linux-kernel - Re: [PATCH] mm: Re-allow pinning of zero pfns

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Date:   Fri, 24 Jun 2022 11:55:25 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        akpm@...ux-foundation.org, minchan@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, paulmck@...nel.org,
        jhubbard@...dia.com, joaodias@...gle.com
Subject: Re: [PATCH] mm: Re-allow pinning of zero pfns


Jason Gunthorpe <jgg@...dia.com> writes:

> On Fri, Jun 24, 2022 at 10:11:01AM +1000, Alistair Popple wrote:
>
>> > Hum.. Alistair, maybe you should look at this as well, I'm struggling
>> > alot to understand how it is safe to drop the reference on the page
>> > but hold a pointer to it on the movable_page_list - sure it was
>> > isolated - but why does that mean it won't be concurrently unmapped
>> > and freed?
>>
>> folio_isolate_lru() takes a reference on the page so you're safe from it
>> being freed. If it gets unmapped it will be freed when the matching
>> putback_movable_pages() is called.
>
> Hm, I guess I didn't dig deep enough into that call chain..
>
>> > Anyhow, it looks like the problem is the tortured logic in this
>> > function, what do you think about this:
>>
>> At a glance it seems reasonable, although I fear it might conflict with
>> my changes for device coherent migration. Agree the whole
>> check_and_migrate_movable_pages() logic is pretty tortured though, and I
>> don't think I'm making it better so would be happy to try cleaning it up
>> futher once the device coherent changes are in.
>
> OK, can I leave this patch with you then? I have no way to test it..

Yep, no worries.

> Thanks,
> Jason