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

[Muchun Song]

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.

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
>