+ mm-migrate_devicec-copy-pte-dirty-bit-to-page.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/migrate_device.c: copy pte dirty bit to page
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-migrate_devicec-copy-pte-dirty-bit-to-page.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-migrate_devicec-copy-pte-dirty-bit-to-page.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Alistair Popple <apopple@nvidia.com>
Subject: mm/migrate_device.c: copy pte dirty bit to page
Date: Tue, 16 Aug 2022 17:39:24 +1000

migrate_vma_setup() has a fast path in migrate_vma_collect_pmd() that
installs migration entries directly if it can lock the migrating page. 
When removing a dirty pte the dirty bit is supposed to be carried over to
the underlying page to prevent it being lost.

Currently migrate_vma_*() can only be used for private anonymous mappings.
That means loss of the dirty bit usually doesn't result in data loss
because these pages are typically not file-backed.  However pages may be
backed by swap storage which can result in data loss if an attempt is made
to migrate a dirty page that doesn't yet have the PageDirty flag set.

In this case migration will fail due to unexpected references but the
dirty pte bit will be lost.  If the page is subsequently reclaimed data
won't be written back to swap storage as it is considered uptodate,
resulting in data loss if the page is subsequently accessed.

Prevent this by copying the dirty bit to the page when removing the pte to
match what try_to_migrate_one() does.

Link: https://lkml.kernel.org/r/6e77914685ede036c419fa65b6adc27f25a6c3e9.1660635033.git-series.apopple@nvidia.com
Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reported-by: Huang Ying <ying.huang@intel.com>
Reviewed-by: Huang Ying <ying.huang@intel.com>
Cc: Alex Sierra <alex.sierra@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Paul Mackerras <paulus@ozlabs.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/migrate_device.c |   21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

--- a/mm/migrate_device.c~mm-migrate_devicec-copy-pte-dirty-bit-to-page
+++ a/mm/migrate_device.c
@@ -7,6 +7,7 @@
 #include <linux/export.h>
 #include <linux/memremap.h>
 #include <linux/migrate.h>
+#include <linux/mm.h>
 #include <linux/mm_inline.h>
 #include <linux/mmu_notifier.h>
 #include <linux/oom.h>
@@ -61,7 +62,7 @@ static int migrate_vma_collect_pmd(pmd_t
 	struct migrate_vma *migrate = walk->private;
 	struct vm_area_struct *vma = walk->vma;
 	struct mm_struct *mm = vma->vm_mm;
-	unsigned long addr = start, unmapped = 0;
+	unsigned long addr = start;
 	spinlock_t *ptl;
 	pte_t *ptep;
 
@@ -193,11 +194,10 @@ again:
 			bool anon_exclusive;
 			pte_t swp_pte;
 
+			flush_cache_page(vma, addr, pte_pfn(*ptep));
+			pte = ptep_clear_flush(vma, addr, ptep);
 			anon_exclusive = PageAnon(page) && PageAnonExclusive(page);
 			if (anon_exclusive) {
-				flush_cache_page(vma, addr, pte_pfn(*ptep));
-				ptep_clear_flush(vma, addr, ptep);
-
 				if (page_try_share_anon_rmap(page)) {
 					set_pte_at(mm, addr, ptep, pte);
 					unlock_page(page);
@@ -205,12 +205,14 @@ again:
 					mpfn = 0;
 					goto next;
 				}
-			} else {
-				ptep_get_and_clear(mm, addr, ptep);
 			}
 
 			migrate->cpages++;
 
+			/* Set the dirty flag on the folio now the pte is gone. */
+			if (pte_dirty(pte))
+				folio_mark_dirty(page_folio(page));
+
 			/* Setup special migration page table entry */
 			if (mpfn & MIGRATE_PFN_WRITE)
 				entry = make_writable_migration_entry(
@@ -242,9 +244,6 @@ again:
 			 */
 			page_remove_rmap(page, vma, false);
 			put_page(page);
-
-			if (pte_present(pte))
-				unmapped++;
 		} else {
 			put_page(page);
 			mpfn = 0;
@@ -257,10 +256,6 @@ next:
 	arch_leave_lazy_mmu_mode();
 	pte_unmap_unlock(ptep - 1, ptl);
 
-	/* Only flush the TLB if we actually modified any entries */
-	if (unmapped)
-		flush_tlb_range(walk->vma, start, end);
-
 	return 0;
 }
 
_

Patches currently in -mm which might be from apopple@nvidia.com are

mm-migrate_devicec-copy-pte-dirty-bit-to-page.patch
mm-gupc-simplify-and-fix-check_and_migrate_movable_pages-return-codes.patch