Re: [PATCH v3 4/4] io_uring: add support for zone-append

Kanchan Joshi
2020-07-20T16:50:31Z

On Fri, Jul 10, 2020 at 7:41 PM Kanchan Joshi <joshiiitr@gmail.com> wrote:
>
> On Fri, Jul 10, 2020 at 7:21 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Fri, Jul 10, 2020 at 02:49:32PM +0100, Christoph Hellwig wrote:
> > > On Fri, Jul 10, 2020 at 02:48:24PM +0100, Matthew Wilcox wrote:
> > > > If we're going to go the route of changing the CQE, how about:
> > > >
> > > > struct io_uring_cqe {
> > > > __u64 user_data; /* sqe->data submission passed back */
> > > > - __s32 res; /* result code for this event */
> > > > - __u32 flags;
> > > > + union {
> > > > + struct {
> > > > + __s32 res; /* result code for this event */
> > > > + __u32 flags;
> > > > + };
> > > > + __s64 res64;
> > > > + };
> > > > };
> > > >
> > > > then we don't need to change the CQE size and it just depends on the SQE
> > > > whether the CQE for it uses res+flags or res64.
> > >
> > > How do you return a status code or short write when you just have
> > > a u64 that is needed for the offset?
> >
> > it's an s64 not a u64 so you can return a negative errno. i didn't
> > think we allowed short writes for objects-which-have-a-pos.
>
> If we are doing this for zone-append (and not general cases), "__s64
> res64" should work -.
> 64 bits = 1 (sign) + 23 (bytes-copied: cqe->res) + 40
> (written-location: chunk_sector bytes limit)

And this is for the scheme when single CQE is used with bits
refactoring into "_s64 res64" instead of res/flags.

41 bits for zone-append completion = in bytes, sufficient to cover
chunk_sectors size zone
1+22 bits for zone-append bytes-copied = can cover 4MB bytes copied
(single I/O is capped at 4MB in NVMe)

+ * zone-append specific flags
+#define APPEND_OFFSET_BITS (41)
+#define APPEND_RES_BITS (23)
+
+/*
* IO completion data structure (Completion Queue Entry)
*/
struct io_uring_cqe {
- __u64 user_data; /* sqe->data submission passed back */
- __s32 res; /* result code for this event */
- __u32 flags;
+ __u64 user_data; /* sqe->data submission passed back */
+ union {
+ struct {
+ __s32 res; /* result code for
this event */
+ __u32 flags;
+ };
+ /* Alternate for zone-append */
+ struct {
+ union {
+ /*
+ * kernel uses this to store append result
+ * Most significant 23 bits to return number of
+ * bytes or error, and least significant 41 bits
+ * to return zone-relative offset in bytes
+ * */
+ __s64 res64;
+ /*for user-space ease, kernel does not use*/
+ struct {
+#if defined(__LITTLE_ENDIAN_BITFIELD)
+ __u64 append_offset :
APPEND_OFFSET_BITS;
+ __s32 append_res : APPEND_RES_BITS;
+#elif defined(__BIG_ENDIAN_BITFIELD)
+ __s32 append_res : APPEND_RES_BITS;
+ __u64 append_offset :
APPEND_OFFSET_BITS;
+#endif
+ }__attribute__ ((__packed__));
+ };
+ };
+ };
};

--
Joshi