mm: save soft-dirty bits on swapped pages

Andy Lutomirski reported that if a page with _PAGE_SOFT_DIRTY bit set
get swapped out, the bit is getting lost and no longer available when
pte read back.

To resolve this we introduce _PTE_SWP_SOFT_DIRTY bit which is saved in
pte entry for the page being swapped out.  When such page is to be read
back from a swap cache we check for bit presence and if it's there we
clear it and restore the former _PAGE_SOFT_DIRTY bit back.

One of the problem was to find a place in pte entry where we can save
the _PTE_SWP_SOFT_DIRTY bit while page is in swap.  The _PAGE_PSE was
chosen for that, it doesn't intersect with swap entry format stored in
pte.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index cd356df4f71a..83325b80142b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1236,6 +1236,7 @@ int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                            swp_entry_to_pte(make_hwpoison_entry(page)));
         } else if (PageAnon(page)) {
                 swp_entry_t entry = { .val = page_private(page) };
+                pte_t swp_pte;
 
                 if (PageSwapCache(page)) {
                         /*
@@ -1264,7 +1265,10 @@ int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                         BUG_ON(TTU_ACTION(flags) != TTU_MIGRATION);
                         entry = make_migration_entry(page, pte_write(pteval));
                 }
-                set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
+                swp_pte = swp_entry_to_pte(entry);
+                if (pte_soft_dirty(pteval))
+                        swp_pte = pte_swp_mksoft_dirty(swp_pte);
+                set_pte_at(mm, address, pte, swp_pte);
                 BUG_ON(pte_file(*pte));
         } else if (IS_ENABLED(CONFIG_MIGRATION) &&
                    (TTU_ACTION(flags) == TTU_MIGRATION)) {