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

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Nov 13, 2020 at 6:16 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 13 Nov 2020 17:57:02 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
>
> > On Fri, Nov 13, 2020 at 5:18 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > On Fri, 13 Nov 2020 17:09:37 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > > > > > 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
> > > > > > . 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
> > > > 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.
>
> Ah, OK, return value.  Maybe process_madvise() shouldn't have done that
> and should have simply returned 0 on success, like madvise().
>
> I guess a special "nuke whole address space" command is OK.  But, again
> in the search for generality, the ability to nuke very large amounts of
> address space (but not the entire address space) would be better.
>
> The process_madvise() return value issue could be addressed by adding a
> process_madvise() mode which return 0 on success.
>
> And I guess the MAX_RW_COUNT issue is solvable by adding an
> import_iovec() arg to say "don't check that".  Along those lines.
>
> It's all sounding a bit painful (but not *too* painful).  But to
> reiterate, I do think that adding the ability for a process to shoot
> down a large amount of another process's memory is a lot more generally
> useful than tying it to SIGKILL, agree?

I see. So you are suggesting a mode where process_madvise() can
operate on large areas spanning multiple VMAs. This slightly differs
from option 4 in the previous RFC which suggested a special mode that
operates on the *entire* mm of the process. I agree, your suggestion
is more generic.

>
> > >
> > > > 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.
>
> OK.  I wouldn't expect performance to be very different (and things can
> be sped up if so), but the API usefulness might be an issue.  Using
> process_madvise() (or similar) makes it a two-step operation, whereas
> tying it to SIGKILL&&TASK_UNINTERRUPTIBLE provides a more precise tool.
> Any thoughts on this?
>