[Xen-devel] [PATCH V2 4/7] mm/gup: Add FOLL_LONGTERM capability to GUP fast

[ira.weiny@xxxxxxxxx]

From: Ira Weiny <ira.weiny@xxxxxxxxx>

DAX pages were previously unprotected from longterm pins when users
called get_user_pages_fast().

Use the new FOLL_LONGTERM flag to check for DEVMAP pages and fall
back to regular GUP processing if a DEVMAP page is encountered.

Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
---
 mm/gup.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 6f32d36b3c5b..f7e759c523bb 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1439,6 +1439,9 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, 
unsigned long end,
                        goto pte_unmap;
 
                if (pte_devmap(pte)) {
+                       if (unlikely(flags & FOLL_LONGTERM))
+                               goto pte_unmap;
+
                        pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
                        if (unlikely(!pgmap)) {
                                undo_dev_pagemap(nr, nr_start, pages);
@@ -1578,8 +1581,11 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, 
unsigned long addr,
        if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
                return 0;
 
-       if (pmd_devmap(orig))
+       if (pmd_devmap(orig)) {
+               if (unlikely(flags & FOLL_LONGTERM))
+                       return 0;
                return __gup_device_huge_pmd(orig, pmdp, addr, end, pages, nr);
+       }
 
        refs = 0;
        page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
@@ -1904,8 +1910,20 @@ int get_user_pages_fast(unsigned long start, int 
nr_pages,
                start += nr << PAGE_SHIFT;
                pages += nr;
 
-               ret = get_user_pages_unlocked(start, nr_pages - nr, pages,
-                                             gup_flags);
+               if (gup_flags & FOLL_LONGTERM) {
+                       down_read(&current->mm->mmap_sem);
+                       ret = __gup_longterm_locked(current, current->mm,
+                                                   start, nr_pages - nr,
+                                                   pages, NULL, gup_flags);
+                       up_read(&current->mm->mmap_sem);
+               } else {
+                       /*
+                        * retain FAULT_FOLL_ALLOW_RETRY optimization if
+                        * possible
+                        */
+                       ret = get_user_pages_unlocked(start, nr_pages - nr,
+                                                     pages, gup_flags);
+               }
 
                /* Have to be a bit careful with return values */
                if (nr > 0) {
-- 
2.20.1


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