Re: [PATCH v3 0/4] mm: clarify nofail memory allocation

[Vlastimil Babka <vbabka@suse.cz>]

On 8/27/24 09:50, Barry Song wrote:
> On Tue, Aug 27, 2024 at 7:38 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>>
>> Ugh, wasn't aware, well spotted. So it means there at least shouldn't be
>> existing users of __GFP_NOFAIL with order > 1 :)
>>
>> But also the check is in the hotpath, even before trying the pcplists, so we
>> could move it to __alloc_pages_slowpath() while extending it?
> 
> Agreed. I don't think it is reasonable to check the order and flags in
> two different places especially rmqueue() has already had
> gfp_flags & __GFP_NOFAIL operation and order > 1
> overhead.
> 
> We can at least extend the current check to make some improvement
> though I still believe Michal's suggestion of implementing OOPS_ON is a
> better approach to pursue, as it doesn't crash the entire system
> while ensuring the problematic process is terminated.

Linus made clear it's not a mm concern. If e.g. hardening people want to
pursuit that instead, they can.

BTW I think BUG_ON already works like this, if possible only the calling
process is terminated. panic happens in case of being in a irq context, or
due to panic_on_oops. Which the security people are setting to 1 anyway and
OOPS_ON would have to observe it too. So AFAICS the only difference from
BUG_ON would be not panic in the irq context, if panic_on_oops isn't set.
(as for "no mm locks held" I think it's already satisfied at the points we
check for __GFP_NOFAIL).