[PATCH v2] mm:fix gup_pud_range

[Qiujun Huang <hqjagain@gmail.com>]

sorry for not processing for a long time. I met it again.

patch v1   https://lkml.org/lkml/2019/9/20/656

do_machine_check()
  do_memory_failure()
    memory_failure()
      hw_poison_user_mappings()
        try_to_unmap()
          pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));

...and now we have a swap entry that indicates that the page entry
refers to a bad (and poisoned) page of memory, but gup_fast() at this
level of the page table was ignoring swap entries, and incorrectly
assuming that "!pxd_none() == valid and present".

And this was not just a poisoned page problem, but a generaly swap entry
problem. So, any swap entry type (device memory migration, numa migration,
or just regular swapping) could lead to the same problem.

Fix this by checking for pxd_present(), instead of pxd_none().

Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
---
 mm/gup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index 7646bf9..9c41670 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2237,7 +2237,7 @@ static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
 		pud_t pud = READ_ONCE(*pudp);
 
 		next = pud_addr_end(addr, end);
-		if (pud_none(pud))
+		if (unlikely(!pud_present(pud)))
 			return 0;
 		if (unlikely(pud_huge(pud))) {
 			if (!gup_huge_pud(pud, pudp, addr, next, flags,
-- 
1.8.3.1