[RFCv2 08/13] mm/gup: Add FOLL_ALLOW_POISONED

[Kirill A. Shutemov]

The new flag allows to bypass check if the page is poisoned and get
reference on it.

Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
---
 include/linux/mm.h |  1 +
 mm/gup.c           | 29 ++++++++++++++++++-----------
 2 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ecdf8a8cd6ae..378a94481fd1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2802,6 +2802,7 @@ struct page *follow_page(struct vm_area_struct *vma, 
unsigned long address,
 #define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */
 #define FOLL_PIN       0x40000 /* pages must be released via unpin_user_page */
 #define FOLL_FAST_ONLY 0x80000 /* gup_fast: prevent fall-back to slow gup */
+#define FOLL_ALLOW_POISONED 0x100000 /* bypass poison check */
 
 /*
  * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
diff --git a/mm/gup.c b/mm/gup.c
index e4c224cd9661..dd3b79b03eb5 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -384,22 +384,29 @@ static struct page *follow_page_pte(struct vm_area_struct 
*vma,
        ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
        pte = *ptep;
        if (!pte_present(pte)) {
-               swp_entry_t entry;
+               swp_entry_t entry = pte_to_swp_entry(pte);
+
+               if (pte_none(pte))
+                       goto no_page;
+
                /*
                 * KSM's break_ksm() relies upon recognizing a ksm page
                 * even while it is being migrated, so for that case we
                 * need migration_entry_wait().
                 */
-               if (likely(!(flags & FOLL_MIGRATION)))
-                       goto no_page;
-               if (pte_none(pte))
-                       goto no_page;
-               entry = pte_to_swp_entry(pte);
-               if (!is_migration_entry(entry))
-                       goto no_page;
-               pte_unmap_unlock(ptep, ptl);
-               migration_entry_wait(mm, pmd, address);
-               goto retry;
+               if (is_migration_entry(entry) && (flags & FOLL_MIGRATION)) {
+                       pte_unmap_unlock(ptep, ptl);
+                       migration_entry_wait(mm, pmd, address);
+                       goto retry;
+               }
+
+               if (is_hwpoison_entry(entry) && (flags & FOLL_ALLOW_POISONED)) {
+                       page = hwpoison_entry_to_page(entry);
+                       get_page(page);
+                       goto out;
+               }
+
+               goto no_page;
        }
        if ((flags & FOLL_NUMA) && pte_protnone(pte))
                goto no_page;
-- 
2.26.3