Re: [DISCUSSION] anon_vma root lock contention and per anon_vma lock

[Matthew Wilcox <willy@infradead.org>]

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.

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.

Maybe this isn't practical, but I thought it worth suggesting.