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

[Alistair Popple]

Peter Xu <peterx@xxxxxxxxxx> writes:

> 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@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 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)?

Mainly because I agree with your original comments, that it would be
better to keep the batching of TLB flushing if possible. After the
discussion I don't think there is any issues with HW pte dirty bits
here. There are already other cases where HW needs to get that right
anyway (eg. zap_pte_range).

> 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?

Isn't that already the case? Patch 1 moves the TLB flush before the PTL
as suggested, patch 2 atomically copies the dirty bit without changing
any TLB flushing.

>>
>>  			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
>>