linux-kernel - Re: [PATCH V7 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE

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Date:   Thu, 17 Sep 2020 12:00:06 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     kan.liang@...ux.intel.com, peterz@...radead.org, mingo@...hat.com,
        acme@...nel.org, linux-kernel@...r.kernel.org
Cc:     mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, eranian@...gle.com, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com, mpe@...erman.id.au,
        benh@...nel.crashing.org, paulus@...ba.org
Subject: Re: [PATCH V7 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

On 9/17/20 6:52 AM, kan.liang@...ux.intel.com wrote:
> +	mm = current->mm;
> +	if (!mm) {
> +		/*
> +		 * For kernel threads and the like, use init_mm so that
> +		 * we can find kernel memory.
> +		 */
> +		mm = &init_mm;
> +	}

I think it might be better to use current->active_mm instead of
current->mm.  Kernel threads can "take over" the mm of the threads that
switched to them, so they're not actually using all of the page tables
from the init_mm all the time.

It's not _common_, thought.  The only time that I think they can diverge
is when vmalloc PGD sync'ing needs to be done, and there's even an
effort to remove that.

But, it's probably more more precise to just use ->active_mm since
that's what will actually be more consistent with the values loaded into
CR3.

I _think_ ->active_mm is always non-NULL, too.

One last concern as I look at this: I wish it was a bit more
future-proof.  There are lots of weird things folks are trying to do
with the page tables, like Address Space Isolation.  For instance, if
you get a perf NMI when running userspace, current->mm->pgd is
*different* than the PGD that was in use when userspace was running.
It's close enough today, but it might not stay that way.  But I can't
think of any great ways to future proof this code, other than spitting
out an error message if too many of the page table walks fail when they
shouldn't.