linux-kernel - Re: [PATCH 1/5] mm: Introduce mm_struct.has

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Date:   Fri, 25 Sep 2020 17:13:21 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On Fri, Sep 25, 2020 at 12:56:05PM -0700, Linus Torvalds wrote:
> So I think we can simply add a
> 
>         if (page_mapcount(page) != 1)
>                 return false;
> 
> to page_maybe_dma_pinned(), and that very naturally protects against
> the "is the page count perhaps elevated due to a lot of forking?"

How about the MAP_SHARED case where the page is pinned by some process but also
shared (so mapcount can be >1)?

> And honestly, since this is all getting fairly late in the rc, and it
> took longer than I thought, I think we should do the GFP_ATOMIC
> approach for now - not great, but since it only triggers for this case
> that really should never happen anyway, I think it's probably the best
> thing for 5.9, and we can improve on things later.

Sorry for that.  Maybe I should have moved even faster.

Would the ATOMIC version always work?  I mean, I thought it could fail anytime,
so any fork() can start to fail for the tests too.

PS. I do plan to post a GFP_KERNEL version soon today, no matter for this
release or the next one.

-- 
Peter Xu