Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page

[Peter Xu <peterx@redhat.com>]

On Wed, Aug 24, 2022 at 01:03:38PM +1000, 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>
> 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 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 6a5ef9f..51d9afa 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,
>  			anon_exclusive = PageAnon(page) && PageAnonExclusive(page);
>  			if (anon_exclusive) {
>  				flush_cache_page(vma, addr, pte_pfn(*ptep));
> -				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);
>  			}

I remember that in v2 both flush_cache_page() and ptep_get_and_clear() are
moved above the condition check so they're called unconditionally.  Could
you explain the rational on why it's changed back (since I think v2 was the
correct approach)?

The other question is if we want to split the patch, would it be better to
move the tlb changes to patch 1, and leave the dirty bit fix in patch 2?

>  
>  			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(
> -- 
> git-series 0.9.1
> 

-- 
Peter Xu