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

[Vlastimil Babka <vbabka@suse.cz>]

On 8/22/24 11:34, Linus Torvalds wrote:
> On Thu, 22 Aug 2024 at 17:27, David Hildenbrand <david@redhat.com> wrote:
>>
>> To me, that implies that if you pass in MAX_ORDER+1 the VM will "retry
>> infinitely". if that implies just OOPSing or actually be in a busy loop,
>> I don't care. It could effectively happen with MAX_ORDER as well, as
>> stated. But certainly not BUG_ON.
> 
> No BUG_ON(), but also no endless loop.
> 
> Just return NULL for bogus users. Really. Give a WARN_ON_ONCE() to
> make it easy to find offenders, and then let them deal with it.

Right now we give the WARN_ON_ONCE() (for !can_direct_reclaim) only when
we're about to actually return NULL, so the memory has to be depleted
already. To make it easier to find the offenders much more reliably, we
should consider doing it sooner, but also not add unnecessary overhead to
allocator fastpaths just because of the potentially buggy users. So either
always in __alloc_pages_slowpath(), which should be often enough (unless the
system never needs to wake up kswapd to reclaim) but with negligible enough
overhead, or on every allocation but only with e.g. CONFIG_DEBUG_VM?

> Don't take it upon yourself to say "we have to deal with any amount of
> stupidity".
> 
> The MM layer is not some slave to users. The MM layer is one of the
> most core pieces of code in the kernel, and as such the MM layer is
> damn well in charge.
> 
> Nobody has the right to say "I will not deal with allocation
> failures". The MM should not bend over backwards over something like
> that.
> 
> Seriously. Get a spine already, people. Tell random drivers that claim
> that they cannot deal with errors to just f-ck off.
> 
> And you don't do it by looping forever, and you don't do it by killing
> the kernel. You do it by ignoring their bullying tactics.
> 
> Then you document the *LIMITED* cases where you actually will try forever.
> 
> This discussion has gone on for too damn long.
> 
>               Linus