huang ying <huang.ying.caritas@xxxxxxxxx> writes:
> On Tue, Aug 16, 2022 at 3:39 PM Alistair Popple <apopple@xxxxxxxxxx> 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@xxxxxxxxxx>
>> Acked-by: Peter Xu <peterx@xxxxxxxxxx>
>> Reported-by: Huang Ying <ying.huang@xxxxxxxxx>
>> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
>> Cc: stable@xxxxxxxxxxxxxxx
>>
>> ---
>>
>> 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 | 21 ++++++++-------------
>> 1 file changed, 8 insertions(+), 13 deletions(-)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index 27fb37d..e2d09e5 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>
>> @@ -61,7 +62,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> 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 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> bool anon_exclusive;
>> pte_t swp_pte;
>>
>> + flush_cache_page(vma, addr, pte_pfn(*ptep));
>> + pte = ptep_clear_flush(vma, addr, ptep);
>
> Although I think it's possible to batch the TLB flushing just before
> unlocking PTL. The current code looks correct.
I think you might be right but I'd rather deal with batch TLB flushing
as a separate change that implements it for normal migration as well
given we don't seem to do it there either.
> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Thanks.
> Best Regards,
> Huang, Ying
>
>> 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 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> 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 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> */
>> page_remove_rmap(page, vma, false);
>> put_page(page);
>> -
>> - if (pte_present(pte))
>> - unmapped++;
>> } else {
>> put_page(page);
>> mpfn = 0;
>> @@ -257,10 +256,6 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> 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;
>> }
>>
>>
>> base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136
>> --
>> git-series 0.9.1
>>
