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

[Qi Zheng]

On 7/19/21 7:28 PM, David Hildenbrand 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.
>
> 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.
>
> 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.

Good point, maybe we can wait a bit in the free_pte_table() in the added
optimiztion patch if the frequency of immediate reuse is high.

> 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.
>
> 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.