linux-kernel - Re: [PATCH v3 31/35] lib: add memory allocations report in show

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Message-ID: <CAJuCfpEARb8t8pc8WVZYB=yPk6G_kYGmJTMOdgiMHaYYKW3fUA@mail.gmail.com>
Date: Mon, 19 Feb 2024 09:17:36 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in show_mem()

On Thu, Feb 15, 2024 at 3:56 PM Kent Overstreet
<kent.overstreet@...ux.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.

> 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
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.