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

[Minchan Kim <minchan@kernel.org>]

On Fri, Nov 13, 2020 at 06:16:32PM -0800, Andrew Morton 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 agree the direction but I think it's the general problem for every
APIs have supported iovec and not sure process_madvise is special to
chage it.
IOW, it wouldn't be a problem to support *entire address space* special
mode but not sure to support *large amount of address space* at the cost
of breaking existing iovec scheme.