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From:   David Wysochanski <dwysocha@redhat.com>
Date:   Mon, 14 Nov 2022 12:34:26 -0500
Message-ID: <CALF+zOnd+7YsofF3K7r8hdki8H-B5jWYnZfFpyO=nGzfxi=B2A@mail.gmail.com>
Subject: Re: [PATCH v9 3/5] NFS: Convert buffered read paths to use netfs when
 fscache is enabled
To:     Benjamin Maynard <benmaynard@google.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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On Mon, Nov 14, 2022 at 11:04 AM Benjamin Maynard <benmaynard@google.com> w=
rote:
>
> 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?us=
p=3Dsharing.
>
> Hopefully this is clearer than my explanations below (let me know if
> you'd prefer me to share an alternative way).
>
Yes, that's very helpful.  Let me think about this one as I'm not sure.
As Jeff says we may need tracepoints to track it down if I cannot repro
it and/or nothing comes to mind.

> 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?us=
p=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.
>
I'm pretty sure the above you're hitting the drop_caches /
"fscache read optimisation" issue #1 I mentioned.

I see dhowells just posted a v2 version of his previous patch:
https://lore.kernel.org/linux-mm/166844174069.1124521.10890506360974169994.=
stgit@warthog.procyon.org.uk/

I started with 6.1-rc5, added the above dhowells latest patch for that issu=
e,
and then my 5 patches on top.  Then I added a small patch to utilize
dhowells patch to ensure the read optimisation is removed.  I ran my
unit test that has been failing all along and as expected it passes with
these patches.  I pushed the series to github:
https://github.com/DaveWysochanskiRH/kernel/commits/nfs-fscache-netfs
https://github.com/DaveWysochanskiRH/kernel/commit/42f58f3d36d83839022dc261=
7bb6c2d1b09db65f

I will also email you the series of patches on top of 6.1-rc5 so you
can just apply from your mailbox if you want.


> Hopefully something I am doing wrong on my end, but I can't figure out wh=
at.
>
> Kind Regards
> Benjamin Maynard
>
>
> On Mon, 14 Nov 2022 at 13:47, David Wysochanski <dwysocha@redhat.com> wro=
te:
> >
> > 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 o=
f
> > > 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 th=
e
> > 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 t=
o
> > > 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) s=
o
> > > 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,acre=
gmin=3D600,acregmax=3D600,acdirmin=3D600,acdirmax=3D600,hard,nocto,proto=3D=
tcp,nconnect=3D16,timeo=3D600,retrans=3D2,sec=3Dsys,mountaddr=3D10.0.0.49,m=
ountvers=3D3,mountport=3D37485,mountproto=3Dtcp,fsc,local_lock=3Dnone,addr=
=3D10.0.0.49
> > >
> > > 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,har=
d,proto=3Dtcp,timeo=3D600,retrans=3D2,sec=3Dsys,mountaddr=3D10.0.0.3,mountv=
ers=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?us=
p=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_=
check,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_=
check,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.co=
m> 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@redh=
at.com> wrote:
> > > > >>
> > > > >> On Fri, Oct 28, 2022 at 12:59 PM Trond Myklebust <trondmy@kernel=
.org> 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@kerne=
l.org>
> > > > >>>> wrote:
> > > > >>>>>
> > > > >>>>> On Mon, 2022-10-17 at 06:52 -0400, Dave Wysochanski wrote:
> > > > >>>>>> Convert the NFS buffered read code paths to corresponding ne=
tfs
> > > > >>>>>> 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 defi=
ne 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 =
call
> > > > >>>>>> to
> > > > >>>>>> netfs_subreq_terminated() which conveys either how many byte=
s
> > > > >>>>>> were
> > > > >>>>>> read
> > > > >>>>>> successfully, or an error.  Note that issue_read() is called=
 with
> > > > >>>>>> a
> > > > >>>>>> structure, netfs_io_subrequest, which defines the IO request=
ed,
> > > > >>>>>> and
> > > > >>>>>> contains a start and a length (both in bytes), and assumes t=
he
> > > > >>>>>> underlying
> > > > >>>>>> netfs will return a either an error on the whole region, or =
the
> > > > >>>>>> number
> > > > >>>>>> of bytes successfully read.
> > > > >>>>>>
> > > > >>>>>> The NFS IO path is page based and the main APIs are the pgio=
 APIs
> > > > >>>>>> defined
> > > > >>>>>> in pagelist.c.  For the pgio APIs, there is no way for the c=
aller
> > > > >>>>>> to
> > > > >>>>>> know how many RPCs will be sent and how the pages will be br=
oken
> > > > >>>>>> up
> > > > >>>>>> into underlying RPCs, each of which will have their own
> > > > >>>>>> completion
> > > > >>>>>> and
> > > > >>>>>> return code.  In contrast, netfs is subrequest based, a sing=
le
> > > > >>>>>> subrequest may contain multiple pages, and a single subreque=
st is
> > > > >>>>>> initiated with issue_read() and terminated with
> > > > >>>>>> netfs_subreq_terminated().
> > > > >>>>>> Thus, to utilze the netfs APIs, NFS needs some way to accomm=
odate
> > > > >>>>>> the netfs API requirement on the single response to the whol=
e
> > > > >>>>>> subrequest, while also minimizing disruptive changes to the =
NFS
> > > > >>>>>> 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 =
and
> > > > >>>>>> number
> > > > >>>>>> of bytes successfully transferred in the structure, and upda=
te
> > > > >>>>>> these
> > > > >>>>>> values
> > > > >>>>>> as each RPC completes.  The refcount on the structure is use=
d 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 (whi=
ch
> > > > >>>>>> signals
> > > > >>>>>> the final outstanding RPC is complete) in
> > > > >>>>>> nfs_netfs_read_completion(),
> > > > >>>>>> call netfs_subreq_terminated() with either the final error v=
alue
> > > > >>>>>> (if
> > > > >>>>>> one or more READs complete with an error) or the number of b=
ytes
> > > > >>>>>> successfully transferred (if all RPCs complete successfully)=
.
> > > > >>>>>> Note
> > > > >>>>>> that when all RPCs complete successfully, the number of byte=
s
> > > > >>>>>> transferred
> > > > >>>>>> is capped to the length of the subrequest.  Capping the
> > > > >>>>>> transferred
> > > > >>>>>> length
> > > > >>>>>> to the subrequest length prevents "Subreq overread" warnings=
 from
> > > > >>>>>> netfs.
> > > > >>>>>> This is due to the "aligned_len" in nfs_pageio_add_page(), a=
nd
> > > > >>>>>> 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=
 the
> > > > >>>>> fscache functionality, so that we can call that if and when i=
t 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 allocati=
ons,
> > > > >>>>> extra
> > > > >>>>> indirect calls, and extra bloat to the inode).
> > > > >>>>>
> > > > >>>> I think I understand what you're saying but I'm not sure.  Let=
 me
> > > > >>>> 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 obje=
cting
> > > > >>>> 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 no=
w
> > > > >>>> have to use netfs, specifically:
> > > > >>>> - call into netfs via either netfs_read_folio or netfs_readahe=
ad
> > > > >>>> - if fscache is enabled, then the IO can be satisfied from fsc=
ache
> > > > >>>> - if fscache is not enabled, or some of the IO cannot be satis=
fied
> > > > >>>> from the cache, then NFS is called back via netfs_issue_read
> > > > >>>> and we use the normal NFS read pageio interface.  This require=
s
> > > > >>>> 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=
 to the
> > > > >>> NFS I/O paths.
> > > > >>>
> > > > >> Got it.
> > > > >>
> > > > >>> I'm willing to consider solutions that are specific only to the=
 fscache
> > > > >>> use case (i.e. when the 'fsc' mount option is specified). Howev=
er when
> > > > >>> I perform a normal NFS mount, and do I/O, then I don't want to =
see
> > > > >>> extra memory allocations, extra indirect calls and larger inode
> > > > >>> footprints.
> > > > >>>
> > > > >>> IOW: I want the code to optimise for the case of standard NFS, =
not 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=
 my
> > > > >> 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 d=
on'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 su=
re
> > > > >>>> 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 VF=
S inode
> > > > >>>> */
> > > > >>>> +#else
> > > > >>>>       struct inode            vfs_inode;
> > > > >>>> +#endif
> > > > >>>> +
> > > > >>>
> > > > >>> Ideally, I'd prefer no extra size. I can live with it up to a c=
ertain
> > > > >>> point, however for now NFS is not unconditionally opting into t=
he netfs
> > > > >>> project. If we're to ever do that, then I want to see streamlin=
ed 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 t=
hink
> > > > >> I can make it work.
> > > > >>
> > > > >> I thought we could change to something like the below, without a=
n
> > > > >> 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=
 inode */
> > > > >> +#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=
 NFS_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 =
WIP
> > > > > 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 transfe=
rred_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 remo=
te 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 =
data */
> > > > > +};
> > > > > +
> > > > > /*
> > > > > * 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 =
*ops)
> > > > > {
> > > > > -       ctx->ops =3D ops;
> > > > > -       ctx->remote_i_size =3D i_size_read(&ctx->inode);
> > > > > +       ctx->netfs =3D kzalloc(sizeof(struct netfs_info)), GFP_KE=
RNEL);
> > > > > +       /* 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 *nfs=
i) {}
> > > > >>>> +static inline void nfs_netfs_initiate_read(struct nfs_pgio_he=
ader
> > > > >>>> *hdr) {}
> > > > >>>> +static inline void nfs_netfs_read_completion(struct nfs_pgio_=
header
> > > > >>>> *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 cal=
l
> > > > >>>> 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 functional=
ity that
> > > > >>> is needed from the netfs code so that it can be optimised. Howe=
ver I'm
> > > > >>> not interested enough in the cachefs functionality to work on t=
hat
> > > > >>> myself. ...and as I indicated above, I might be OK with opting =
into 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_net=
fs_*
> > > > >> functions into a netfs.c file as a starting point.  Or that can =
maybe
> > > > >> 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, str=
uct
> > > > >>>> 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_contr=
ol
> > > > >>>> *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 f=
scache
> > > > >> is not enabled.  As far as I know these work to avoid calling ne=
tfs
> > > > >> 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_=
mds);
> > > > >>>>
> > > > >>>> static void nfs_readpage_release(struct nfs_page *req, int err=
or)
> > > > >>>> {
> > > > >>>> -       struct inode *inode =3D d_inode(nfs_req_openctx(req)->=
dentry);
> > > > >>>>       struct page *page =3D req->wb_page;
> > > > >>>>
> > > > >>>> -       dprintk("NFS: read done (%s/%llu %d@%lld)\n", inode->i=
_sb-
> > > > >>>>> s_id,
> > > > >>>> -               (unsigned long long)NFS_FILEID(inode), req->wb=
_bytes,
> > > > >>>> -               (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 =
code is
> > > > >>> going to need to change when we move it to use folios natively =
anyway.
> > > > >>>
> > > > >> Ok, how about I make it conditional on whether fscache is config=
ured
> > > > >> 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 -ETI=
MEDOUT)
> > > > >>               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_=
data,
> > > > >>>> 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 =
that.
> > > > >>
> > > > >>>>
> > > > >>>>
> > > > >>>> @@ -846,6 +851,9 @@ void nfs_pageio_init(struct nfs_pageio_des=
criptor
> > > > >>>> *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 mini=
mal
> > > > >>>>> overhead, and should certainly not increase the overhead that=
 we
> > > > >>>>> already have. Will this be addressed in future iterations of =
these
> > > > >>>>> patches?
> > > > >>>>>
> > > > >>>>
> > > > >>>> I will do what I can to satisfy what you want, either by fixin=
g up
> > > > >>>> this patch or follow-on patches.  Hopefully the above question=
s
> > > > >>>> 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
> > > >
> > > >
> > >
> >
>