Re: iomap: writeback ioend/bio allocation deadlock risk

[Christoph Hellwig <hch@lst.de>]

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.