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@xxxxxxxxxx> 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
