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

[David Hildenbrand <david@redhat.com>]

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@redhat.com
> 
> 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.

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.

-- 
Thanks,

David / dhildenb