On Thu, Mar 18, 2021 at 05:57:06PM +0100, Vlastimil Babka wrote: > On 3/11/21 7:14 PM, David Hildenbrand wrote: > > Hi folks, > > > > I was wondering, is there any mechanism that reclaims basically empty page > > tables in a running process? > > > > Like: When I MADV_DONTNEED a huge range, there could be plenty of basically > > empty (e.g., all entries invalid) page tables we could reclaim. As soon as we > > zap a complete PMD we could reclaim (depending on the architecture) a whole page. > > > > Zapping on the PMD level might make most impact I guess. > > > > For 1 GB, we need 262144 4k pages. If we assume each PTE is 8 bytes, we need a > > total of 8 MB for the lowest level page tables (PTE). > > > > OTOH, we would need 512 PMD entries - a single 4k page. Zapping 1 TB would mean > > we can free up another 4MB - rather a corner case and we can live with that. > > > > > > Of course, the same might apply to other cases where we can restore all page > > table content from the VMA again. One example would be after MADV_FREE zapped a > > whole range of entries we marked. > > I don't think we have such mechanism, but IIRC I've heard the idea mentioned > before, probably from Michal Hocko. Definitely an interesting research project > idea to evaluate the cost vs benefits of that. > It might lead to interesting interactions with lockless page table walking with implications on the mmap_lock as well. Balbir
