[RFC PATCH] mm/rmap.c: finer hwpoison granularity for PTE-mapped THP

[Wei Yang <richardw.yang@linux.intel.com>]

Currently we behave differently between PMD-mapped THP and PTE-mapped
THP on memory_failure.

User detected difference:

    For PTE-mapped THP, the whole 2M range will trigger MCE after
    memory_failure(), while only 4K range for PMD-mapped THP will.

Direct reason:

    All the 512 PTE entry will be marked as hwpoison entry for a PTE-mapped
    THP while only one PTE will be marked for a PMD-mapped THP.

Root reason:

    The root cause is PTE-mapped page doesn't need to split pmd which skip
    the SPLIT_FREEZE process. This makes try_to_unmap_one() do its job when
    the THP is not splited. And since page is HWPOISON, all the entries in
    THP is marked as hwpoison entry.

    While for the PMD-mapped THP, SPLIT_FREEZE will save migration entry to
    pte and this skip try_to_unmap_one() before THP splited. And then only
    the affected 4k page is marked as hwpoison entry.

This patch tries to provide a finer granularity for PTE-mapped THP by
only mark the affected subpage as hwpoison entry when THP is not
split.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

---
This complicates the picture a little, while I don't find a better way to
improve. 

Also I may miss some case or not handle this properly.

Look forward your comments.
---
 mm/rmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index b3e381919835..90229917dd64 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1554,10 +1554,11 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 				set_huge_swap_pte_at(mm, address,
 						     pvmw.pte, pteval,
 						     vma_mmu_pagesize(vma));
-			} else {
+			} else if (!PageAnon(page) || page == subpage) {
 				dec_mm_counter(mm, mm_counter(page));
 				set_pte_at(mm, address, pvmw.pte, pteval);
-			}
+			} else
+				goto freeze;
 
 		} else if (pte_unused(pteval) && !userfaultfd_armed(vma)) {
 			/*
@@ -1579,6 +1580,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 			swp_entry_t entry;
 			pte_t swp_pte;
 
+freeze:
 			if (arch_unmap_one(mm, vma, address, pteval) < 0) {
 				set_pte_at(mm, address, pvmw.pte, pteval);
 				ret = false;
-- 
2.17.1