Re: [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages

[Jason Gunthorpe <jgg@xxxxxxxx>]

On Fri, Sep 23, 2022 at 02:11:03PM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2022-09-23 13:53, Jason Gunthorpe wrote:
> > On Fri, Sep 23, 2022 at 01:08:31PM -0600, Logan Gunthorpe wrote:
> > I'm encouraging Dan to work on better infrastructure in pgmap core
> > because every pgmap implementation has this issue currently.
> > 
> > For that reason it is probably not so relavent to this series.
> > 
> > Perhaps just clarify in the commit message that the FOLL_LONGTERM
> > restriction is to copy DAX until the pgmap page refcounts are fixed.
> 
> Ok, I'll add that note.
> 
> Per the fix for the try_grab_page(), to me it doesn't fit well in 
> try_grab_page() without doing a bunch of cleanup to change the
> error handling, and the same would have to be added to try_grab_folio().
> So I think it's better to leave it where it was, but move it below the 
> respective grab calls. Does the incremental patch below look correct?

Oh? I was thinking of just a very simple thing:

--- a/mm/gup.c
+++ b/mm/gup.c
@@ -225,6 +225,11 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
                node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
        }
 
+       if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page))) {
+               gup_put_folio(page_folio(page), 1, flags);
+              return false;
+       }
+
        return true;
 }


> I am confused about what happens if neither FOLL_PIN or FOLL_GET 
> are set (which the documentation for try_grab_x() says is possible, but
> other documentation suggests that FOLL_GET is automatically set). 
> In which case it'd be impossible to do the check if we can't 
> access the page.

try_grab_page is operating under the PTL so it can probably touch the
page OK (though perhaps we don't need to even check anything)

try_grab_folio cannot be called without PIN/GET, so like this perhaps:

@@ -123,11 +123,14 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
  */
 struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
 {
+       struct folio *folio;
+
+       if (WARN_ON((flags & (FOLL_GET | FOLL_PIN)) == 0))
+               return NULL;
+
        if (flags & FOLL_GET)
-               return try_get_folio(page, refs);
+               folio = try_get_folio(page, refs);
        else if (flags & FOLL_PIN) {
-               struct folio *folio;
-
                /*
                 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
                 * right zone, so fail and let the caller fall back to the slow
@@ -160,11 +163,14 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
                                        refs * (GUP_PIN_COUNTING_BIAS - 1));
                node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs);
 
-               return folio;
        }
 
-       WARN_ON_ONCE(1);
-       return NULL;
+       if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page))) {
+               gup_put_folio(page, 1, flags);
+               return NULL;
+       }
+
+       return folio;
 }

Jason