Re: [RFC]: userspace memory reaping

[minchan]

On Wed, Oct 14, 2020 at 09:57:20AM -0700, Suren Baghdasaryan wrote:
> On Wed, Oct 14, 2020 at 5:09 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > [Sorry for a late reply]
> >
> > On Mon 14-09-20 17:45:44, Suren Baghdasaryan wrote:
> > > + linux-kernel@xxxxxxxxxxxxxxx
> > >
> > > On Mon, Sep 14, 2020 at 5:43 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > > >
> > > > Last year I sent an RFC about using oom-reaper while killing a
> > > > process: https://patchwork.kernel.org/cover/10894999. During LSFMM2019
> > > > discussion https://lwn.net/Articles/787217 a couple of alternative
> > > > options were discussed with the most promising one (outlined in the
> > > > last paragraph of https://lwn.net/Articles/787217) suggesting to use a
> > > > remote version of madvise(MADV_DONTNEED) operation to force memory
> > > > reclaim of a killed process. With process_madvise() making its way
> > > > through reviews (https://patchwork.kernel.org/patch/11747133/), I
> > > > would like to revive this discussion and get feedback on several
> > > > possible options, their pros and cons.
> >
> > Thanks for reviving this!
> 
> Thanks for your feedback!
> 
> >
> > > > The need is similar to why oom-reaper was introduced - when a process
> > > > is being killed to free memory we want to make sure memory is freed
> > > > even if the victim is in uninterruptible sleep or is busy and reaction
> > > > to SIGKILL is delayed by an unpredictable amount of time. I
> > > > experimented with enabling process_madvise(MADV_DONTNEED) operation
> > > > and using it to force memory reclaim of the target process after
> > > > sending SIGKILL. Unfortunately this approach requires the caller to
> > > > read proc/pid/maps to extract the list of VMAs to pass as an input to
> > > > process_madvise().
> >
> > Well I would argue that this is not really necessary. You can simply
> > call process_madvise with the full address range and let the kernel
> > operated only on ranges which are safe to tear down asynchronously.
> > Sure that would require some changes to the existing code to not fail
> > on those ranges if they contain incompatible vmas but that should be
> > possible. If we are worried about backward compatibility then a
> > dedicated flag could override.
> >
> 
> IIUC this is very similar to the last option I proposed. I think this
> is doable if we treat it as a special case. process_madvise() return
> value not being able to handle a large range would still be a problem.
> Maybe we can return MAX_INT in those cases?

Or, maybe we could just return 0 if the operation succeeds without any
error.

> 
> > [...]
> >
> > > > While the objective is to guarantee forward progress even when the
> > > > victim cannot terminate, we still want this mechanism to be efficient
> > > > because we perform these operations to relieve memory pressure before
> > > > it affects user experience.
> > > >
> > > > Alternative options I would like your feedback are:
> > > > 1. Introduce a dedicated process_madvise(MADV_DONTNEED_MM)
> > > > specifically for this case to indicate that the whole mm can be freed.
> >
> > This shouldn't be any different from madvise on the full address range,
> > right?
> >
> 
> Yep, just a matter of choosing the most appropriate API.


I agree full range or just NULL passing to indicate entire address
space would be better than introducing a new advise in that we could
avoid MADV_PAGEOUT_MM, MADV_COLD_MM.

> 
> > > > 2. A new syscall to efficiently obtain a vector of VMAs (start,
> > > > length, flags) of the process instead of reading /proc/pid/maps. The
> > > > size of the vector is still limited by UIO_MAXIOV (1024), so several
> > > > calls might be needed to query larger number of VMAs, however it will
> > > > still be an order of magnitude more efficient than reading
> > > > /proc/pid/maps file in 4K or smaller chunks.
> >
> > While this might be interesting for other usecases - userspace memory
> > management in general - I do not think it is directly related to this
> > particular feature.
> >
> 
> True but such a syscall would be useful for other use cases, like
> MADV_COLD/MADV_PAGEOUT that Minchan was working on. Maybe we can kill
> more than one bird here? Minchan, any thought?

Generally, it could be helpful but I don't see it as desperate at this
moment.