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

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Message-ID: <20200924154750.GG79898@xz-x1>
Date:   Thu, 24 Sep 2020 11:47:50 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Yang Shi <shy828301@...il.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 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.

-- 
Peter Xu