Re: [PATCH v2] dma-buf/heaps: system_heap: Avoid DoS by limiting single allocations to half of all memory

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 6 Apr 2023 16:27:28 -0700 "T.J. Mercier" <tjmercier@google.com> wrote:

> > When you say "decide what's the largest reasonable size", I think it
> > is difficult as with the variety of RAM sizes and buffer sizes I don't
> > think there's a fixed limit. Systems with more ram will use larger
> > buffers for image/video capture buffers.  And yes, you're right that
> > ram/2-1 in a single allocation is just as broken, but I'm not sure how
> > to establish a better guard rail.
> >
> > thanks
> > -john
> 
> I like ENOMEM with the len / PAGE_SIZE > totalram_pages() check and
> WARN_ON. We know for sure that's an invalid request, and it's pretty
> cheap to check as opposed to trying a bunch of reclaim before failing.

Well, if some buggy caller has gone and requested eleventy bigabytes of
memory, doing a lot of reclaiming before failing isn't really a problem
- we don't want to optimize for this case!

> For buffers smaller than that I agree with John in that I'm not sure
> there's a definitive threshold.

Well...  why do we want to do _anything_ here?  Why cater for buggy
callers?  I think it's because "dma-buf behaves really badly with very
large allocation requests".  Again, can we fix that instead?