Re: [PATCH 1/1] RFC: add pidfd_send_signal flag to reclaim mm while killing a process

On Fri, Nov 13, 2020 at 5:18 PM Andrew Morton [off-list ref] wrote:

On Fri, 13 Nov 2020 17:09:37 -0800 Suren Baghdasaryan [off-list ref] wrote:

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Seems to me that the ability to reap another process's memory is a
generally useful one, and that it should not be tied to delivering a
signal in this fashion.

And we do have the new process_madvise(MADV_PAGEOUT).  It may need a
few changes and tweaks, but can't that be used to solve this problem?

Thank you for the feedback, Andrew. process_madvise(MADV_DONTNEED) was
one of the options recently discussed in
https://lore.kernel.org/linux-api/CAJuCfpGz1kPM3G1gZH+09Z7aoWKg05QSAMMisJ7H5MdmRrRhNQ@mail.gmail.com (local)
. The thread describes some of the issues with that approach but if we
limit it to processes with pending SIGKILL only then I think that
would be doable.

Why would it be necessary to read /proc/pid/maps?  I'd have thought
that a starting effort would be

        madvise((void *)0, (void *)-1, MADV_PAGEOUT)

(after translation into process_madvise() speak).  Which is equivalent
to the proposed process_madvise(MADV_DONTNEED_MM)?

Yep, this is very similar to option #3 in
https://lore.kernel.org/linux-api/CAJuCfpGz1kPM3G1gZH+09Z7aoWKg05QSAMMisJ7H5MdmRrRhNQ@mail.gmail.com (local)
and I actually have a tested prototype for that.

Why is the `vector=NULL' needed?  Can't `vector' point at a single iovec
which spans the whole address range?

That would be the option #4 from the same discussion and the issues
noted there are "process_madvise return value can't handle such a
large number of bytes and there is MAX_RW_COUNT limit on max number of
bytes one process_madvise call can handle". In my prototype I have a
special handling for such "bulk operation" to work around the
MAX_RW_COUNT limitation.

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If that's the
preferred method then I can post it quite quickly.

I assume you've tested that prototype.  How did its usefulness compare
with this SIGKILL-based approach?

Just to make sure I understand correctly your question, you are asking
about performance comparison of:

// approach in this RFC
pidfd_send_signal(SIGKILL, SYNC_REAP_MM)

vs

// option #4 in the previous RFC
kill(SIGKILL); process_madvise(vector=NULL, MADV_DONTNEED);

If so, I have results for the current RFC approach but the previous
approach was testing on an older device, so don't have
apples-to-apples comparison results at the moment. I can collect the
data for fair comparison if desired, however I don't expect a
noticeable performance difference since they both do pretty much the
same thing (even on different devices my results are quite close). I
think it's more a question of which API would be more appropriate.