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

[Peter Xu]

On Fri, Aug 26, 2022 at 11:02:58AM +1000, Alistair Popple wrote:
> 
> 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);

Correct.

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

To me anon exclusive only shows this mm exclusively owns this page. I
didn't quickly figure out why that requires different handling on tlb
flushs.  Did I perhaps miss something?

Thanks,

-- 
Peter Xu