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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Peter Xu <peterx@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
	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: Thu, 17 Sep 2020 08:25:38 -0300
Message-ID: <20200917112538.GD8409@ziepe.ca> (raw)
In-Reply-To: <20200916184619.GB40154@xz-x1>

On Wed, Sep 16, 2020 at 02:46:19PM -0400, Peter Xu wrote:

> My understanding is this may only work for the case when the fork()ed child
> quitted before we reach here (so we still have mapcount==1 for the
> page).  

Yes

> What if not?  Then mapcount will be greater than 1, and cow will
> still trigger.  Is that what we want?

That doesn't work today anyhow, so it is fine continuing to be broken.

> Another problem is that, aiui, one of the major change previous patch proposed
> is to avoid using lock_page() so that we never block in this path.

I saw you mention this before, but it looks like the change was to
lift some of the atomc_reads out of the lock and avoid the lock if
they indicate failure, checking also for page_maybe_dma_pinned()
outside the lock just means the rare case of FOLL_PIN we will take the
lock again.

> Maybe even more complicated, because "correctness" should be even harder
> than "best effort reuse" since it can cause data corruption if we didn't do it
> right...

The only correct way is for the application to avoid write protect on
FOLL_PIN pages. The purpose here is to allow applications that hadn't
hit "bad luck" and failed to keep working.

Another thought is to insert a warning print here as well that the
program is working improperly? At least it would give a transition
period to evaluate the extent of the problem.

We are thinking it is going to be a notable regression.

I botched the last version of the patch, here is something a bit
better.

Does it seem like it could be OK? I know very little about this part
of the kernel

Thanks,
Jason

diff --git a/mm/memory.c b/mm/memory.c
index 469af373ae76e1..332de777854f8b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2889,6 +2889,24 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
 	return ret;
 }
 
+static bool cow_needed(struct vm_fault *vmf)
+{
+	int total_map_swapcount;
+
+	if (!reuse_swap_page(vmf->page, &total_map_swapcount))
+		return true;
+
+	if (total_map_swapcount == 1) {
+		/*
+		 * The page is all ours. Move it to our anon_vma so the rmap
+		 * code will not search our parent or siblings.  Protected
+		 * against the rmap code by the page lock.
+		 */
+		page_move_anon_rmap(vmf->page, vmf->vma);
+	}
+	return false;
+}
+
 /*
  * This routine handles present pages, when users try to write
  * to a shared page. It is done by copying the page to a new address
@@ -2942,13 +2960,27 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		struct page *page = vmf->page;
 
 		/* PageKsm() doesn't necessarily raise the page refcount */
-		if (PageKsm(page) || page_count(page) != 1)
+		if (PageKsm(page))
 			goto copy;
+		if (page_count(page) != 1) {
+			/*
+			 * If the page is DMA pinned we can't rely on the
+			 * above to know if there are other CPU references as
+			 * page_count() will be elevated by the
+			 * pin. Needlessly copying the page will cause the DMA
+			 * pin to break, try harder to avoid that.
+			 */
+			if (!page_maybe_dma_pinned(page))
+				goto copy;
+		}
+
 		if (!trylock_page(page))
 			goto copy;
 		if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
-			unlock_page(page);
-			goto copy;
+			if (cow_needed(vmf)) {
+				unlock_page(page);
+				goto copy;
+			}
 		}
 		/*
 		 * Ok, we've got the only map reference, and the only

  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
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 [this message]
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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