Date:   Fri, 31 Mar 2023 09:51:40 +0900
From:   Jaewon Kim <jaewon31.kim@...sung.com>
To:     "T.J. Mercier" <tjmercier@...gle.com>
CC:     "jstultz@...gle.com" <jstultz@...gle.com>,
        "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jaewon31.kim@...il.com" <jaewon31.kim@...il.com>
Subject: RE: [PATCH] dma-buf/heaps: c9e8440eca61 staging: ion: Fix overflow
 and list bugs in system heap:

>On Wed, Mar 29, 2023 at 5:41?PM Jaewon Kim <jaewon31.kim@...sung.com> wrote:
>>
>> >On Tue, Mar 28, 2023 at 8:13?PM Jaewon Kim <jaewon31.kim@...sung.com> wrote:
>> >>
>> >> >On Tue, Mar 28, 2023 at 5:58?AM Jaewon Kim <jaewon31.kim@...sung.com> wrote:
>> >> >>
>> >> >> Normal free:212600kB min:7664kB low:57100kB high:106536kB
>> >> >>   reserved_highatomic:4096KB active_anon:276kB inactive_anon:180kB
>> >> >>   active_file:1200kB inactive_file:0kB unevictable:2932kB
>> >> >>   writepending:0kB present:4109312kB managed:3689488kB mlocked:2932kB
>> >> >>   pagetables:13600kB bounce:0kB free_pcp:0kB local_pcp:0kB
>> >> >>   free_cma:200844kB
>> >> >> Out of memory and no killable processes...
>> >> >> Kernel panic - not syncing: System is deadlocked on memory
>> >> >>
>> >> >> An OoM panic was reported, there were only native processes which are
>> >> >> non-killable as OOM_SCORE_ADJ_MIN.
>> >> >>
>> >> >> After looking into the dump, I've found the dma-buf system heap was
>> >> >> trying to allocate a huge size. It seems to be a signed negative value.
>> >> >>
>> >> >> dma_heap_ioctl_allocate(inline)
>> >> >>     |  heap_allocation = 0xFFFFFFC02247BD38 -> (
>> >> >>     |    len = 0xFFFFFFFFE7225100,
>> >> >>
>> >> >> Actually the old ion system heap had policy which does not allow that
>> >> >> huge size with commit c9e8440eca61 ("staging: ion: Fix overflow and list
>> >> >> bugs in system heap"). We need this change again. Single allocation
>> >> >> should not be bigger than half of all memory.
>> >> >>
>> >> >> Signed-off-by: Jaewon Kim <jaewon31.kim@...sung.com>
>> >> >> ---
>> >> >>  drivers/dma-buf/heaps/system_heap.c | 3 +++
>> >> >>  1 file changed, 3 insertions(+)
>> >> >>
>> >> >> diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
>> >> >> index e8bd10e60998..4c1ef2ecfb0f 100644
>> >> >> --- a/drivers/dma-buf/heaps/system_heap.c
>> >> >> +++ b/drivers/dma-buf/heaps/system_heap.c
>> >> >> @@ -351,6 +351,9 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap,
>> >> >>         struct page *page, *tmp_page;
>> >> >>         int i, ret = -ENOMEM;
>> >> >>
>> >> >> +       if (len / PAGE_SIZE > totalram_pages() / 2)
>> >> >> +               return ERR_PTR(-ENOMEM);
>> >> >> +
>> >> >
>> >> >Instead of policy like that, would __GFP_RETRY_MAYFAIL on the system
>> >> >heap's LOW_ORDER_GFP flags also avoid the panic, and eventually fail
>> >> >the allocation request?
>> >>
>> >> Hello T.J.
>> >>
>> >> Thank you for your opinion.
>> >> The __GFP_RETRY_MAYFAIL on LOW_ORDER_GFP seems to work.
>> >>
>> >> page allocation failure: order:0, mode:0x144dc2(GFP_HIGHUSER|__GFP_RETRY_MAYFAIL|__GFP_COMP|__GFP_ZERO)
>> >> Node 0 active_anon:120kB inactive_anon:43012kB active_file:36kB inactive_file:788kB
>> >>
>> >> I tried to test it, and the allocation stopped at very low file cache situation without OoM panic
>> >> as we expected. The phone device was freezing for few seconds though.
>> >>
>> >> We can avoid OoM panic through either totalram_pages() / 2 check or __GFP_RETRY_MAYFAIL.
>> >>
>> >> But I think we still need the totalram_pages() / 2 check so that we don't have to suffer
>> >> the freezing in UX perspective. We may kill some critical processes or users' recent apps.
>> >>
>> >> Regarding __GFP_RETRY_MAYFAIL, I think it will help us avoid OoM panic. But I'm worried
>> >> about low memory devices which still need OoM kill to get memory like in camera scenarios.
>> >>
>> >> So what do you think?
>> >>
>> >Hey Jaewon, thanks for checking! The totalram_pages() / 2 just feels
>> >somewhat arbitrary. On the lowest memory devices I'm aware of that use
>> >the system heap it would take a single buffer on the order of several
>> >hundred megabytes to exceed that, so I guess the simple check is fine
>> >here until someone says they just can't live without a buffer that
>> >big!
>> >
>> >Reviewed-by: T.J. Mercier <tjmercier@...gle.com>
>>
>> Hello T.J.
>>
>> Thank you for your Reviewed-by.
>>
>> I also think the totalram_pages() / 2 doesn't look perfect, but I think
>> we need it.
>>
>> By the way I'm a little confused on a single buffer. Please help me to be clear.
>> Do you mean we may need to reconsider the totalram_pages() / 2 some day,
>> if camera may request a huge memory for a single camera buffer? Then I hope
>> the device has also huge total memory to support that high quality camera.
>>
>Right, it's only a problem if a very low memory device wants a very
>large buffer. IDK why anyone would want to do that.
>
>> And if possible, could you give your idea about __GFP_RETRY_MAYFAIL regarding
>> what I said? I think OoM kill doesn't seem to occur that often thanks to LMKD kill.
>> And I also want to avoid OoM panic, so I'd like to apply it.
>
>Yeah even with the totalram_pages() / 2 check, a process could trigger
>the panic by consuming all available memory by allocating multiple
>buffers. (As long as that allocating process doesn't get oom killed
>first, and it allocates faster than LMKD can kill it.) So to prevent
>users of the system heap from crashing the system, I think it's still
>worth adding __GFP_RETRY_MAYFAIL.
>

Correct, I exactly think like you. I'd love to apply __GFP_RETRY_MAYFAIL
to this system heap in memory management perspective. But I think we are
not ready to apply because of that oom-kill-needed situations. Let me just
apply totalram_pages() / 2 this time. And I hope we have chance to discuss
this __GFP_RETRY_MAYFAIL some day later. I'm going to keep thinking about
it with monitoring this situations.

>> But what if there is a situation which still need OoM kill to get memory. I just
>> thought policy of __GFP_RETRY_MAYFAIL could be changed to allow OoM kill but return
>> NULL when there was a victim process.
>
>I'm not sure exactly what you mean here, but it might be nice to have
>a way to allow oom kills but not panics if a victim can't be found
>(and then fail the allocation request). Looks like that'd be possible
>by changing alloc_pages to conditionally set oom_control->order = -1
>for some new GFP flag, but not sure if that's worth it. As you
>mentioned, that'd be a super slow allocation. So I don't think that's
>a state we'd really want to be operating in.
>

Oh sorry I missed 'not', let me correct like 'when there was NOT a victim
process.' Anyway you already got what I meant. And I found the code
is_sysrq_oom regarding oom_control->order = -1. Yes it could be possible,
but as you said, I think we don't really want that so far. We need to think
more.

It was great discussion, I've apprecited it.

>> Thank you
>> Jaewon Kim

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