Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230405185650.239f9721f066aa480e83d543@linux-foundation.org>
 <20230405172524.e25b62e1c548a95564b1d324@linux-foundation.org>
 <20230406000854.25764-1-jaewon31.kim@samsung.com> <20230406014419epcms1p3f285b6e3fdbb1457db1bcbaab9e863be@epcms1p3>
 <20230406021712epcms1p216f274040d25d18380668ffbfa809c48@epcms1p2>
 <20230405200923.9b0dca2165ef3335a0f6b112@linux-foundation.org>
 <CANDhNCq+mhhXjW7huYoQwUq18sRCcn6HjbPqh7mU9KrGyKeLfA@mail.gmail.com>
 <CABdmKX0ZnQX5BPEJjqxwUsx=c8ykcX8Bjitak50DAYvfti6VFw@mail.gmail.com>
 <20230406163822.faae6a90b3aa4942df6e7442@linux-foundation.org>
 <CGME20230406000841epcas1p3630010a770682be0f1d540a448f3e00e@epcms1p4>
 <CABdmKX3NzTWHa3K_rna1iY+UERUrYK1Rq9WqUt9VQRaAKZsWwg@mail.gmail.com> <20230407022419epcms1p424f1f8a7100aeccac62651ce25cd6140@epcms1p4>
In-Reply-To: <20230407022419epcms1p424f1f8a7100aeccac62651ce25cd6140@epcms1p4>
From:   "T.J. Mercier" <tjmercier@google.com>
Date:   Thu, 6 Apr 2023 22:12:34 -0700
Message-ID: <CABdmKX3qBrOU6K=JXZhU8=oi=quxBqBnVaKiV5p=4uCTLLsRPA@mail.gmail.com>
Subject: Re: [PATCH v2] dma-buf/heaps: system_heap: Avoid DoS by limiting
 single allocations to half of all memory
To:     jaewon31.kim@samsung.com
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        John Stultz <jstultz@google.com>,
        "sumit.semwal@linaro.org" <sumit.semwal@linaro.org>,
        "daniel.vetter@ffwll.ch" <daniel.vetter@ffwll.ch>,
        "hannes@cmpxchg.org" <hannes@cmpxchg.org>,
        "mhocko@kernel.org" <mhocko@kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jaewon31.kim@gmail.com" <jaewon31.kim@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Thu, Apr 6, 2023 at 7:24=E2=80=AFPM Jaewon Kim <jaewon31.kim@samsung.com=
> wrote:
>
> >On Thu, Apr 6, 2023 at 4:38?PM Andrew Morton <akpm@linux-foundation.org>=
 wrote:
> >>
> >> 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 d=
on'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 th=
at
> >> > > 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 failin=
g.
> >>
> >> Well, if some buggy caller has gone and requested eleventy bigabytes o=
f
> >:)
> >> memory, doing a lot of reclaiming before failing isn't really a proble=
m
> >> - 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_R=
ETRY_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 inva=
lid
> huge size over ram should be avoided.
>
> I agree on that mm should reclaim even for the large size. But that recla=
im
> process may affect system performance or user convenience. In that perspe=
ctive
> I thought ram / 2 was reasonable, but yes not a golden value. I hope to u=
se
> just ram size as sanity check.
>
> Additionally if all agree, we may be able to apply __GFP_RETRY_MAYFAIL to=
o.
>
> BR