linux-kernel - Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page during isolation

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Date:   Wed, 3 Feb 2021 14:00:04 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Joao Martins <joao.m.martins@...cle.com>
Cc:     Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, vbabka@...e.cz, mhocko@...e.com,
        david@...hat.com, osalvador@...e.de, dan.j.williams@...el.com,
        sashal@...nel.org, tyhicks@...ux.microsoft.com,
        iamjoonsoo.kim@....com, mike.kravetz@...cle.com,
        rostedt@...dmis.org, mingo@...hat.com, peterz@...radead.org,
        mgorman@...e.de, willy@...radead.org, rientjes@...gle.com,
        jhubbard@...dia.com, linux-doc@...r.kernel.org,
        ira.weiny@...el.com, linux-kselftest@...r.kernel.org,
        jmorris@...ei.org
Subject: Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page
 during isolation

On Wed, Feb 03, 2021 at 04:13:21PM +0000, Joao Martins wrote:

> If check_and_migrate_movable_pages() is meant to migrate unpinned
> pages, then rather than pinning+unpinning+moving, perhaps it would
> be called in __get_user_pages() place where we are walking page
> tables and know if it's a PUD/PMD and can skip all the subpages and
> just record and migrate those instead. Was that your thinking?

I think a reasonable approach is to detect non-pinnable pages while
walking the VMAs, when found isolate them and thread them on a linked
list.

When the VMA walk is done you'll have a linked list of isolated pages
that need migration. So the check_and_migrate_movable_pages() gets
split into the top half being diffused in the VMA walk and the bottom
half still called after __get_user_pages() returns.

Jason