Re: [PATCH v4 3/4] mm/migrate_device.c: Copy pte dirty bit to page

David Hildenbrand
2022-09-02T06:53:30Z

On 02.09.22 02:35, Alistair Popple wrote:
> 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.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Acked-by: Peter Xu <peterx@redhat.com>
> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
> Reported-by: "Huang, Ying" <ying.huang@intel.com>
> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
> Cc: stable@vger.kernel.org
>
> ---
>
> Changes for v4:
>
> - Added Reviewed-by
>
> Changes for v3:
>
> - Defer TLB flushing
> - Split a TLB flushing fix into a separate change.
>
> Changes for v2:
>
> - Fixed up Reported-by tag.
> - Added Peter's Acked-by.
> - Atomically read and clear the pte to prevent the dirty bit getting
> set after reading it.
> - Added fixes tag
> ---
> mm/migrate_device.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 4cc849c..dbf6c7a 100644
> --- a/mm/migrate_device.c
> +++ b/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>
> @@ -196,7 +197,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> flush_cache_page(vma, addr, pte_pfn(*ptep));
> anon_exclusive = PageAnon(page) && PageAnonExclusive(page);
> if (anon_exclusive) {
> - ptep_clear_flush(vma, addr, ptep);
> + pte = ptep_clear_flush(vma, addr, ptep);
>
> if (page_try_share_anon_rmap(page)) {
> set_pte_at(mm, addr, ptep, pte);
> @@ -206,11 +207,15 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> goto next;
> }
> } else {
> - ptep_get_and_clear(mm, addr, ptep);
> + pte = 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(

This matches what we do in try_to_unmap_one()

Acked-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb