Re: [PATCH 03/21] fs/bdev: Add atomic write support info to statx

[John Garry]

On 01/10/2023 14:23, Bart Van Assche wrote:
> On 9/29/23 15:49, Eric Biggers wrote:
> > On Fri, Sep 29, 2023 at 10:27:08AM +0000, John Garry wrote:
> > > diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
> > > index 7cab2c65d3d7..c99d7cac2aa6 100644
> > > --- a/include/uapi/linux/stat.h
> > > +++ b/include/uapi/linux/stat.h
> > > @@ -127,7 +127,10 @@ struct statx {
> > >       __u32    stx_dio_mem_align;    /* Memory buffer alignment for 
> > > direct I/O */
> > >       __u32    stx_dio_offset_align;    /* File offset alignment for 
> > > direct I/O */
> > >       /* 0xa0 */
> > > -    __u64    __spare3[12];    /* Spare space for future expansion */
> > > +    __u32    stx_atomic_write_unit_max;
> > > +    __u32    stx_atomic_write_unit_min;
> >
> > Maybe min first and then max?  That seems a bit more natural, and a 
> > lot of the
> > code you've written handle them in that order.

ok, I think it's fine to reorder

> > > +#define STATX_ATTR_WRITE_ATOMIC        0x00400000 /* File supports 
> > > atomic write operations */
> >
> > How would this differ from stx_atomic_write_unit_min != 0?

Yeah, I suppose that we can just not set this for the case of 
stx_atomic_write_unit_min == 0.

> Is it even possible that stx_atomic_write_unit_min == 0? My understanding
> is that all Linux filesystems rely on the assumption that writing a single
> logical block either succeeds or does not happen, even if a power failure
> occurs between writing and reading a logical block.

Maybe they do rely on this, but is it particularly interesting?

BTW, I would not like to provide assurances that every storage media 
produced writes logical blocks atomically.

Thanks,
John