Re: [PATCH 0/7] Free user PTE page table pages

[Muchun Song]

On Mon, Jul 19, 2021 at 8:42 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> On Mon, Jul 19, 2021 at 7:28 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > On 19.07.21 09:34, David Hildenbrand wrote:
> > > On 18.07.21 06:30, Qi Zheng wrote:
> > >> Hi,
> > >>
> > >> This patch series aims to free user PTE page table pages when all PTE entries
> > >> are empty.
> > >>
> > >> The beginning of this story is that some malloc libraries(e.g. jemalloc or
> > >> tcmalloc) usually allocate the amount of VAs by mmap() and do not unmap those VAs.
> > >> They will use madvise(MADV_DONTNEED) to free physical memory if they want.
> > >> But the page tables do not be freed by madvise(), so it can produce many
> > >> page tables when the process touches an enormous virtual address space.
> > >
> > > ... did you see that I am actually looking into this?
> > >
> > > https://lkml.kernel.org/r/bae8b967-c206-819d-774c-f57b94c4b362@xxxxxxxxxx
> > >
> > > and have already spent a significant time on it as part of my research,
> > > which is *really* unfortunate and makes me quite frustrated at the
> > > beginning of the week alreadty ...
> > >
> > > Ripping out page tables is quite difficult, as we have to stop all page
> > > table walkers from touching it, including the fast_gup, rmap and page
> > > faults. This usually involves taking the mmap lock in write. My approach
> > > does page table reclaim asynchronously from another thread and do not
> > > rely on reference counts.
> >
>
> Hi David,
>
> > FWIW, I had a quick peek and I like the simplistic approach using
> > reference counting, although it seems to come with a price. By hooking
> > using pte_alloc_get_map_lock() instead of pte_alloc_map_lock, we can
> > handle quite some cases easily.
>
> Totally agree.
>
> >
> > There are cases where we might immediately see a reuse after discarding
> > memory (especially, with virtio-balloon free page reporting), in which
> > case it's suboptimal to immediately discard instead of waiting a bit if
> > there is a reuse. However, the performance impact seems to be
> > comparatively small.
> >
> > I do wonder if the 1% overhead you're seeing is actually because of
> > allcoating/freeing or because of the reference count handling on some
> > hot paths.
>
> Qi Zheng has compared the results collected by using the "perf top"
> command. The LRU lock is more contended with this patchset applied.
> I think the reason is that this patchset will free more pages (including
> PTE page table pages). We don't see the overhead caused by reference
> count handling.

Sorry for the confusion. I am wrong. The PTE page table page does
not add to LRU list, so it should not be the LRU lock. We actually see
that _raw_spin_unlock_irqrestore is hotter than before. I guess it is
zone lock.

>
> Thanks,
>
> Muchun
>
> >
> > I'm primarily looking into asynchronous reclaim, because it somewhat
> > makes sense to only reclaim (+ pay a cost) when there is really need to
> > reclaim memory -- similar to our shrinker infrastructure.
> >
> > --
> > Thanks,
> >
> > David / dhildenb
> >