Re: [PATCH] target: pscsi: avoid OOM in pscsi_map_sg()

[Martin Wilck <mwilck@suse.com>]

On Tue, 2021-03-23 at 16:29 +0000, Christoph Hellwig wrote:
> On Tue, Mar 23, 2021 at 05:22:03PM +0100, mwilck@suse.com wrote:
> > Avoid this by simply not resetting nr_pages to 0 after allocating the
> > bio. This way, the client receives an IO error when it tries to send
> > requests exceeding the devices max_sectors_kb, and eventually gets
> > it right. The client must still limit max_sectors_kb e.g. by an udev
> > rule if (like in my case) the driver doesn't report valid block
> > limits, otherwise it encounters I/O errors.
> 
> FYI, I think the what you did here is correct, but not enough.
> When pscsi_get_bio (that is bio_kmalloc) fails, this function needs
> to unwind and return an error insted of blindly retrying the
> allocation,
> else we can't recover from a memory shortage.

I can try to do that, but in the tests I ran, I never observed
bio_kmalloc() failing. I saw it eat all memory, and various processes
being killed by the OOM killer, before the system eventually panicked
with OOM. It takes only fractions of a second until this happens:

[   57.356238] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/,task=lightdm-gtk-gre,pid=4586,uid=481
[   57.369635] Out of memory: Killed process 4586 (lightdm-gtk-gre) total-vm:1035752kB, anon-rss:0kB, file-rss:2936kB, shmem-rss:0kB
...
[   57.698656] Node 0 Normal free:54828kB min:55008kB low:68760kB high:82512kB active_anon:4kB inactive_anon:0kB active_file:936kB inactive_file:1164kB unevictable:58036kB writepending:720kB present:13631488kB managed:13310252kB mlocked:58036kB kernel_stack:5808kB pagetables:8972kB bounce:0kB free_pcp:564kB local_pcp:0kB free_cma:0kB
...
[   57.818978] Unreclaimable slab info:
[   58.254160] kmalloc-192         15683546KB   15683546KB

(system has 16GiB memory).

Martin