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 H+KvQjqO YdZVSmfhUPsZQVGpRXRrCW5AzjHTnTi6HkN1WDmVLyTKw9Eszfpk4iI8YEU370aTcTgop/1RCCCGxxWwZhvX7G363bA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Sep 12, 2025 at 01:37:35PM +0200, Jann Horn wrote: > On Fri, Sep 12, 2025 at 6:49 AM Lorenzo Stoakes > > > folio_referenced() only wants to look at mappings of a single folio, > > > right? And it only uses the anon_vma of that folio? So as long as we > > > can guarantee that the folio can't concurrently change which anon_vma > > > it is associated with, folio_referenced() really only cares about the > > > specific anon_vma that the folio is associated with, and the anon_vmas > > > of other folios in the VMAs we traverse are irrelevant? > > > > Right yeah, true. But the AVC's link you to 'related' VMA's which are > > across the hierarchy. > > > > I think really the refined way of saying this is - yes, you could, but > > you're then putting the weight on the VMA side, and the VMA side is > > being invoked _all the time_. > > Ah, fair. > > I guess one approach would be to do something hazard-pointer-ish? Like > a semaphore-like thing in the root anon_vma that contains a normal > reader count, a hazard-pointer reader count (limited to some small > number like 2 or 4), and a writer count (up to 1), combined with a > limited number of hazard pointer slots; where a writer can ignore the > hazard-pointer reader count if none of the hazard pointers match any > anon_vma it wants to look at (but readers still always have to wait > for writers). The write-locking fastpath would just be a normal > "atomically add N if zero" just like with normal locking, and only the > case where there actually are hazard-pointer readers would make the > locking more expensive... Ohhh nice idea! Will look into that :) > > But inventing more artisanal locking schemes is probably not a great idea... > Well, sometimes it's valid! I will come up with some 'stupid' solution first so we can analyse it and shoot out an RFC. Also thanks Barry for raising this - this is an important issue and we do need to figure out a way to attack it. I think a combination of incremental work with the current anon_vma impl and also adjusting the design for the new anon_vma approach I'm working on is I think the way forward here. Anyway will send out an RFC soon! :) Cheers, Lorenzo