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Message-ID: <CAJfpegsgHE0MkZLFgE4yrZXO5ThDxCj85-PjizrXPRC2CceT1g@mail.gmail.com>
Date: Wed, 26 Aug 2020 16:06:35 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-nvdimm <linux-nvdimm@...ts.01.org>,
virtio-fs-list <virtio-fs@...hat.com>,
Stefan Hajnoczi <stefanha@...hat.com>,
"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v3 11/18] fuse: implement FUSE_INIT map_alignment field
On Thu, Aug 20, 2020 at 12:21 AM Vivek Goyal <vgoyal@...hat.com> wrote:
>
> The device communicates FUSE_SETUPMAPPING/FUSE_REMOVMAPPING alignment
> constraints via the FUST_INIT map_alignment field. Parse this field and
> ensure our DAX mappings meet the alignment constraints.
>
> We don't actually align anything differently since our mappings are
> already 2MB aligned. Just check the value when the connection is
> established. If it becomes necessary to honor arbitrary alignments in
> the future we'll have to adjust how mappings are sized.
>
> The upshot of this commit is that we can be confident that mappings will
> work even when emulating x86 on Power and similar combinations where the
> host page sizes are different.
>
> Signed-off-by: Stefan Hajnoczi <stefanha@...hat.com>
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> ---
> fs/fuse/fuse_i.h | 5 ++++-
> fs/fuse/inode.c | 18 ++++++++++++++++--
> include/uapi/linux/fuse.h | 4 +++-
> 3 files changed, 23 insertions(+), 4 deletions(-)
>
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 478c940b05b4..4a46e35222c7 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -47,7 +47,10 @@
> /** Number of dentries for each connection in the control filesystem */
> #define FUSE_CTL_NUM_DENTRIES 5
>
> -/* Default memory range size, 2MB */
> +/*
> + * Default memory range size. A power of 2 so it agrees with common FUSE_INIT
> + * map_alignment values 4KB and 64KB.
> + */
> #define FUSE_DAX_SZ (2*1024*1024)
> #define FUSE_DAX_SHIFT (21)
> #define FUSE_DAX_PAGES (FUSE_DAX_SZ/PAGE_SIZE)
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index b82eb61d63cc..947abdd776ca 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -980,9 +980,10 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_args *args,
> {
> struct fuse_init_args *ia = container_of(args, typeof(*ia), args);
> struct fuse_init_out *arg = &ia->out;
> + bool ok = true;
>
> if (error || arg->major != FUSE_KERNEL_VERSION)
> - fc->conn_error = 1;
> + ok = false;
> else {
> unsigned long ra_pages;
>
> @@ -1045,6 +1046,13 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_args *args,
> min_t(unsigned int, FUSE_MAX_MAX_PAGES,
> max_t(unsigned int, arg->max_pages, 1));
> }
> + if ((arg->flags & FUSE_MAP_ALIGNMENT) &&
> + (FUSE_DAX_SZ % (1ul << arg->map_alignment))) {
This just obfuscates "arg->map_alignment != FUSE_DAX_SHIFT".
So the intention was that userspace can ask the kernel for a
particular alignment, right?
In that case kernel can definitely succeed if the requested alignment
is smaller than the kernel provided one, no? It would also make
sense to make this a two way negotiation. I.e. send the largest
alignment (FUSE_DAX_SHIFT in this implementation) that the kernel can
provide in fuse_init_in. In that case the only error would be if
userspace ignored the given constraints.
Am I getting not getting something?
> + pr_err("FUSE: map_alignment %u incompatible"
> + " with dax mem range size %u\n",
> + arg->map_alignment, FUSE_DAX_SZ);
> + ok = false;
> + }
> } else {
> ra_pages = fc->max_read / PAGE_SIZE;
> fc->no_lock = 1;
> @@ -1060,6 +1068,11 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_args *args,
> }
> kfree(ia);
>
> + if (!ok) {
> + fc->conn_init = 0;
> + fc->conn_error = 1;
> + }
> +
> fuse_set_initialized(fc);
> wake_up_all(&fc->blocked_waitq);
> }
> @@ -1082,7 +1095,8 @@ void fuse_send_init(struct fuse_conn *fc)
> FUSE_WRITEBACK_CACHE | FUSE_NO_OPEN_SUPPORT |
> FUSE_PARALLEL_DIROPS | FUSE_HANDLE_KILLPRIV | FUSE_POSIX_ACL |
> FUSE_ABORT_ERROR | FUSE_MAX_PAGES | FUSE_CACHE_SYMLINKS |
> - FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA;
> + FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
> + FUSE_MAP_ALIGNMENT;
> ia->args.opcode = FUSE_INIT;
> ia->args.in_numargs = 1;
> ia->args.in_args[0].size = sizeof(ia->in);
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index 373cada89815..5b85819e045f 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -313,7 +313,9 @@ struct fuse_file_lock {
> * FUSE_CACHE_SYMLINKS: cache READLINK responses
> * FUSE_NO_OPENDIR_SUPPORT: kernel supports zero-message opendir
> * FUSE_EXPLICIT_INVAL_DATA: only invalidate cached pages on explicit request
> - * FUSE_MAP_ALIGNMENT: map_alignment field is valid
> + * FUSE_MAP_ALIGNMENT: init_out.map_alignment contains log2(byte alignment) for
> + * foffset and moffset fields in struct
> + * fuse_setupmapping_out and fuse_removemapping_one.
fuse_setupmapping_in
Thanks,
Miklos
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