On Fri, May 21, 2021 at 11:27:54AM +0800, Ming Lei wrote: > if %__gfp_direct_reclaim is set then bio_alloc will always be able to > allocate a bio. this is due to the mempool guarantees. to make this work, > callers must never allocate more than 1 bio at a time from the general pool. > callers that need to allocate more than 1 bio must always submit the > previously allocated bio for io before attempting to allocate a new one. > failure to do so can cause deadlocks under memory pressure. > > 1) more than one ioends can be allocated from 'iomap_ioend_bioset' > before submitting them all, so mempoll guarantee can't be made, which can > be observed frequently in writeback over ext4 > > 2) more than one chained bio(allocated from fs_bio_set) via iomap_chain_bio, > which is easy observed when writing big files on XFS: The comment describing bio_alloc_bioset is actually wrong. Allocating more than 1 at a time is perfectly fine, it just can't be more than the pool_size argument passed to bioset_init. iomap_ioend_bioset is sized to make sure we can always complete up to 4 pages, and the list is only used inside a page, so we're fine. fs_bio_set always has two entries to allow exactly for the common chain and submit pattern.
