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 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 > > > Signed-off-by: Vivek Goyal > > > --- > > > 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