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List-Unsubscribe: On Mon, Sep 15, 2025 at 12:53:20AM +0100, Matthew Wilcox wrote: > On Thu, Sep 11, 2025 at 07:17:01PM +1200, Barry Song wrote: > > 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. > > > > On Android, this issue becomes more severe since many processes are > > descendants of zygote. > > I'm not nearly as familiar with anon_vma as, well, the rest of you > are. As I understand this situation, usually after fork(), a process > calls exec() and the VMAs evaporate. Android is different in that after > the zygotecalls fork(), there is no exec() and so the VMAs stay COW. Oh really, wasn't aware of this... > > I wonder if we could fix this by adding a new syscall: > > mremap(addr, size, size, MREMAP_COW_NOW); > > That would create a new VMA that contains the COWed pages from the > old VMA, but crucially no longer attached to the anon_vma root of > the zygote. You wouldn't want to call this for every VMA, of course. > Just the ones which are likely to be fully COWed. Hm, I'm not sure how this would work. So the folio->mapping would point at the zygote's anon_vma, which would have AVC's to the zygote + the child. after this call you have a new VMA that surely would need that same anon_vma referencing it via an AVC unless you intend to actually CoW the folios to new folios that reference the new VMA"s, which I guess is what you mean? This is essentially doing a CoW _and_ saying 'hey we are definitely actually CoWing the _whole range_ in the VMA so can safely no longer link to the zygote'. I mean firstly I think the interface is definitely not right, I don't know where you'd be mremap()'ing to and from. I think it'd need to be more like an madvise(), one that you'd have to restrict to a whole VMA. But I really don't love this idea, I think we'd be solving a specific issue for Android while leaving a genuine problem that exists in the anon_vma logic alone, I'd far rather we attack things at a fundamental level. Also I think figuring out which bits are likely to get CoW'd or not will be non-trivial. Presumably Google are doing this zygote stuff to take advantage of CoW, and wouldn't want the overhead of copying data all that much. > > Maybe this isn't practical, but I thought it worth suggesting. > Yeah I'm not sure this is the right approach. But you have managed to get 'cow now' stuck in my head ;) Cheers, Lorenzo