Re: [PATCH mm] vmalloc: back off when the current task is OOM-killed

[Michal Hocko]

On Mon 27-09-21 12:36:15, Vasily Averin wrote:
> On 9/24/21 10:55 AM, Michal Hocko wrote:
> > On Thu 23-09-21 09:49:57, Vasily Averin wrote:
[...]
> >> Hypothetically, cancelled vmalloc called inside some filesystem's transaction
> >> forces its rollback, that in own turn it can call own vmalloc.
> > 
> > Do you have any specific example?
> 
> No, it was pure hypothetical assumption.
> I was thinking about it over the weekend, and decided that:
> a) such kind of issue (i.e. vmalloc call in rollback after failed vmalloc)
>    is very unlikely
> b) if it still exists -- it must have own failback with kmalloc(NOFAIL) 
>    or just accept/ignore such failure and should not lead to critical failures.
>    If this still happen -- ihis is a bug, we should detect and fix it ASAP.

I would even argue that nobody should rely on vmalloc suceeding. The
purpose of the allocator is to allow larger allocations and we do not
guarantee anything even for small reqests.

> >> Should we perhaps interrupt the first vmalloc only?
> > 
> > This doesn't make much sense to me TBH. It doesn't address the very
> > problem you are describing in the changelog.
> 
> Last question:
> how do you think, should we perhaps, instead, detect such vmallocs 
> (called in rollback after failed vmalloc) and generate a warnings,
> to prevent such kind of problems in future?

We do provide an allocation failure splat unless the request is
explicitly __GFP_NOWARN IIRC.
-- 
Michal Hocko
SUSE Labs