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Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned
From:   John Hubbard <jhubbard@nvidia.com>
To:     Peter Xu <peterx@redhat.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
CC:     Jason Gunthorpe <jgg@ziepe.ca>,
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References: <20200921211744.24758-1-peterx@redhat.com>
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On 9/21/20 4:53 PM, John Hubbard wrote:
> On 9/21/20 2:17 PM, Peter Xu wrote:
>> (Commit message collected from Jason Gunthorpe)
>>
>> Reduce the chance of false positive from page_maybe_dma_pinned() by keep=
ing
>=20
> Not yet, it doesn't. :)=C2=A0 More:
>=20
>> track if the mm_struct has ever been used with pin_user_pages(). mm_stru=
cts
>> that have never been passed to pin_user_pages() cannot have a positive
>> page_maybe_dma_pinned() by definition. This allows cases that might driv=
e up
>> the page ref_count to avoid any penalty from handling dma_pinned pages.
>>
>> Due to complexities with unpining this trivial version is a permanent st=
icky
>> bit, future work will be needed to make this a counter.
>=20
> How about this instead:
>=20
> Subsequent patches intend to reduce the chance of false positives from
> page_maybe_dma_pinned(), by also considering whether or not a page has
> even been part of an mm struct that has ever had pin_user_pages*()


arggh, correction: please make that:

     "...whether or not a page is part of an mm struct that...".

(Present tense.) Otherwise, people start wondering about the checkered past
of a page's past lives, and it badly distracts from the main point here. :)


thanks,
--=20
John Hubbard
NVIDIA