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

[Peter Xu <peterx@redhat.com>]

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.

-- 
Peter Xu