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Message-ID: <CAJnrk1ZRSoMN+jan5D9d3UYWnTVxc_5KVaBtP7JV2b+0skrBfg@mail.gmail.com>
Date: Thu, 1 May 2025 22:48:15 -0700
From: Joanne Koong <joannelkoong@...il.com>
To: John Groves <John@...ves.net>
Cc: Dan Williams <dan.j.williams@...el.com>, Miklos Szeredi <miklos@...redb.hu>, 
	Bernd Schubert <bschubert@....com>, John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>, 
	Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>, 
	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	"Darrick J . Wong" <djwong@...nel.org>, Luis Henriques <luis@...lia.com>, 
	Randy Dunlap <rdunlap@...radead.org>, Jeff Layton <jlayton@...nel.org>, 
	Kent Overstreet <kent.overstreet@...ux.dev>, Petr Vorel <pvorel@...e.cz>, 
	Brian Foster <bfoster@...hat.com>, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev, 
	linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	Amir Goldstein <amir73il@...il.com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
	Stefan Hajnoczi <shajnocz@...hat.com>, Josef Bacik <josef@...icpanda.com>, 
	Aravind Ramesh <arramesh@...ron.com>, Ajay Joshi <ajayjoshi@...ron.com>
Subject: Re: [RFC PATCH 12/19] famfs_fuse: Plumb the GET_FMAP message/response

On Sun, Apr 20, 2025 at 6:34 PM John Groves <John@...ves.net> wrote:
>
> Upon completion of a LOOKUP, if we're in famfs-mode we do a GET_FMAP to
> retrieve and cache up the file-to-dax map in the kernel. If this
> succeeds, read/write/mmap are resolved direct-to-dax with no upcalls.
>
> Signed-off-by: John Groves <john@...ves.net>
> ---
>  fs/fuse/dir.c             | 69 +++++++++++++++++++++++++++++++++++++++
>  fs/fuse/fuse_i.h          | 36 +++++++++++++++++++-
>  fs/fuse/inode.c           | 15 +++++++++
>  include/uapi/linux/fuse.h |  4 +++
>  4 files changed, 123 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index bc29db0117f4..ae135c55b9f6 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -359,6 +359,56 @@ bool fuse_invalid_attr(struct fuse_attr *attr)
>         return !fuse_valid_type(attr->mode) || !fuse_valid_size(attr->size);
>  }
>
> +#define FMAP_BUFSIZE 4096
> +
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +static void
> +fuse_get_fmap_init(
> +       struct fuse_conn *fc,
> +       struct fuse_args *args,
> +       u64 nodeid,
> +       void *outbuf,
> +       size_t outbuf_size)
> +{
> +       memset(outbuf, 0, outbuf_size);

I think we can skip the memset here since kcalloc will zero out the
memory automatically when the fmap_buf gets allocated

> +       args->opcode = FUSE_GET_FMAP;
> +       args->nodeid = nodeid;
> +
> +       args->in_numargs = 0;
> +
> +       args->out_numargs = 1;
> +       args->out_args[0].size = FMAP_BUFSIZE;
> +       args->out_args[0].value = outbuf;
> +}
> +
> +static int
> +fuse_get_fmap(struct fuse_mount *fm, struct inode *inode, u64 nodeid)
> +{
> +       size_t fmap_size;
> +       void *fmap_buf;
> +       int err;
> +
> +       pr_notice("%s: nodeid=%lld, inode=%llx\n", __func__,
> +                 nodeid, (u64)inode);
> +       fmap_buf = kcalloc(1, FMAP_BUFSIZE, GFP_KERNEL);
> +       FUSE_ARGS(args);
> +       fuse_get_fmap_init(fm->fc, &args, nodeid, fmap_buf, FMAP_BUFSIZE);
> +
> +       /* Send GET_FMAP command */
> +       err = fuse_simple_request(fm, &args);

I'm assuming the fmap_buf gets freed in a later patch, but for this
one we'll probably need a kfree(fmap_buf) here in the meantime?

> +       if (err) {
> +               pr_err("%s: err=%d from fuse_simple_request()\n",
> +                      __func__, err);
> +               return err;
> +       }
> +
> +       fmap_size = args.out_args[0].size;
> +       pr_notice("%s: nodei=%lld fmap_size=%ld\n", __func__, nodeid, fmap_size);
> +
> +       return 0;
> +}
> +#endif
> +
>  int fuse_lookup_name(struct super_block *sb, u64 nodeid, const struct qstr *name,
>                      struct fuse_entry_out *outarg, struct inode **inode)
>  {
> @@ -404,6 +454,25 @@ int fuse_lookup_name(struct super_block *sb, u64 nodeid, const struct qstr *name
>                 fuse_queue_forget(fm->fc, forget, outarg->nodeid, 1);
>                 goto out;
>         }
> +
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +       if (fm->fc->famfs_iomap) {
> +               if (S_ISREG((*inode)->i_mode)) {
> +                       /* Note Lookup returns the looked-up inode in the attr
> +                        * struct, but not in outarg->nodeid !
> +                        */
> +                       pr_notice("%s: outarg: size=%d nodeid=%lld attr.ino=%lld\n",
> +                                __func__, args.out_args[0].size, outarg->nodeid,
> +                                outarg->attr.ino);
> +                       /* Get the famfs fmap */
> +                       fuse_get_fmap(fm, *inode, outarg->attr.ino);

I agree with Darrick's comment about fetching the mappings only if the
file gets opened. I wonder though if we could bundle the open with the
get_fmap so that we don't have to do an additional request / incur 2
extra context switches. This seems feasible to me. When we send the
open request, we could check if fc->famfs_iomap is set and if so, set
inarg.open_flags to include FUSE_OPEN_GET_FMAP and set outarg.value to
an allocated buffer that holds both struct fuse_open_out and the
fmap_buf and adjust outarg.size accordingly. Then the server could
send both the open and corresponding fmap data in the reply.

> +               } else
> +                       pr_notice("%s: no get_fmap for non-regular file\n",
> +                                __func__);
> +       } else
> +               pr_notice("%s: fc->dax_iomap is not set\n", __func__);
> +#endif
> +
>         err = 0;
>
>   out_put_forget:
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 931613102d32..437177c2f092 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -193,6 +193,10 @@ struct fuse_inode {
>         /** Reference to backing file in passthrough mode */
>         struct fuse_backing *fb;
>  #endif
> +
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +       void *famfs_meta;
> +#endif
>  };
>
>  /** FUSE inode state bits */
> @@ -942,6 +946,8 @@ struct fuse_conn {
>  #endif
>
>  #if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +       struct rw_semaphore famfs_devlist_sem;
> +       struct famfs_dax_devlist *dax_devlist;
>         char *shadow;
>  #endif
>  };
> @@ -1432,11 +1438,14 @@ void fuse_free_conn(struct fuse_conn *fc);
>
>  /* dax.c */
>
> +static inline int fuse_file_famfs(struct fuse_inode *fi); /* forward */
> +
>  /* This macro is used by virtio_fs, but now it also needs to filter for
>   * "not famfs"
>   */
>  #define FUSE_IS_VIRTIO_DAX(fuse_inode) (IS_ENABLED(CONFIG_FUSE_DAX)    \
> -                                       && IS_DAX(&fuse_inode->inode))
> +                                       && IS_DAX(&fuse_inode->inode)   \
> +                                       && !fuse_file_famfs(fuse_inode))
>
>  ssize_t fuse_dax_read_iter(struct kiocb *iocb, struct iov_iter *to);
>  ssize_t fuse_dax_write_iter(struct kiocb *iocb, struct iov_iter *from);
> @@ -1547,4 +1556,29 @@ extern void fuse_sysctl_unregister(void);
>  #define fuse_sysctl_unregister()       do { } while (0)
>  #endif /* CONFIG_SYSCTL */
>
> +/* famfs.c */
> +static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
> +                                                      void *meta)
> +{
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +       return xchg(&fi->famfs_meta, meta);
> +#else
> +       return NULL;
> +#endif
> +}
> +
> +static inline void famfs_meta_free(struct fuse_inode *fi)
> +{
> +       /* Stub wil be connected in a subsequent commit */
> +}
> +
> +static inline int fuse_file_famfs(struct fuse_inode *fi)
> +{
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +       return (fi->famfs_meta != NULL);

Does this need to be "return READ_ONCE(fi->famfs_meta) != NULL"?

> +#else
> +       return 0;
> +#endif
> +}
> +
>  #endif /* _FS_FUSE_I_H */
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 7f4b73e739cb..848c8818e6f7 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -117,6 +117,9 @@ static struct inode *fuse_alloc_inode(struct super_block *sb)
>         if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
>                 fuse_inode_backing_set(fi, NULL);
>
> +       if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
> +               famfs_meta_set(fi, NULL);

"fi->famfs_meta = NULL;" looks simpler here

> +
>         return &fi->inode;
>
>  out_free_forget:
> @@ -138,6 +141,13 @@ static void fuse_free_inode(struct inode *inode)
>         if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
>                 fuse_backing_put(fuse_inode_backing(fi));
>
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +       if (S_ISREG(inode->i_mode) && fi->famfs_meta) {
> +               famfs_meta_free(fi);
> +               famfs_meta_set(fi, NULL);
> +       }
> +#endif
> +
>         kmem_cache_free(fuse_inode_cachep, fi);
>  }
>
> @@ -1002,6 +1012,11 @@ void fuse_conn_init(struct fuse_conn *fc, struct fuse_mount *fm,
>         if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
>                 fuse_backing_files_init(fc);
>
> +       if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)) {
> +               pr_notice("%s: Kernel is FUSE_FAMFS_DAX capable\n", __func__);
> +               init_rwsem(&fc->famfs_devlist_sem);
> +       }

Should we only init this if the server chooses to opt into famfs (eg
if their init reply sets the FUSE_DAX_FMAP flag)? This imo seems to
belong more in process_init_reply().


Thanks,
Joanne
> +
>         INIT_LIST_HEAD(&fc->mounts);
>         list_add(&fm->fc_entry, &fc->mounts);
>         fm->fc = fc;
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index f9e14180367a..d85fb692cf3b 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -652,6 +652,10 @@ enum fuse_opcode {
>         FUSE_TMPFILE            = 51,
>         FUSE_STATX              = 52,
>
> +       /* Famfs / devdax opcodes */
> +       FUSE_GET_FMAP           = 53,
> +       FUSE_GET_DAXDEV         = 54,
> +
>         /* CUSE specific operations */
>         CUSE_INIT               = 4096,
>
> --
> 2.49.0
>

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