Hello Guys,
I found there may be two deadlock risk under memory pressure wrt.
ioend/bio allocation in iomap writeback code wrt. bio_alloc_bioset():
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 old bio is submitted _after_ the new allocation
- submission on old chained bio can't make forward progress because all chained
bios can only be freed after the whole ioend is completed, see iomap_finish_ioend()
Both looks not hard to fix, just want to make sure they are real issues?
Thanks,
Ming
