Re: Page zapping and page table reclaim

[Balbir Singh <bsingharora@gmail.com>]

On Fri, Mar 19, 2021 at 01:44:55PM +0100, David Hildenbrand wrote:
> On 19.03.21 00:53, Balbir Singh wrote:
> > 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.
> > 
> 
> I think if lockless page table walks have to be able with THP code swapping
> populated page tables by a PMD back and forth, swapping an unpopulated page
> table by an invalid PMD entry might be quite similar. At least it feels like
> both approaches would rely on similar mechanisms / locking. :)
>

Yes, but then I suspect you always need destruct page tables by RCU.
 
> I'm planning on looking into this, but not sure when I'll have time to
> prototype something up.
> 
>

Thanks,
Balbir Singh.