Re: [PATCH v9 3/5] NFS: Convert buffered read paths to use netfs when fscache is enabled

Benjamin Maynard
2022-10-31T17:43:20Z

Just wanted to add that I am really keen to see some form of these
patches merged.

I am using FS-Cache for the NFS re-exporting use-case.

I (and a fair few others) are impacted heavily by the IO performance
bottleneck when using FS-Cache, so much so that we are currently stuck
on v5.16 of the kernel.

--
Ben

On Sun, 30 Oct 2022 at 23:26, 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.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@kernel.org>
> > > > wrote:
> > > > >
> > > > > On Mon, 2022-10-17 at 06:52 -0400, Dave Wysochanski wrote:
> > > > > > Convert the NFS buffered read code paths to corresponding netfs
> > > > > > 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 call
> > > > > > to
> > > > > > netfs_subreq_terminated() which conveys either how many bytes
> > > > > > were
> > > > > > read
> > > > > > successfully, or an error. Note that issue_read() is called with
> > > > > > 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 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 caller
> > > > > > to
> > > > > > know how many RPCs will be sent and how the pages will be broken
> > > > > > 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 accommodate
> > > > > > the netfs API requirement on the single response to the whole
> > > > > > 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 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 value
> > > > > > (if
> > > > > > one or more READs complete with an error) or the number of bytes
> > > > > > 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 from
> > > > > > 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 the
> > > > > 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 allocations,
> > > > > 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 objecting
> > > > 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 fscache
> > > > - if fscache is not enabled, or some of the IO cannot be satisfied
> > > > 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 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). However 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 = 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 certain
> > > 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 think
> > 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 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?
>
> 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 transferred_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 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 = ops;
> - ctx->remote_i_size = i_size_read(&ctx->inode);
> + ctx->netfs = kzalloc(sizeof(struct netfs_info)), GFP_KERNEL);
> + /* FIXME: Check for NULL */
> + ctx->netfs->ops = ops;
> + ctx->netfs->remote_i_size = i_size_read(&ctx->inode);
> #if IS_ENABLED(CONFIG_FSCACHE)
> - ctx->cache = NULL;
> + ctx->netfs->cache = 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_header
> > > > *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 = 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 functionality that
> > > is needed from the netfs code so that it can be optimised. However I'm
> > > not interested enough in the cachefs functionality to work on that
> > > 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_netfs_*
> > 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, struct
> > > > folio *folio)
> > > > if (NFS_STALE(inode))
> > > > goto out_unlock;
> > > >
> > > > + ret = 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 = nfs_netfs_readahead(ractl);
> > > > + if (!ret)
> > > > + goto out;
> > > > +
> > > >
> > The above wrappers should prevent any additional overhead when fscache
> > is not enabled. As far as I know these work to avoid calling netfs
> > 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 error)
> > > > {
> > > > - struct inode *inode = d_inode(nfs_req_openctx(req)->dentry);
> > > > struct page *page = 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 != -
> > > > 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 configured
> > 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 = req->wb_page;
> >
> > if (nfs_error_is_fatal_on_server(error) && error != -ETIMEDOUT)
> > 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 = 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 = mirror->pg_count;
> > > > hdr->io_completion = desc->pg_io_completion;
> > > > hdr->dreq = desc->pg_dreq;
> > > > +#ifdef CONFIG_NFS_FSCACHE
> > > > + if (desc->pg_netfs)
> > > > + hdr->netfs = 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_descriptor
> > > > *desc,
> > > > desc->pg_lseg = NULL;
> > > > desc->pg_io_completion = NULL;
> > > > desc->pg_dreq = NULL;
> > > > +#ifdef CONFIG_NFS_FSCACHE
> > > > + desc->pg_netfs = NULL;
> > > > +#endif
> > > >
> > > >
> > > > @@ -1360,6 +1369,9 @@ int nfs_pageio_resend(struct
> > > > nfs_pageio_descriptor *desc,
> > > >
> > > > desc->pg_io_completion = hdr->io_completion;
> > > > desc->pg_dreq = hdr->dreq;
> > > > +#ifdef CONFIG_NFS_FSCACHE
> > > > + desc->pg_netfs = hdr->netfs;
> > > > +#endif
> > >
> > > Those all need wrapper functions instead of embedding #ifdefs.
> > >
> > Ok.
> >
> >
> >
> > > >
> > > >
> > > > > My expectation is that the standard I/O path should have minimal
> > > > > 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 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
> > >
> > >
>