Re: iomap: writeback ioend/bio allocation deadlock risk

[Christoph Hellwig <hch@lst.de>]

On Fri, May 21, 2021 at 03:31:05PM +0800, Ming Lei wrote:
> > 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.
> 
> The number itself does not matter, because there isn't any limit on how
> many ioends can be allocated before submitting, for example, it can be
> observed that 64 ioends is allocated before submitting when writing
> 5GB file to ext4. So far the reserved pool size is 32.

How do you manage to allocate iomap ioends when writing to ext4?  ext4
doesn't use iomap for buffered I/O.

> > fs_bio_set always has two entries to allow exactly for the common
> > chain and submit pattern.
> 
> It is easy to trigger dozens of chained bios in one ioend when writing
> big file to XFS.

Yes, we can still have one chained bio per ioend, so we need a bioset
with the same size as iomap_ioend_bioset.  That still should not be
dozends for a common setup, though.