Re: [PATCH V2] block: make segment size limit workable for > 4K PAGE_SIZE

[Ming Lei <ming.lei@redhat.com>]

On Thu, Feb 13, 2025 at 10:23:02AM +0000, John Garry wrote:
> On 13/02/2025 09:58, Ming Lei wrote:
> > On Thu, Feb 13, 2025 at 08:45:18AM +0000, John Garry wrote:
> > > On 10/02/2025 09:03, Ming Lei wrote:
> > > > PAGE_SIZE is applied in some block device queue limits, this way is
> > > > very fragile and is wrong:
> > > > 
> > > > - queue limits are read from hardware, which is often one readonly
> > > > hardware property
> > > > 
> > > > - PAGE_SIZE is one config option which can be changed during build time.
> > > > 
> > > > In RH lab, it has been found that max segment size of some mmc card is
> > > > less than 64K, then this kind of card can't work in case of 64K PAGE_SIZE.
> > > > 
> > > > Fix this issue by using BLK_MIN_SEGMENT_SIZE in related code for dealing
> > > > with queue limits and checking if bio needn't split. Define BLK_MIN_SEGMENT_SIZE
> > > > as 4K(minimized PAGE_SIZE).
> > > Please note that blk_queue_max_quaranteed_bio() for atomic writes assumes
> > > that we can fit a PAGE_SIZE in a segment. I suppose that if the
> > > max_segment_size < PAGE_SIZE is supported, then the calculation there needs
> > > to change.
> > It isn't related with blk_queue_max_guaranteed_bio() which calculates the max
> > allowed ubuf bytes which can fit in a bio, so PAGE_SIZE has to be used here.
> > 
> > BLK_MIN_SEGMENT_SIZE is just one hint which can be used to check if one
> > bvec can fit in single segment quickly, otherwise the normal split code
> > path is run into.
> 
> So consider we have PAGE_SIZE > 4k and max_segment_size=4k, if an iovec has
> PAGE_SIZE then a bvec can also have PAGE_SIZE but then we need to split into
> multiple segments, right?

Yes, hardware limit needs to be respected.

Looks one write atomic application trouble in case of 64K page size,
and it can't work w/wo this patchset.


Thanks, 
Ming