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

[Alistair Popple]

Peter Xu <peterx@xxxxxxxxxx> writes:

> On Fri, Aug 26, 2022 at 08:21:44AM +1000, Alistair Popple wrote:
>>
>> 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).
>
> Yes tlb batching was kept, thanks for doing that way.  Though if only apply
> patch 1 we'll have both ptep_clear_flush() and batched flush which seems to
> be redundant.
>
>>
>> > 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.
>
> IMHO it's cleaner to have patch 1 fix batch flush, replace
> ptep_clear_flush() with ptep_get_and_clear() and update pte properly.

Which ptep_clear_flush() are you referring to? This one?

			if (anon_exclusive) {
				flush_cache_page(vma, addr, pte_pfn(*ptep));
				ptep_clear_flush(vma, addr, ptep);

My understanding is that we need to do a flush for anon_exclusive.

> No strong opinions on the layout, but I still think we should drop the
> redundant ptep_clear_flush() above, meanwhile add the flush_cache_page()
> properly for !exclusive case too.

Good point, we need flush_cache_page() for !exclusive. Will add.

> Thanks,