Re: [PATCH 1/2] mm/migrate_device.c: Copy pte dirty bit to page

[Alistair Popple <apopple@nvidia.com>]

huang ying <huang.ying.caritas@gmail.com> writes:

> Hi, Alistair,
>
> On Fri, Aug 12, 2022 at 1:23 PM Alistair Popple <apopple@nvidia.com> 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>
>> Reported-by: Peter Xu <peterx@redhat.com>
>> ---
>>  mm/migrate_device.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index 27fb37d..d38f8a6 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>
>> @@ -211,6 +212,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>
>>                         migrate->cpages++;
>>
>> +                       /* Set the dirty flag on the folio now the pte is gone. */
>> +                       if (pte_dirty(pte))
>> +                               folio_mark_dirty(page_folio(page));
>> +
>
> I think that this isn't sufficient to fix all issues.  Firstly, "pte"
> is assigned at the begin of the loop, before the PTE is cleared via
> ptep_clear_flush() or ptep_get_and_clear().  That is, the pte isn't
> changed atomically.  Between "pte" assignment and PTE clear, the PTE
> may become dirty.  That is, we need to update pte when we clear the
> PTE.

Oh good catch, thanks. Will fix.

> And I don't know why we use ptep_get_and_clear() to clear PTE if
> (!anon_exclusive).  Why don't we need to flush the TLB?

We do the TLB flush at the end if anything was modified:

	/* Only flush the TLB if we actually modified any entries */
	if (unmapped)
		flush_tlb_range(walk->vma, start, end);

Obviously I don't think that will work correctly now given we have to
read the dirty bits and clear the PTE atomically. I assume it was
originally written this way for some sort of performance reason.

 - Alistair

> Best Regards,
> Huang, Ying
>
>>                         /* Setup special migration page table entry */
>>                         if (mpfn & MIGRATE_PFN_WRITE)
>>                                 entry = make_writable_migration_entry(
>>
>> base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136
>> --
>> git-series 0.9.1
>>