Re: [PATCH v2 2/2] mm: Remember young/dirty bit for page migrations

[Alistair Popple]

Peter Xu <peterx@xxxxxxxxxx> writes:

> On Wed, Aug 10, 2022 at 08:53:49AM +0800, Huang, Ying wrote:
>> Peter Xu <peterx@xxxxxxxxxx> writes:
>>
>> > On Tue, Aug 09, 2022 at 04:40:12PM +0800, Huang, Ying wrote:
>> [snip]
>> >
>> >> I don't find pte_dirty() is synced to PageDirty() as in
>> >> try_to_migrate_one().  Is it a issue in the original code?
>> >
>> > I think it has?  There is:
>> >
>> > 		/* Set the dirty flag on the folio now the pte is gone. */
>> > 		if (pte_dirty(pteval))
>> > 			folio_mark_dirty(folio);
>> >
>>
>> Sorry, my original words are confusing.  Yes, there's dirty bit syncing
>> in try_to_migrate_one().  But I don't find that in migrate_device.c
>>
>>  $ grep dirty mm/migrate_device.c
>> 				if (pte_soft_dirty(pte))
>> 					swp_pte = pte_swp_mksoft_dirty(swp_pte);
>> 				if (pte_swp_soft_dirty(pte))
>> 					swp_pte = pte_swp_mksoft_dirty(swp_pte);
>> 			entry = pte_mkwrite(pte_mkdirty(entry));
>>
>> I guess that migrate_device.c is used to migrate between CPU visible
>> page to CPU un-visible page (device visible), so the rule is different?
>
> IIUC migrate_vma_collect() handles migrations for both directions (RAM <->
> device mem).

That's correct.

> Yeah, indeed I also didn't see how migrate_vma_collect_pmd() handles the
> carry-over of pte dirty to page dirty, which looks a bit odd.  I also don't
> see why the dirty bit doesn't need to be maintained, e.g. when a previous
> page was dirty then after migration of ram->dev->ram it seems to be clean
> with current code.

That's a bug - it does need to be maintained. migrate_vma_*() currently
only works with anonymous private mappings. We could still loose data if
we attempt (but fail) to migrate a page that has been swapped in from
disk though, depending on the precise sequence.

Will post a fix for this, thanks for pointing it out.

> Another scenario is, even if the page was clean, as long as page migrated
> to device mem, device DMAed to the page, then page migrated back to RAM.  I
> also didn't see how we could detect the DMAs and set pte/page dirty
> properly after migrated back.

That would be up to the driver, unless we assume the page is always
dirty which is probably not a bad default. In practice I don't think
this will currently be a problem as any pages migrated to the device
won't have pages allocated in swap and this only works with private
anonymous mappings. But I think we should fix it anyway so will include
it in the fix.

> Copy Alistair and Jason..

Thanks. I will take a look at this series too, but probably won't get to
it until next week.

 - Alistair