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List-Unsubscribe: On Thu, Sep 11, 2025 at 07:17:01PM +1200, Barry Song wrote: > Hi All, > > I’m aware that Lokesh started a discussion on the concurrency issue > between usefaultfd_move and memory reclamation [1]. However, my > concern is different, so I’m starting a separate discussion. > > In the process tree, many processes may share anon_vma->root, even if > they don’t share the anon_vma itself. This causes serious lock contention > between memory reclamation (which calls folio_referenced and try_to_unmap) > and other processes calling fork(), exit(), mprotect(), etc. Well, when you say lock contention, I mean - we need to have a lock that is held over the entire fork tree, as we are cloning references to them. This is at the anon_vma level - so the folio might be exclusive, but other folios there might not be. Note that I'm working on a radical rework of anon_vma's at the moment (time is not in my favour given other tasks + review workload, but it _is_ happening). So I'm interested to gather real world usecase data on how best to implement things and this is interesting re: that. My proposed approach would use something like ranged locks. It's a bit fuzzy right now so definitely interested in putting some meat on that. > > On Android, this issue becomes more severe since many processes are > descendants of zygote. > > Memory reclamation path: > folio_lock_anon_vma_read > > mprotect path: > mprotect > split_vma > anon_vma_clone > > fork / copy_process path: > copy_process > dup_mmap > anon_vma_fork > > exit path: > exit_mmap > free_pgtables > unlink_anon_vmas > > To be honest, memory reclamation—especially folio_referenced()—is a > problem. It is called very frequently and can block other important > user threads waiting for the anon_vma root lock, causing UI lag. > > I have a rough idea: since the vast majority of anon folios are actually > exclusive (I observed almost 98% of Android anon folios fall into this > category), they don’t need to iterate the anon_vma tree. They belong to > a single process, and even for rmap, it is per-process. > > I propose introducing a per-anon_vma lock. For exclusive folios whose > anon_vma is not shared, we could use this per-anon_vma lock. I'm not sure how adding _more_ locks is going to reduce contention :) and the anon_vma's are all linked to their parents etc. etc. so it's simply not ok to hold one lock and not the others when making changes. > folio_referenced declares that it will begin reading, and Lokesh’s > folio_lock may also help maintain folios as exclusive, so I am > somewhat in favor of his RFC. Any thread writing to such an anon_vma Will reply on his latest re: Lokesh's approach. > would take the per-vma write lock, and possibly also the anon_vma > root write lock. If folio_referenced fails to declare the per-vma lock, > it can fall back to the global anon_vma->root read mutex, similar to > mmap_lock. Again, we actually _need_ to hold a lock over this range. So you can't just hold the root and a descendent it has to be all. > > I haven’t carefully considered this or written any code yet—just a > very rough idea. Sorry if it comes across as too naive. It's fine, though I do wish we'd have a _little_ less workload this cycle, can barely breath at the moment, but that's not your fault ;) I do wonder whether part of the problem here is keeping anon_vma's connected to parents whwen they don't need to be. Right now, even if you entirely CoW everything in a VMA, we are still attached to parents with all the overhead. That's something I can look at. But also perhaps worth considering how we approach the whole clone thing. My (very early) anon_vma rework would do away with anon_vma_chain's altogether and make forking simpler. There'd be a per-mm object that connects to others via (probably) interval tree edges for ranges that are CoW, so splitting for instance would be easier. Early days with it though... > > [1] https://lore.kernel.org/linux-mm/CA+EESO4Z6wtX7ZMdDHQRe5jAAS_bQ-POq5+4aDx5jh2DvY6UHg@mail.gmail.com/ > > Thanks > Barry > Cheers, Lorenzo