On Thu, Apr 6, 2023 at 7:24 PM Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> wrote: > > >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). > > I think T. J. also agree with me on what I shared. > if (len / PAGE_SIZE > totalram_pages()) return ERR_PTR(-ENOMEM); > #define LOW_ORDER_GFP (GFP_HIGHUSER | __GFP_ZERO | __GFP_COMP | __GFP_RETRY_MAYFAIL) > Oh yeah, sorry if that wasn't clear. I was referring to your updated check for just totalram_pages() above, not totalram_pages() / 2. > Regarding the dma-buf behavior, I also would like to say that the dma-buf > system heap seems to be designed to allocate that large memory. In mobile > devices, we need that large memory for camera buffers or grahpics > rendendering buffers. So that large memory should be allowed but the invalid > huge size over ram should be avoided. > > I agree on that mm should reclaim even for the large size. But that reclaim > process may affect system performance or user convenience. In that perspective > I thought ram / 2 was reasonable, but yes not a golden value. I hope to use > just ram size as sanity check. > > Additionally if all agree, we may be able to apply __GFP_RETRY_MAYFAIL too. > > BR
