On Mon, Dec 04, 2023 at 01:13:55PM +0000, John Garry wrote: > > > > > > > I added this here (as opposed to the caller), as I was not really worried > > > about speeding up the failure path. Are you saying to call even earlier in > > > submission path? > > atomic_write_unit_min is one hardware property, and it should be checked > > in blk_queue_atomic_write_unit_min_sectors() from beginning, then you > > can avoid this check every other where. > > ok, but we still need to ensure in the submission path that the block device > actually supports atomic writes - this was the initial check. Then you may add one helper bdev_support_atomic_write(). > > > > > > > > + if (pos % atomic_write_unit_min_bytes) > > > > > + return false; > > > > > + if (iov_iter_count(iter) % atomic_write_unit_min_bytes) > > > > > + return false; > > > > > + if (!is_power_of_2(iov_iter_count(iter))) > > > > > + return false; > > > > > + if (iov_iter_count(iter) > atomic_write_unit_max_bytes) > > > > > + return false; > > > > > + if (pos % iov_iter_count(iter)) > > > > > + return false; > > > > I am a bit confused about relation between atomic_write_unit_max_bytes and > > > > atomic_write_max_bytes. > > > I think that naming could be improved. Or even just drop merging (and > > > atomic_write_max_bytes concept) until we show it to improve performance. > > > > > > So generally atomic_write_unit_max_bytes will be same as > > > atomic_write_max_bytes, however it could be different if: > > > a. request queue nr hw segments or other request queue limits needs to > > > restrict atomic_write_unit_max_bytes > > > b. atomic_write_unit_max_bytes does not need to be a power-of-2 and > > > atomic_write_max_bytes does. So essentially: > > > atomic_write_unit_max_bytes = rounddown_pow_of_2(atomic_write_max_bytes) > > > > > plug merge often improves sequential IO perf, so if the hardware supports > > this way, I think 'atomic_write_max_bytes' should be supported from the > > beginning, such as: > > > > - user space submits sequential N * (4k, 8k, 16k, ...) atomic writes, all can > > be merged to single IO request, which is issued to driver. > > > > Or > > > > - user space submits sequential 4k, 4k, 8k, 16K, 32k, 64k atomic writes, all can > > be merged to single IO request, which is issued to driver. > > Right, we do expect userspace to use a fixed block size, but we give scope > in the API to use variable size. Maybe it is enough to just take atomic_write_unit_min_bytes only, and allow length to be N * atomic_write_unit_min_bytes. But it may violate atomic write boundary? > > > > > The hardware should recognize unit size by start LBA, and check if length is > > valid, so probably the interface might be relaxed to: > > > > 1) start lba is unit aligned, and this unit is in the supported unit > > range(power_2 in [unit_min, unit_max]) > > > > 2) length needs to be: > > > > - N * this_unit_size > > - <= atomic_write_max_bytes > > Please note that we also need to consider: > - any atomic write boundary (from NVMe) Can you provide actual NVMe boundary value? Firstly natural aligned write won't cross boundary, so boundary should be >= write_unit_max, see blow code from patch 10/21: +static bool bio_straddles_atomic_write_boundary(loff_t bi_sector, + unsigned int bi_size, + unsigned int boundary) +{ + loff_t start = bi_sector << SECTOR_SHIFT; + loff_t end = start + bi_size; + loff_t start_mod = start % boundary; + loff_t end_mod = end % boundary; + + if (end - start > boundary) + return true; + if ((start_mod > end_mod) && (start_mod && end_mod)) + return true; + + return false; +} + Then if the WRITE size is <= boundary, the above function should return false, right? Looks like it is power_of(2) & aligned atomic_write_max_bytes? > - virt boundary (from NVMe) virt boundary is applied on bv_offset and bv_len, and NVMe's virt bounary is (4k - 1), it shouldn't be one issue in reality. > > And, as I mentioned elsewhere, I am still not 100% comfortable that we don't > pay attention to regular max_sectors_kb... max_sectors_kb should be bigger than atomic_write_max_bytes actually, then what is your concern? Thanks, Ming