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

[Logan Gunthorpe <logang@xxxxxxxxxxxx>]

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?

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.

I'm assuming that seeing there are other accesses to the page in these
two instances of try_grab_x() that these spots will always have FOLL_GET
or  FOLL_PIN set and thus this isn't an issue. Another reason not
to push the check into try_grab_x().

Logan

--

diff --git a/mm/gup.c b/mm/gup.c
index 108848b67f6f..f05ba3e8e29a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -601,12 +601,6 @@ static struct page *follow_page_pte(struct vm_area_struct >
                goto out;
        }
 
-       if (unlikely(!(flags & FOLL_PCI_P2PDMA) &&
-                    is_pci_p2pdma_page(page))) {
-               page = ERR_PTR(-EREMOTEIO);
-               goto out;
-       }
-
        VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
                       !PageAnonExclusive(page), page);
 
@@ -615,6 +609,13 @@ static struct page *follow_page_pte(struct vm_area_struct >
                page = ERR_PTR(-ENOMEM);
                goto out;
        }
+
+       if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page))) {
+               gup_put_folio(page_folio(page), 1, flags);
+               page = ERR_PTR(-EREMOTEIO);
+               goto out;
+       }
+
        /*
         * We need to make the page accessible if and only if we are going
         * to access its content (the FOLL_PIN case).  Please see
@@ -2392,14 +2393,16 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr,>
                VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
                page = pte_page(pte);
 
-               if (unlikely(!(flags & FOLL_PCI_P2PDMA) &&
-                            is_pci_p2pdma_page(page)))
-                       goto pte_unmap;
-
                folio = try_grab_folio(page, 1, flags);
                if (!folio)
                        goto pte_unmap;
 
+               if (unlikely(!(flags & FOLL_PCI_P2PDMA) &&
+                            is_pci_p2pdma_page(page))) {
+                       gup_put_folio(folio, 1, flags);
+                       goto pte_unmap;
+               }
+
                if (unlikely(page_is_secretmem(page))) {
                        gup_put_folio(folio, 1, flags);
                        goto pte_unmap;