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 >
