Re: [PATCH 3/5] userfualtfd: Replace lru_cache functions with folio_add functions

[Peter Xu <peterx@redhat.com>]

On Thu, Nov 03, 2022 at 10:34:38AM -0700, Axel Rasmussen wrote:
> On Wed, Nov 2, 2022 at 1:44 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Wed, Nov 02, 2022 at 07:21:19PM +0000, Matthew Wilcox wrote:
> > > On Wed, Nov 02, 2022 at 03:02:35PM -0400, Peter Xu wrote:
> > > > Does the patch attached look reasonable to you?
> > >
> > > Mmm, no.  If the page is in the swap cache, this will be "true".
> >
> > It will not happen in practise, right?
> >
> > I mean, shmem_get_folio() should have done the swap-in, and we should have
> > the page lock held at the meantime.
> >
> > For anon, mcopy_atomic_pte() is the only user and it's passing in a newly
> > allocated page here.
> >
> > >
> > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > > > index 3d0fef3980b3..650ab6cfd5f4 100644
> > > > --- a/mm/userfaultfd.c
> > > > +++ b/mm/userfaultfd.c
> > > > @@ -64,7 +64,7 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > > >     pte_t _dst_pte, *dst_pte;
> > > >     bool writable = dst_vma->vm_flags & VM_WRITE;
> > > >     bool vm_shared = dst_vma->vm_flags & VM_SHARED;
> > > > -   bool page_in_cache = page->mapping;
> > > > +   bool page_in_cache = page_mapping(page);
> > >
> > > We could do:
> > >
> > >       struct page *head = compound_head(page);
> > >       bool page_in_cache = head->mapping && !PageMappingFlags(head);
> >
> > Sounds good to me, but it just gets a bit complicated.
> >
> > If page_mapping() doesn't sound good, how about we just pass that over from
> > callers?  We only have three, so quite doable too.
> 
> For what it's worth, I think I like Matthew's version better than the
> original patch. This is because, although page_mapping() looks simpler
> here, looking into the definition of page_mapping() I feel it's
> handling several cases, not all of which are relevant here (or, as
> Matthew points out, would actually be wrong if it were possible to
> reach those cases here).
> 
> It's not clear to me what is meant by "pass that over from callers"?
> Do you mean, have callers pass in true/false for page_in_cache
> directly?

Yes.

> 
> That could work, but I still think I prefer Matthew's version slightly
> better, if only because this function already takes a lot of
> arguments.

IMHO that's not an issue, we can merge them into flags, cleaning things
alongside.

The simplest so far is still just to use page_mapping() to me, but no
strong opinion here.

If to go with Matthew's patch, it'll be great if we can add a comment
showing what we're doing (something like "Unwrapped page_mapping() but
avoid looking into swap cache" would be good enough to me).

Thanks,

-- 
Peter Xu