Re: [PATCH 2/6] mm: Call wp_page_copy() under the VMA lock

[Matthew Wilcox <willy@infradead.org>]

On Wed, Sep 27, 2023 at 03:38:38PM -0700, Suren Baghdasaryan wrote:
> On Tue, Sep 26, 2023 at 10:25 PM Matthew Wilcox (Oracle)
> <willy@infradead.org> wrote:
> > It is usually safe to call wp_page_copy() under the VMA lock.  The only
> > unsafe situation is when no anon_vma has been allocated for this VMA,
> > and we have to look at adjacent VMAs to determine if their anon_vma can
> > be shared.  Since this happens only for the first COW of a page in this
> > VMA, the majority of calls to wp_page_copy() do not need to fall back
> > to the mmap_sem.
> >
> > Add vmf_anon_prepare() as an alternative to anon_vma_prepare() which
> > will return RETRY if we currently hold the VMA lock and need to allocate
> > an anon_vma.  This lets us drop the check in do_wp_page().
> >
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > ---
> >  mm/memory.c | 39 ++++++++++++++++++++++++++-------------
> >  1 file changed, 26 insertions(+), 13 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 97f860d6cd2a..cff78c496728 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3042,6 +3042,21 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
> >         count_vm_event(PGREUSE);
> >  }
> >
> > +static vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
> > +{
> > +       struct vm_area_struct *vma = vmf->vma;
> > +
> > +       if (likely(vma->anon_vma))
> > +               return 0;
> > +       if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> 
> I don't think the above condition will happen today because
> lock_vma_under_rcu() returns NULL and do_page_fault() falls back to
> taking mmap_lock when !vma->anon_vma
> (https://elixir.bootlin.com/linux/v6.6-rc3/source/mm/memory.c#L5428).
> We would need to narrow down that check in lock_vma_under_rcu() to
> make this work here.

That's only for anon VMAs.  For file-backed VMAs, we can get here ...

handle_pte_fault()
        if (vmf->flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)) {
                if (!pte_write(entry))
                        return do_wp_page(vmf);

ie we we have a MAP_PRIVATE of a file, first take a read-fault on it,
then write to it.  That causes us to allocate an anon page in this
file-backed VMA, so we need an anon_vma to exist.