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From:   Alistair Popple <apopple@nvidia.com>
To:     Peter Xu <peterx@redhat.com>
Cc:     "Huang, Ying" <ying.huang@intel.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        huang ying <huang.ying.caritas@gmail.com>,
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        linuxppc-dev@lists.ozlabs.org, stable@vger.kernel.org
Subject: Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page
Date:   Thu, 25 Aug 2022 11:24:03 +1000
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Alistair Popple <apopple@nvidia.com> writes:

> Peter Xu <peterx@redhat.com> writes:

>>> But it's kind of against a pure "optimization" in that if trylock failed,
>>> we'll clear the mpfn so the src[i] will be zero at last.  Then will we
>>> directly give up on this page, or will we try to lock_page() again
>>> somewhere?
>
> That comment is out dated. We used to try locking the page again but
> that was removed by ab09243aa95a ("mm/migrate.c: remove
> MIGRATE_PFN_LOCKED"). See
> https://lkml.kernel.org/r/20211025041608.289017-1-apopple@nvidia.com
>
> Will post a clean-up for it.

By the way it's still an optimisation because in most cases we can avoid
calling try_to_migrate() and walking the rmap altogether if we install
the migration entries here. But I agree the comment is misleading.

>>> The future unmap op is also based on this "cpages", not "npages":
>>>
>>> 	if (args->cpages)
>>> 		migrate_vma_unmap(args);
>>>
>>> So I never figured out how this code really works.  It'll be great if you
>>> could shed some light to it.
>>>
>>> Thanks,
>>>
>>> --
>>> Peter Xu