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 11:12:07AM +0000, John Garry wrote:
> On 13/02/2025 10:35, Ming Lei wrote:
> > > > 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.
> 
> I think that we need to take max_segment_size into account in
> blk_queue_max_guaranteed_bio(), like:
> 
> static unsigned int blk_queue_max_guaranteed_bio(struct queue_limits *lim)
> {
> 	unsigned int max_segments = min(BIO_MAX_VECS, lim->max_segments);
> 	unsigned int length;
> 
> 	length = min(max_segments, 2) * lim->logical_block_size;
> 	if (max_segments > 2)
> 		length += (max_segments - 2) * min(PAGE_SIZE, lim->max_segment_size);
> 
> 	return length;
> }
> 
> Note that blk_queue_max_guaranteed_bio() is only really relevant to dio, so
> assumes that the iov_iter follows the bdev dio rules

It can't work because ITER_UBUF from pwritev2(iovcnt=1) is virtually-contiguous,
and the middle segment size has to be PAGE_SIZE.


Thanks,
Ming