On Thu, Apr 6, 2023 at 4:38 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, 6 Apr 2023 16:27:28 -0700 "T.J. Mercier" <tjmercier@xxxxxxxxxx> 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! > The issue I see is that it could delay other non-buggy callers, or cause reclaim that wouldn't have happened if we just outright rejected a known-bad allocation request from the beginning. > > 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? > There are a variety of different allocation strategies used by different exporters so I don't think there's one dma-buf thing we could fix for slow, large allocations in general. For the system_heap in this patch it's really just alloc_pages. I'm saying I don't think the kernel should ever ask alloc_pages for more memory than exists on the system, which seems like a pretty reasonable sanity check to me. Given that, I don't think we should do anything for buffers smaller than totalram_pages() (except maybe to prevent OOM panics via __GFP_RETRY_MAYFAIL when we attempt to exhaust system memory on any request - valid or otherwise).
