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Re: [Virtio-fs] [PATCH 07/24] DAX: virtio-fs: Add vhost-user slave comma
From: |
Chirantan Ekbote |
Subject: |
Re: [Virtio-fs] [PATCH 07/24] DAX: virtio-fs: Add vhost-user slave commands for mapping |
Date: |
Mon, 15 Feb 2021 19:35:53 +0900 |
On Wed, Feb 10, 2021 at 4:04 AM Dr. David Alan Gilbert (git)
<dgilbert@redhat.com> wrote:
>
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> +
> +typedef struct {
> + /* Offsets within the file being mapped */
> + uint64_t fd_offset[VHOST_USER_FS_SLAVE_ENTRIES];
> + /* Offsets within the cache */
> + uint64_t c_offset[VHOST_USER_FS_SLAVE_ENTRIES];
> + /* Lengths of sections */
> + uint64_t len[VHOST_USER_FS_SLAVE_ENTRIES];
> + /* Flags, from VHOST_USER_FS_FLAG_* */
> + uint64_t flags[VHOST_USER_FS_SLAVE_ENTRIES];
> +} VhostUserFSSlaveMsg;
> +
Is it too late to change this? This struct allocates space for up to
8 entries but most of the time the server will only try to set up one
mapping at a time so only 32 out of the 256 bytes in the message are
actually being used. We're just wasting time memcpy'ing bytes that
will never be used. Is there some reason this can't be dynamically
sized? Something like:
typedef struct {
/* Number of mapping requests */
uint16_t num_requests;
/* `num_requests` mapping requests */
MappingRequest requests[];
} VhostUserFSSlaveMsg;
typedef struct {
/* Offset within the file being mapped */
uint64_t fd_offset;
/* Offset within the cache */
uint64_t c_offset;
/* Length of section */
uint64_t len;
/* Flags, from VHOST_USER_FS_FLAG_* */
uint64_t flags;
} MappingRequest;
The current pre-allocated structure both wastes space when there are
fewer than 8 requests and requires extra messages to be sent when
there are more than 8 requests. I realize that in the grand scheme of
things copying 224 extra bytes is basically not noticeable but it just
irks me that we could fix this really easily before it gets propagated
to too many other places.
Chirantan
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> 2.29.2
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- [PATCH 03/24] DAX: libvhost-user: Allow popping a queue element with bad pointers, (continued)
- [PATCH 03/24] DAX: libvhost-user: Allow popping a queue element with bad pointers, Dr. David Alan Gilbert (git), 2021/02/09
- [PATCH 04/24] DAX subprojects/libvhost-user: Add virtio-fs slave types, Dr. David Alan Gilbert (git), 2021/02/09
- [PATCH 05/24] DAX: virtio: Add shared memory capability, Dr. David Alan Gilbert (git), 2021/02/09
- [PATCH 06/24] DAX: virtio-fs: Add cache BAR, Dr. David Alan Gilbert (git), 2021/02/09
- [PATCH 07/24] DAX: virtio-fs: Add vhost-user slave commands for mapping, Dr. David Alan Gilbert (git), 2021/02/09
- [PATCH 08/24] DAX: virtio-fs: Fill in slave commands for mapping, Dr. David Alan Gilbert (git), 2021/02/09
- [PATCH 09/24] DAX: virtiofsd Add cache accessor functions, Dr. David Alan Gilbert (git), 2021/02/09
- [PATCH 10/24] DAX: virtiofsd: Add setup/remove mappings fuse commands, Dr. David Alan Gilbert (git), 2021/02/09