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From: Peter Xu <peterx@redhat.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Leon Romanovsky <leonro@nvidia.com>,
	Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"Maya B . Gokhale" <gokhale2@llnl.gov>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Marty Mcfadden <mcfadden8@llnl.gov>,
	Kirill Shutemov <kirill@shutemov.name>,
	Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
	Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Tue, 15 Sep 2020 10:50:40 -0400
Message-ID: <20200915145040.GA2949@xz-x1> (raw)
In-Reply-To: <20200914232851.GH1221970@ziepe.ca>

Hi, all,

I prepared another version of the FOLL_PIN enforced cow patch attached, just in
case it would still be anything close to useful (though now I highly doubt it
considering below...).  I took care of !USERFAULTFD as suggested by Leon, and
also the fast gup path.

However...

On Mon, Sep 14, 2020 at 08:28:51PM -0300, Jason Gunthorpe wrote:
> 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)

... Now I'm more confused on what has happened.

If we're with FORCE|WRITE, iiuc it should guarantee that the page will trigger
COW during gup even if it is shared, so no problem on the gup side.  Then I'm
quite confused on why the write bit is not set when cow triggered.

E.g., in wp_page_copy(), if I'm not wrong, the write bit is only controlled by
(besides the fix patch, though I believe the rdma test should have nothing to
do with uffd-wp after all so it should be the same anyways):

        entry = maybe_mkwrite(pte_mkdirty(entry), vma);

It means, as long as the rdma region has VM_WRITE set (which I think of no
reason on why it shouldn't...), then it should have the write bit in the COWed
page entry.  If so, the page should be stable and I don't undersdand why
another COW could even trigger and how the code path in the "trial cow" patch
is triggered.

Or, the VMA is without VM_WRITE due to some reason?  Sorry I probably know
nothing about RDMA, more information on that side might help too. E.g., is the
hardware going to walk the software process page table too when doing RDMA (or
is IOMMU page table used, or none)?

Thanks,

-- 
Peter Xu


  parent reply index

Thread overview: 93+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-21 23:49 [PATCH 0/4] mm: Simplfy cow handling Peter Xu
2020-08-21 23:49 ` [PATCH 1/4] mm: Trial do_wp_page() simplification Peter Xu
2020-08-24  8:36   ` Kirill Tkhai
2020-08-24 14:30     ` Jan Kara
2020-08-24 15:37       ` Kirill Tkhai
2020-08-24 18:22         ` Linus Torvalds
2020-09-01  7:01           ` Hugh Dickins
2020-09-14 14:38   ` Jason Gunthorpe
2020-09-14 17:32     ` Linus Torvalds
2020-09-14 18:34       ` Peter Xu
2020-09-14 21:15         ` Peter Xu
2020-09-14 22:55           ` Jason Gunthorpe
2020-09-14 22:59             ` Linus Torvalds
2020-09-14 23:28               ` Jason Gunthorpe
2020-09-15  0:19                 ` Linus Torvalds
2020-09-15 14:50                 ` Peter Xu [this message]
2020-09-15 15:17                   ` Peter Xu
2020-09-15 16:05                   ` Jason Gunthorpe
2020-09-15 18:29                     ` Jason Gunthorpe
2020-09-15 19:13                       ` Peter Xu
2020-09-15 19:38                         ` Jason Gunthorpe
2020-09-15 21:33                           ` Peter Xu
2020-09-15 23:22                             ` Jason Gunthorpe
2020-09-16  1:50                               ` John Hubbard
2020-09-16 17:48                                 ` Jason Gunthorpe
2020-09-16 18:46                                   ` Peter Xu
2020-09-17 11:25                                     ` Jason Gunthorpe
2020-09-17 18:11                                       ` Linus Torvalds
2020-09-17 19:38                                         ` Jason Gunthorpe
2020-09-17 19:51                                           ` Linus Torvalds
2020-09-18 16:40                                             ` Peter Xu
2020-09-18 17:16                                               ` Linus Torvalds
2020-09-18 19:57                                                 ` Peter Xu
2020-09-18 17:32                                               ` Jason Gunthorpe
2020-09-18 20:40                                                 ` Peter Xu
2020-09-18 20:59                                                   ` Linus Torvalds
2020-09-19  0:28                                                     ` Jason Gunthorpe
2020-09-18 21:06                                                   ` John Hubbard
2020-09-19  0:01                                                     ` Jason Gunthorpe
2020-09-21  8:35                                                       ` Jan Kara
2020-09-21 12:03                                                         ` Jason Gunthorpe
2020-09-21 13:42                                               ` Michal Hocko
2020-09-21 14:18                                                 ` Peter Xu
2020-09-21 14:28                                                   ` Michal Hocko
2020-09-21 14:38                                                     ` Tejun Heo
2020-09-21 14:43                                                       ` Christian Brauner
2020-09-21 14:55                                                         ` Michal Hocko
2020-09-21 15:04                                                           ` Christian Brauner
2020-09-21 16:06                                                             ` Michal Hocko
2020-09-23  7:53                                                               ` Michal Hocko
2020-09-21 14:41                                                 ` Christian Brauner
2020-09-21 14:57                                                   ` Michal Hocko
2020-09-21 16:31                                                     ` Peter Xu
2020-09-17 18:14                                       ` Peter Xu
2020-09-17 18:26                                         ` Linus Torvalds
2020-09-17 19:03                                           ` Peter Xu
2020-09-17 19:42                                             ` Linus Torvalds
2020-09-17 19:55                                               ` John Hubbard
2020-09-17 20:06                                               ` Jason Gunthorpe
2020-09-17 20:19                                                 ` John Hubbard
2020-09-17 20:25                                                   ` Jason Gunthorpe
2020-09-17 20:35                                                 ` Linus Torvalds
2020-09-17 21:40                                                   ` Peter Xu
2020-09-17 22:09                                                     ` Jason Gunthorpe
2020-09-17 22:25                                                       ` Linus Torvalds
2020-09-17 22:48                                                       ` Ira Weiny
2020-09-18  9:36                                                         ` Jan Kara
2020-09-18  9:44                                                       ` Jan Kara
2020-09-18 16:19                                             ` Jason Gunthorpe
2020-09-15 10:23           ` Leon Romanovsky
2020-09-15 15:56           ` Jason Gunthorpe
2020-09-15 15:03   ` Oleg Nesterov
2020-09-15 16:18     ` Peter Xu
2020-08-21 23:49 ` [PATCH 2/4] mm/ksm: Remove reuse_ksm_page() Peter Xu
2020-08-21 23:49 ` [PATCH 3/4] mm/gup: Remove enfornced COW mechanism Peter Xu
2020-09-14 14:27   ` Oleg Nesterov
2020-09-14 17:59     ` Peter Xu
2020-09-14 19:03       ` Linus Torvalds
2020-08-21 23:49 ` [PATCH 4/4] mm: Add PGREUSE counter Peter Xu
2020-08-22 16:14   ` Linus Torvalds
2020-08-24  0:24     ` Peter Xu
2020-08-22 16:05 ` [PATCH 0/4] mm: Simplfy cow handling Linus Torvalds
2020-08-23 23:58   ` Peter Xu
2020-08-24  8:38 ` Kirill Tkhai
2020-08-27 14:15 ` Peter Xu
2021-02-02 14:40 [PATCH 1/4] mm: Trial do_wp_page() simplification Gal Pressman
2021-02-02 16:31 ` Peter Xu
2021-02-02 16:44   ` Jason Gunthorpe
2021-02-02 17:05     ` Peter Xu
2021-02-02 17:13       ` Jason Gunthorpe
2021-02-03 12:43         ` Gal Pressman
2021-02-03 14:00           ` Jason Gunthorpe
2021-02-03 14:47             ` Gal Pressman

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