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

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Message-ID: <20220624013415.GI4147@nvidia.com>
Date:   Thu, 23 Jun 2022 22:34:15 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...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

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..

Thanks,
Jason