linux-kernel - Re: [PATCH 1/2] perf/core: Bail out early if the request AUX area is out of bound

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Message-ID: <20230612102537.GF217089@leoy-huanghe.lan>
Date:   Mon, 12 Jun 2023 18:25:37 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Shuai Xue <xueshuai@...ux.alibaba.com>
Cc:     alexander.shishkin@...ux.intel.com, peterz@...radead.org,
        mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
        jolsa@...nel.org, namhyung@...nel.org, irogers@...gle.com,
        adrian.hunter@...el.com, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        Baolin Wang <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH 1/2] perf/core: Bail out early if the request AUX area is
 out of bound

On Mon, Jun 12, 2023 at 06:05:02PM +0800, Leo Yan wrote:
> On Mon, Jun 12, 2023 at 04:35:07PM +0800, Shuai Xue wrote:
> 
> [...]
> 
> > > Furthermore, I believe the AUX trace pages are only mapped for VMA
> > > (continuous virtual address), the kernel will defer to map to physical
> > > pages (which means it's not necessarily continuous physical pages)
> > > when handling data abort caused by accessing the pages.
> > 
> > I don't know why the rb->aux_pages is limit to allocated with continuous physical pages.
> > so I just add a check to avoid oops and report a proper error code -EINVAL to
> > user.
> > 
> > I would like to use vmalloc() family to replace kmalloc() so that we could support
> > allocate a more large AUX area if it is not necessarily continuous physical pages.
> > Should we remove the restriction?
> 
> As you said, we are now able to support a maximum AUX trace buffer
> size of up to 2GiB, and AUX trace buffer is per CPU wise.

Ouch, I reviewed my notes and correct myself:

For per thread mode, perf tool only allocates one generic ring buffer
and one AUX ring buffer for the whole session; for the system wide mode,
perf allocates the generic ring buffer and the AUX ring buffer per CPU
wise.

> Seems to me, 2GiB AUX buffer per CPU is big enough for most tracing
> scenarios, right?  Except you can provide profiling scenario which
> must use bigger buffer size.

But I think this question is still valid.

> Another factor is the allocation of buffers from kmalloc() offers better
> performance compared to allocation from vmalloc(), this is also
> important for perf core layer.
> 
> Thanks,
> Leo