Date:   Wed, 26 Aug 2020 18:34:08 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        virtio-fs-list <virtio-fs@...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 Wed, Aug 26, 2020 at 11:51:42AM -0400, Vivek Goyal wrote:
> On Wed, Aug 26, 2020 at 04:06:35PM +0200, Miklos Szeredi wrote:
> > 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?
> 
> My understanding is that device will specify alignment for
> the foffset/moffset fields in fuse_setupmapping_in/fuse_removemapping_one.
> And DAX mapping can be any size meeting that alignment contraint.
> 
> > 
> > In that case kernel can definitely succeed if the requested alignment
> > is smaller than the kernel provided one, no? 
> 
> Yes. So if map_alignemnt is 64K and DAX mapping size is 2MB, that's just
> fine because it meets 4K alignment contraint. Just that we can't use
> 4K size DAX mapping in that case.
> 
> > 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.
> 
> We could make it two way negotiation if it helps. So if we support
> multiple mapping sizes in future, say 4K, 64K, 2MB, 1GB. So idea is
> to send alignment of largest mapping size to device/user_space (1GB)
> in this case? And that will allow device to choose an alignment
> which best fits its needs?
> 
> But problem here is that sending (log2(1GB)) does not mean we support
> all the alignments in that range. For example, if device selects say
> 256MB as minimum alignment, kernel might not support it.
> 
> So there seem to be two ways to handle this.
> 
> A.Let device be conservative and always specify the minimum aligment
>   it can work with and let guest kernel automatically choose a mapping
>   size which meets that min_alignment contraint.
> 
> B.Send all the mapping sizes supported by kernel to device and then
>   device chooses an alignment as it sees fit. We could probably send
>   a 64bit field and set a bit for every size we support as dax mapping.
>   If we were to go down this path, I think in that case client should
>   respond back with exact mapping size we should use (and not with
>   minimum alignment).
> 
> I thought intent behind this patch was to implement A.
> 
> Stefan/David, this patch came from you folks. What do you think?

Yes, I agree with Vivek.

The FUSE server is telling the client the minimum alignment for
foffset/moffset. The client can map any size it likes as long as
foffset/moffset meet the alignment constraint. I can't think of a reason
to do two-way negotiation.

Stefan

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