linux-kernel - Re: [PATCH 1/4] mm: Trial do_wp

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Message-ID: <20200914232851.GH1221970@ziepe.ca>
Date:   Mon, 14 Sep 2020 20:28:51 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Xu <peterx@...hat.com>, Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        Kirill Shutemov <kirill@...temov.name>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jan Kara <jack@...e.cz>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

On Mon, Sep 14, 2020 at 03:59:31PM -0700, Linus Torvalds wrote:
> On Mon, Sep 14, 2020 at 3:55 PM Jason Gunthorpe <jgg@...dia.com> wrote:
> >
> > Just as an aside, the RDMA stuff is also supposed to set MADV_DONTFORK
> > on these regions, so I'm a bit puzzled what is happening here.
> 
> Did the fork perhaps happen _before_ , so the pages are shared when
> you do the pin?

Looking at the progam, it seems there are a number of forks for exec
before and after pin_user_pages_fast(), but the parent process always
does waitpid() after the fork.

> MADV_DONTFORK doesn't mean COW doesn't happen. It just means that the
> next fork() won't be copying that memory area.

Yes, this stuff does pin_user_pages_fast() and MADV_DONTFORK
together. It sets FOLL_FORCE and FOLL_WRITE to get an exclusive copy
of the page and MADV_DONTFORK was needed to ensure that a future fork
doesn't establish a COW that would break the DMA by moving the
physical page over to the fork. DMA should stay with the process that
called pin_user_pages_fast() (Is MADV_DONTFORK still needed with
recent years work to GUP/etc? It is a pretty terrible ancient thing)

> That said, it's possible that the test cases do something invalid - or
> maybe we've broken MADV_DONTFORK - and it all just happened to work
> before.

Hmm. If symptoms stop with this patch should we investigate
MADV_DONTFORK?

Thanks,
Jason