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
