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Date: Tue, 20 Feb 2024 19:27:26 +0100
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in show_mem()
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To: Suren Baghdasaryan <surenb@google.com>,
 Kent Overstreet <kent.overstreet@linux.dev>
Cc: Steven Rostedt <rostedt@goodmis.org>, Michal Hocko <mhocko@suse.com>,
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From: Vlastimil Babka <vbabka@suse.cz>
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On 2/19/24 18:17, Suren Baghdasaryan wrote:
> On Thu, Feb 15, 2024 at 3:56 PM Kent Overstreet
> <kent.overstreet@linux.dev> wrote:
>>
>> On Thu, Feb 15, 2024 at 06:27:29PM -0500, Steven Rostedt wrote:
>> > All this, and we are still worried about 4k for useful debugging :-/
> 
> I was planning to refactor this function to print one record at a time
> with a smaller buffer but after discussing with Kent, he has plans to
> reuse this function and having the report in one buffer is needed for
> that.

We are printing to console, AFAICS all the code involved uses plain printk()
I think it would be way easier to have a function using printk() for this
use case than the seq_buf which is more suitable for /proc and friends. Then
all concerns about buffers would be gone. It wouldn't be that much of a code
duplication?

>> Every additional 4k still needs justification. And whether we burn a
>> reserve on this will have no observable effect on user output in
>> remotely normal situations; if this allocation ever fails, we've already
>> been in an OOM situation for awhile and we've already printed out this
>> report many times, with less memory pressure where the allocation would
>> have succeeded.
> 
> I'm not sure this claim will always be true, specifically in the case
> of low-end devices with relatively low amounts of reserves and in the

That's right, GFP_ATOMIC failures can easily happen without prior OOMs.
Consider a system where userspace allocations fill the memory as they
usually do, up to high watermark. Then a burst of packets is received and
handled by GFP_ATOMIC allocations that deplete the reserves and can't cause
OOMs (OOM is when we fail to reclaim anything, but we are allocating from a
context that can't reclaim), so the very first report would be an GFP_ATOMIC
failure and now it can't allocate that buffer for printing.

I'm sure more such scenarios exist, Cc: Tetsuo who I recall was an expert on
this topic.

> presence of a possible quick memory usage spike. We should also
> consider a case when panic_on_oom is set. All we get is one OOM
> report, so we get only one chance to capture this report. In any case,
> I don't yet have data to prove or disprove this claim but it will be
> interesting to test it with data from the field once the feature is
> deployed.
> 
> For now I think with Vlastimil's __GFP_NOWARN suggestion the code
> becomes safe and the only risk is to lose this report. If we get cases
> with reports missing this data, we can easily change to reserved
> memory.