Re: can't oom-kill zap the victim's memory?

[Linus Torvalds]

On Tue, Oct 6, 2015 at 9:49 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> The basic fact remains: kernel allocations are so important that
> rather than fail, you should kill user space. Only kernel allocations
> that *explicitly* know that they have fallback code should fail, and
> they should just do the __GFP_NORETRY.

To be clear: "big" orders (I forget if the limit is at order-3 or
order-4) do fail much more aggressively. But no, we do not limit retry
to just order-0, because even small kmalloc sizes tend to often do
order-1 or order-2 just because of memory packing issues (ie trying to
pack into a single page wastes too much memory if the allocation sizes
don't come out right).

So no, order-0 isn't special. 1/2 are rather important too.

[ Checking /proc/slabinfo: it looks like several slabs are order-3,
for things like files_cache, signal_cache and sighand_cache for me at
least. So I think it's up to order-3 that we basically need to
consider "we'll need to shrink user space aggressively unless we have
an explicit fallback for the allocation" ]

            Linus
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