Messages in this thread | | | Date | Mon, 5 Feb 2024 17:00:22 -0500 | From | "Liam R. Howlett" <> | Subject | Re: [PATCH v2 3/3] userfaultfd: use per-vma locks in userfaultfd operations |
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* Lokesh Gidra <lokeshgidra@google.com> [240205 16:55]: ..
> > > > We can take care of anon_vma as well here right? I can take a bool > > > > parameter ('prepare_anon' or something) and then: > > > > > > > > if (vma) { > > > > if (prepare_anon && vma_is_anonymous(vma)) && > > > > !anon_vma_prepare(vma)) { > > > > vma = ERR_PTR(-ENOMEM); > > > > goto out_unlock; > > > > } > > > > > vma_aquire_read_lock(vma); > > > > } > > > > out_unlock: > > > > > mmap_read_unlock(mm); > > > > > return vma; > > > > > } > > > > > > Do you need this? I didn't think this was happening in the code as > > > written? If you need it I would suggest making it happen always and > > > ditch the flag until a user needs this variant, but document what's > > > going on in here or even have a better name. > > > > I think yes, you do need this. I can see calls to anon_vma_prepare() > > under mmap_read_lock() protection in both mfill_atomic_hugetlb() and > > in mfill_atomic(). This means, just like in the pagefault path, we > > modify vma->anon_vma under mmap_read_lock protection which guarantees > > that adjacent VMAs won't change. This is important because > > __anon_vma_prepare() uses find_mergeable_anon_vma() that needs the > > neighboring VMAs to be stable. Per-VMA lock guarantees stability of > > the VMA we locked but not of its neighbors, therefore holding per-VMA > > lock while calling anon_vma_prepare() is not enough. The solution > > Lokesh suggests would call anon_vma_prepare() under mmap_read_lock and > > therefore would avoid the issue. > >
..
> anon_vma_prepare() is also called in validate_move_areas() via move_pages().
Probably worth doing it unconditionally and have a comment as to why it is necessary.
Does this avoid your locking workaround?
Thanks, Liam
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