Re: [PATCH] iomap: Do not use GFP_NORETRY to allocate BIOs

On Mon, Mar 23, 2020 at 08:10:54AM -0700, Matthew Wilcox wrote:

quoted

That looks silly to me.  This just means we'll keep iterating over
small bios for readahead..  Either we just ignore the different gfp
mask, or we need to go all the way and handle errors, although that
doesn't really look nice.

I'm not sure it's silly,

Oh well, I'm not going to be in the way of fixing a bug I added.  So
feel free to go ahead with this and mention it matches mpage_readpages.

although I'd love to see bio_alloc() support
nr_iovecs == 0 meaning "allocate me any size biovec and tell me what
size I got in ->bi_max_vecs".  By allocating a small biovec this time,
we do one allocation rather than two, and maybe by the time we come to
allocate the next readahead bio, kswapd will have succeeded in freeing
up more memory for us.

Sounds easy enough - especially as callers don't need to look at
bi_max_vecs anyway, that is the job of bio_add_page and friends.  That
being said an upper bound still sounds useful - no need to allocate
a a gigantic bio if we know we only need a few pages.