From: Peter Xu <peterx@redhat.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Michal Hocko <mhocko@suse.com>,
Kirill Shutemov <kirill@shutemov.name>,
Jann Horn <jannh@google.com>, Kirill Tkhai <ktkhai@virtuozzo.com>,
Hugh Dickins <hughd@google.com>,
Leon Romanovsky <leonro@nvidia.com>, Jan Kara <jack@suse.cz>,
John Hubbard <jhubbard@nvidia.com>,
Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>,
Jason Gunthorpe <jgg@ziepe.ca>,
Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes
Date: Tue, 22 Sep 2020 14:34:38 -0400 [thread overview]
Message-ID: <20200922183438.GL19098@xz-x1> (raw)
In-Reply-To: <20200922165216.GF11679@redhat.com>
On Tue, Sep 22, 2020 at 06:52:17PM +0200, Oleg Nesterov wrote:
> On 09/22, Peter Xu wrote:
> >
> > On Tue, Sep 22, 2020 at 02:40:14PM +0200, Oleg Nesterov wrote:
> > > On 09/22, Oleg Nesterov wrote:
> > > >
> > > > On 09/21, Peter Xu wrote:
> > > > >
> > > > > @@ -859,6 +989,25 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > > > > spin_needbreak(src_ptl) || spin_needbreak(dst_ptl))
> > > > > break;
> > > > > }
> > > > > +
> > > > > + if (unlikely(data.cow_new_page)) {
> > > > > + /*
> > > > > + * If cow_new_page set, we must be at the 2nd round of
> > > > > + * a previous COPY_MM_BREAK_COW. Try to arm the new
> > > > > + * page now. Note that in all cases page_break_cow()
> > > > > + * will properly release the objects in copy_mm_data.
> > > > > + */
> > > > > + WARN_ON_ONCE(copy_ret != COPY_MM_BREAK_COW);
> > > > > + if (pte_install_copied_page(dst_mm, new, src_pte,
> > > > > + dst_pte, addr, rss,
> > > > > + &data)) {
> > > > > + /* We installed the pte successfully; move on */
> > > > > + progress++;
> > > > > + continue;
> > > >
> > > > I'm afraid I misread this patch too ;)
> > > >
> > > > But it seems to me in this case the main loop can really "leak"
> > > > COPY_MM_BREAK_COW. Suppose the the next 31 pte's are pte_none() and
> > > > need_resched() is true.
> > > >
> > > > No?
> >
> > I still think it's a no...
> >
> > Note that now we'll reset "progress" every time before the do loop, so we'll
> > never reach need_resched() (since progress<32) before pte_install_copied_page()
> > when needed.
>
> Yes. But copy_ret is still COPY_MM_BREAK_COW after pte_install_copied_page().
> Now suppose that the next 31 pte's are pte_none(), progress will be incremented
> every time.
Yes, I think you're right - I'll need to reset that.
>
> > I explicitly put the pte_install_copied_page() into the loop just...
> ...
> > > progress = 0;
> > > + if (unlikely(copy_ret == COPY_MM_BREAK_COW)) {
> > > + /*
> > > + * Note that in all cases pte_install_copied_page()
> > > + * will properly release the objects in copy_mm_data.
> > > + */
> > > + copy_ret = COPY_MM_DONE;
> > > + if (pte_install_copied_page(dst_mm, new, src_pte,
> > > + dst_pte, addr, rss,
> > > + &data)) {
> > > + /* We installed the pte successfully; move on */
> > > + progress++;
> > > + goto next;
> >
> > ... to avoid jumps like this because I think it's really tricky. :)
>
> To me it looks better before the main loop because we know that
> data.cow_new_page != NULL is only possible at the 1st iterattion after
> restart ;)
>
> But I agree, this is subjective, please ignore.
Thanks. For simplicity, I'll keep the code majorly as is. But I'm still open
to change if e.g. someone else still perfers the other way.
> However, I still think
> it is better to rely on the copy_ret == COPY_MM_BREAK_COW check rather
> than data.cow_new_page != NULL.
Yes. Logically we should check both, but now as I'm written it as:
if (unlikely(data.cow_new_page)) {
WARN_ON_ONCE(copy_ret != COPY_MM_BREAK_COW);
...
}
I think it's even safer because it's actually checking both, but also warn if
only cow_new_page is set, which should never happen anyways.
Or I can also do it in inverted order if you think better:
if (unlikely(copy_ret == COPY_MM_BREAK_COW)) {
WARN_ON_ONCE(!data.cow_new_page);
...
}
>
> > > case COPY_MM_SWAP_CONT:
> > > if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0)
> > > return -ENOMEM;
> > > - break;
> > > + copy_ret = COPY_MM_DONE;
> >
> > Kind of a continuation of the discussion from previous patch - I think we'd
> > better reset copy_ret not only for this case, but move it after the switch
> > (just in case there'll be new ones). The new BREAK_COW uses goto so it's quite
> > special.
> >
> > > + goto again;
> >
> > I feel like this could go wrong without the "addr != end" check later, when
> > this is the last pte to check.
>
> How? We know that copy_one_pte() failed and returned COPY_MM_SWAP_CONT
> before addr = end.
I think you're right, again. :)
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2020-09-22 18:34 UTC|newest]
Thread overview: 110+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-21 21:17 [PATCH 0/5] mm: Break COW for pinned pages during fork() Peter Xu
2020-09-21 21:17 ` [PATCH 1/5] mm: Introduce mm_struct.has_pinned Peter Xu
2020-09-21 21:43 ` Jann Horn
2020-09-21 22:30 ` Peter Xu
2020-09-21 22:47 ` Jann Horn
2020-09-22 11:54 ` Jason Gunthorpe
2020-09-22 14:28 ` Peter Xu
2020-09-22 15:56 ` Jason Gunthorpe
2020-09-22 16:25 ` Linus Torvalds
2020-09-21 23:53 ` John Hubbard
2020-09-22 0:01 ` John Hubbard
2020-09-22 15:17 ` Peter Xu
2020-09-22 16:10 ` Jason Gunthorpe
2020-09-22 17:54 ` Peter Xu
2020-09-22 19:11 ` Jason Gunthorpe
2020-09-23 0:27 ` Peter Xu
2020-09-23 13:10 ` Peter Xu
2020-09-23 14:20 ` Jan Kara
2020-09-23 17:12 ` Jason Gunthorpe
2020-09-24 7:44 ` Jan Kara
2020-09-24 14:02 ` Jason Gunthorpe
2020-09-24 14:45 ` Jan Kara
2020-09-23 17:07 ` Jason Gunthorpe
2020-09-24 14:35 ` Peter Xu
2020-09-24 16:51 ` Jason Gunthorpe
2020-09-24 17:55 ` Peter Xu
2020-09-24 18:15 ` Jason Gunthorpe
2020-09-24 18:34 ` Peter Xu
2020-09-24 18:39 ` Jason Gunthorpe
2020-09-24 21:30 ` Peter Xu
2020-09-25 19:56 ` Linus Torvalds
2020-09-25 21:06 ` Linus Torvalds
2020-09-26 0:41 ` Jason Gunthorpe
2020-09-26 1:15 ` Linus Torvalds
2020-09-26 22:28 ` Linus Torvalds
2020-09-27 6:23 ` Leon Romanovsky
2020-09-27 18:16 ` Linus Torvalds
2020-09-27 18:45 ` Linus Torvalds
2020-09-28 12:49 ` Jason Gunthorpe
2020-09-28 16:17 ` Linus Torvalds
2020-09-28 17:22 ` Peter Xu
2020-09-28 17:54 ` Linus Torvalds
2020-09-28 18:39 ` Jason Gunthorpe
2020-09-28 19:29 ` Linus Torvalds
2020-09-28 23:57 ` Jason Gunthorpe
2020-09-29 0:18 ` John Hubbard
2020-09-28 19:36 ` Linus Torvalds
2020-09-28 19:50 ` Linus Torvalds
2020-09-28 22:51 ` Jason Gunthorpe
2020-09-29 0:30 ` Peter Xu
2020-10-08 5:49 ` Leon Romanovsky
2020-09-28 17:13 ` Peter Xu
2020-09-25 21:13 ` Peter Xu
2020-09-25 22:08 ` Linus Torvalds
2020-09-22 18:02 ` John Hubbard
2020-09-22 18:15 ` Peter Xu
2020-09-22 19:11 ` John Hubbard
2020-09-27 0:41 ` [mm] 698ac7610f: will-it-scale.per_thread_ops 8.2% improvement kernel test robot
2020-09-21 21:17 ` [PATCH 2/5] mm/fork: Pass new vma pointer into copy_page_range() Peter Xu
2020-09-21 21:17 ` [PATCH 3/5] mm: Rework return value for copy_one_pte() Peter Xu
2020-09-22 7:11 ` John Hubbard
2020-09-22 15:29 ` Peter Xu
2020-09-22 10:08 ` Oleg Nesterov
2020-09-22 10:18 ` Oleg Nesterov
2020-09-22 15:36 ` Peter Xu
2020-09-22 15:48 ` Oleg Nesterov
2020-09-22 16:03 ` Peter Xu
2020-09-22 16:53 ` Oleg Nesterov
2020-09-22 18:13 ` Peter Xu
2020-09-22 18:23 ` Oleg Nesterov
2020-09-22 18:49 ` Peter Xu
2020-09-23 6:52 ` Oleg Nesterov
2020-09-23 17:16 ` Linus Torvalds
2020-09-23 21:24 ` Linus Torvalds
2020-09-21 21:20 ` [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes Peter Xu
2020-09-21 21:55 ` Jann Horn
2020-09-21 22:18 ` John Hubbard
2020-09-21 22:27 ` Jann Horn
2020-09-22 0:08 ` John Hubbard
2020-09-21 22:27 ` Peter Xu
2020-09-22 11:48 ` Oleg Nesterov
2020-09-22 12:40 ` Oleg Nesterov
2020-09-22 15:58 ` Peter Xu
2020-09-22 16:52 ` Oleg Nesterov
2020-09-22 18:34 ` Peter Xu [this message]
2020-09-22 18:44 ` Oleg Nesterov
2020-09-23 1:03 ` Peter Xu
2020-09-23 20:25 ` Linus Torvalds
2020-09-24 15:08 ` Peter Xu
2020-09-24 11:48 ` Kirill Tkhai
2020-09-24 15:16 ` Peter Xu
2020-09-21 21:20 ` [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork() Peter Xu
2020-09-22 6:41 ` John Hubbard
2020-09-22 10:33 ` Jan Kara
2020-09-22 20:01 ` John Hubbard
2020-09-23 9:22 ` Jan Kara
2020-09-23 13:50 ` Peter Xu
2020-09-23 14:01 ` Jan Kara
2020-09-23 15:44 ` Peter Xu
2020-09-23 20:19 ` John Hubbard
2020-09-24 18:49 ` Peter Xu
2020-09-23 16:06 ` Peter Xu
2020-09-22 12:05 ` Jason Gunthorpe
2020-09-23 15:24 ` Peter Xu
2020-09-23 16:07 ` Yang Shi
2020-09-24 15:47 ` Peter Xu
2020-09-24 17:29 ` Yang Shi
2020-09-23 17:17 ` Jason Gunthorpe
2020-09-23 10:21 ` [PATCH 0/5] mm: Break COW for pinned pages during fork() Leon Romanovsky
2020-09-23 15:37 ` Peter Xu
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