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

[David Hildenbrand]

On 26.08.22 16:32, Peter Xu wrote:
> 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?

GUP-fast is the magic bit, we have to make sure that we won't see new
GUP pins, thus the TLB flush.

include/linux/mm.h:gup_must_unshare() contains documentation.

Without GUP-fast, some things would be significantly easier to handle.


-- 
Thanks,

David / dhildenb