Re: [PATCH RFC 01/16] block: Add atomic write operations to request_queue limits

[Eric Biggers]

On Fri, May 05, 2023 at 08:26:23AM +1000, Dave Chinner wrote:
> > ok, we can do that but would also then make statx field 64b. I'm fine with
> > that if it is wise to do so - I don't don't want to wastefully use up an
> > extra 2 x 32b in struct statx.
> 
> Why do we need specific varibles for DIO atomic write alignment
> limits? We already have direct IO alignment and size constraints in statx(),
> so why wouldn't we just reuse those variables when the user requests
> atomic limits for DIO?
> 
> i.e. if STATX_DIOALIGN is set, we return normal DIO alignment
> constraints. If STATX_DIOALIGN_ATOMIC is set, we return the atomic
> DIO alignment requirements in those variables.....
> 
> Yes, we probably need the dio max size to be added to statx for
> this. Historically speaking, I wanted statx to support this in the
> first place because that's what we were already giving userspace
> with XFS_IOC_DIOINFO and we already knew that atomic IO when it came
> along would require a bound maximum IO size much smaller than normal
> DIO limits.  i.e.:
> 
> struct dioattr {
>         __u32           d_mem;          /* data buffer memory alignment */
>         __u32           d_miniosz;      /* min xfer size                */
>         __u32           d_maxiosz;      /* max xfer size                */
> };
> 
> where d_miniosz defined the alignment and size constraints for DIOs.
> 
> If we simply document that STATX_DIOALIGN_ATOMIC returns minimum
> (unit) atomic IO size and alignment in statx->dio_offset_align (as
> per STATX_DIOALIGN) and the maximum atomic IO size in
> statx->dio_max_iosize, then we don't burn up anywhere near as much
> space in the statx structure....

I don't think that's how statx() is meant to work.  The request mask is a bitmask, and the user can
request an arbitrary combination of different items.  For example, the user could request both
STATX_DIOALIGN and STATX_WRITE_ATOMIC at the same time.  That doesn't work if different items share
the same fields.

- Eric