIME=
DOUT)
> > > >>               SetPageError(page);
> > > >>       if (nfs_page_group_sync_on_bit(req, PG_UNLOCKPAGE))
> > > >> #ifndef CONFIG_NFS_FSCACHE
> > > >>               unlock_page(req->wb_page);
> > > >> #else
> > > >>               nfs_netfs_readpage_release(req);
> > > >> #endif
> > > >>       nfs_release_request(req);
> > > >> }
> > > >>
> > > >>
> > > >> void nfs_netfs_readpage_release(struct nfs_page *req)
> > > >> {
> > > >>   struct inode *inode =3D d_inode(nfs_req_openctx(req)->dentry);
> > > >>
> > > >>   /*
> > > >>    * If fscache is enabled, netfs will unlock pages.
> > > >>    */
> > > >>   if (netfs_inode(inode)->cache)
> > > >>       return;
> > > >>
> > > >>   unlock_page(req->wb_page);
> > > >> }
> > > >>
> > > >>
> > > >>>>       nfs_release_request(req);
> > > >>>> }
> > > >>>>
> > > >>>> @@ -177,6 +170,8 @@ static void nfs_read_completion(struct
> > > >>>> nfs_pgio_header *hdr)
> > > >>>>               nfs_list_remove_request(req);
> > > >>>>               nfs_readpage_release(req, error);
> > > >>>>       }
> > > >>>> +       nfs_netfs_read_completion(hdr);
> > > >>>> +
> > > >>>> out:
> > > >>>>       hdr->release(hdr);
> > > >>>> }
> > > >>>> @@ -187,6 +182,7 @@ static void nfs_initiate_read(struct
> > > >>>> nfs_pgio_header *hdr,
> > > >>>>                             struct rpc_task_setup *task_setup_da=
ta,
> > > >>>> int how)
> > > >>>> {
> > > >>>>       rpc_ops->read_setup(hdr, msg);
> > > >>>> +       nfs_netfs_initiate_read(hdr);
> > > >>>>       trace_nfs_initiate_read(hdr);
> > > >>>> }
> > > >>>>
> > > >>>>
> > > >>>> Are you ok with these additions?  Something like this would
> > > >>>> be required in the case of fscache configured and enabled,
> > > >>>> because we could have some of the data in a read in
> > > >>>> fscache, and some not.  That is the reason for the netfs
> > > >>>> design, and why we need to be able to call the normal
> > > >>>> NFS read IO path (netfs calls into issue_read, and we call
> > > >>>> back via netfs_subreq_terminated)?
> > > >>>>
> > > >>>> @@ -101,6 +101,9 @@ struct nfs_pageio_descriptor {
> > > >>>>       struct pnfs_layout_segment *pg_lseg;
> > > >>>>       struct nfs_io_completion *pg_io_completion;
> > > >>>>       struct nfs_direct_req   *pg_dreq;
> > > >>>> +#ifdef CONFIG_NFS_FSCACHE
> > > >>>> +       void                    *pg_netfs;
> > > >>>> +#endif
> > > >>>>
> > > >>>> @@ -1619,6 +1619,9 @@ struct nfs_pgio_header {
> > > >>>>       const struct nfs_rw_ops *rw_ops;
> > > >>>>       struct nfs_io_completion *io_completion;
> > > >>>>       struct nfs_direct_req   *dreq;
> > > >>>> +#ifdef CONFIG_NFS_FSCACHE
> > > >>>> +       void                    *netfs;
> > > >>>> +#endif
> > > >>>>
> > > >>>>
> > > >>>> And these additions to pagelist.c?
> > > >>>>
> > > >>>> @@ -68,6 +69,10 @@ void nfs_pgheader_init(struct
> > > >>>> nfs_pageio_descriptor *desc,
> > > >>>>       hdr->good_bytes =3D mirror->pg_count;
> > > >>>>       hdr->io_completion =3D desc->pg_io_completion;
> > > >>>>       hdr->dreq =3D desc->pg_dreq;
> > > >>>> +#ifdef CONFIG_NFS_FSCACHE
> > > >>>> +       if (desc->pg_netfs)
> > > >>>> +               hdr->netfs =3D desc->pg_netfs;
> > > >>>> +#endif
> > > >>>
> > > >>> Why the conditional?
> > > >>>
> > > >> Not really needed and I was thinking of removing it, so I'll do th=
at.
> > > >>
> > > >>>>
> > > >>>>
> > > >>>> @@ -846,6 +851,9 @@ void nfs_pageio_init(struct nfs_pageio_descr=
iptor
> > > >>>> *desc,
> > > >>>>       desc->pg_lseg =3D NULL;
> > > >>>>       desc->pg_io_completion =3D NULL;
> > > >>>>       desc->pg_dreq =3D NULL;
> > > >>>> +#ifdef CONFIG_NFS_FSCACHE
> > > >>>> +       desc->pg_netfs =3D NULL;
> > > >>>> +#endif
> > > >>>>
> > > >>>>
> > > >>>> @@ -1360,6 +1369,9 @@ int nfs_pageio_resend(struct
> > > >>>> nfs_pageio_descriptor *desc,
> > > >>>>
> > > >>>>       desc->pg_io_completion =3D hdr->io_completion;
> > > >>>>       desc->pg_dreq =3D hdr->dreq;
> > > >>>> +#ifdef CONFIG_NFS_FSCACHE
> > > >>>> +       desc->pg_netfs =3D hdr->netfs;
> > > >>>> +#endif
> > > >>>
> > > >>> Those all need wrapper functions instead of embedding #ifdefs.
> > > >>>
> > > >> Ok.
> > > >>
> > > >>
> > > >>
> > > >>>>
> > > >>>>
> > > >>>>> My expectation is that the standard I/O path should have minima=
l
> > > >>>>> overhead, and should certainly not increase the overhead that w=
e
> > > >>>>> already have. Will this be addressed in future iterations of th=
ese
> > > >>>>> patches?
> > > >>>>>
> > > >>>>
> > > >>>> I will do what I can to satisfy what you want, either by fixing =
up
> > > >>>> this patch or follow-on patches.  Hopefully the above questions
> > > >>>> will clarify the next steps.
> > > >>>>
> > > >>>
> > > >>> --
> > > >>> Trond Myklebust
> > > >>> Linux NFS client maintainer, Hammerspace
> > > >>> trond.myklebust@hammerspace.com
> > >
> > >
> > >
> > > Trond Myklebust
> > > CTO, Hammerspace Inc
> > > 1900 S Norfolk St, Suite 350 - #45
> > > San Mateo, CA 94403
> > >
> > > www.hammer.space
> > >
> > >
> >
>