From: Peter Xu <peterx@redhat.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
"Maya B . Gokhale" <gokhale2@llnl.gov>,
Linus Torvalds <torvalds@linux-foundation.org>,
Yang Shi <yang.shi@linux.alibaba.com>,
Marty Mcfadden <mcfadden8@llnl.gov>,
Kirill Shutemov <kirill@shutemov.name>,
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 3/4] mm/gup: Remove enfornced COW mechanism
Date: Mon, 14 Sep 2020 13:59:46 -0400 [thread overview]
Message-ID: <20200914175946.GC30881@xz-x1> (raw)
In-Reply-To: <20200914142722.GA21906@redhat.com>
On Mon, Sep 14, 2020 at 04:27:24PM +0200, Oleg Nesterov wrote:
> On 08/21, Peter Xu wrote:
> >
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -381,22 +381,13 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
> > }
> >
> > /*
> > - * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
> > - * but only after we've gone through a COW cycle and they are dirty.
> > + * FOLL_FORCE can write to even unwritable pte's, but only
> > + * after we've gone through a COW cycle and they are dirty.
> > */
> > static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> > {
> > - return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
> > -}
> > -
> > -/*
> > - * A (separate) COW fault might break the page the other way and
> > - * get_user_pages() would return the page from what is now the wrong
> > - * VM. So we need to force a COW break at GUP time even for reads.
> > - */
> > -static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
> > -{
> > - return is_cow_mapping(vma->vm_flags) && (flags & (FOLL_GET | FOLL_PIN));
> > + return pte_write(pte) ||
> > + ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
>
> Do we really need to add the FOLL_FORCE check back?
>
> Afaics, FOLL_COW is only possible if FOLL_FORCE was set.
When I proposed the patch I wanted to add back FOLL_FORCE because the previous
removing of FOLL_FORCE should be related to the enforced COW mechanism where
FOLL_COW can definitely happen without FOLL_FORCE. So when we want to revert
the enforced COW we definitely need to recover this check too as it was. I
didn't think deeper than that.
However now I'm a bit confused on why FOLL_COW must be with FOLL_FORCE even
without the enforced COW... Shouldn't FOLL_COW be able to happen even without
FOLL_FORCE (as long as when a page is shared, and the gup is with WRITE
permission)? Not sure what I've missed, though.
--
Peter Xu
next prev parent reply other threads:[~2020-09-14 18:00 UTC|newest]
Thread overview: 90+ 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
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
2022-02-16 16:59 ` Oded Gabbay
2022-02-16 17:24 ` Oded Gabbay
2022-02-16 19:04 ` Linus Torvalds
2022-02-16 19:20 ` Oded Gabbay
2022-02-16 19:24 ` David Hildenbrand
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 [this message]
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
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