linux-kernel - Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()

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Date:   Thu, 24 Sep 2020 10:29:27 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Hugh Dickins <hughd@...gle.com>,
        Leon Romanovsky <leonro@...dia.com>, Jan Kara <jack@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()

On Thu, Sep 24, 2020 at 8:47 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Wed, Sep 23, 2020 at 09:07:49AM -0700, Yang Shi wrote:
> > For tmpfs map, the pmd split just clears the pmd entry without
> > reinstalling ptes (oppositely anonymous map would reinstall ptes). It
> > looks this patch intends to copy at pte level by splitting pmd. But
> > I'm afraid this may not work for tmpfs mappings.
>
> IIUC that's exactly what we want.
>
> We only want to make sure the pinned tmpfs shared pages will be kept there in
> the parent.  It's not a must to copy the pages to the child, as long as they
> can be faulted in later correctly.

Aha, got your point. Yes, they can be refaulted in later. This is how
the file THP pmd split was designed.

>
> --
> Peter Xu
>