From: Jason Gunthorpe <jgg@ziepe.ca>
To: Minchan Kim <minchan@kernel.org>
Cc: John Hubbard <jhubbard@nvidia.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
John Dias <joaodias@google.com>,
David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page
Date: Tue, 24 May 2022 12:48:31 -0300 [thread overview]
Message-ID: <20220524154831.GC2661880@ziepe.ca> (raw)
In-Reply-To: <Yoz9H95uXbjoKwdC@google.com>
On Tue, May 24, 2022 at 08:43:27AM -0700, Minchan Kim wrote:
> On Tue, May 24, 2022 at 11:19:37AM -0300, Jason Gunthorpe wrote:
> > On Mon, May 23, 2022 at 10:16:58PM -0700, Minchan Kim wrote:
> > > On Mon, May 23, 2022 at 07:55:25PM -0700, John Hubbard wrote:
> > > > On 5/23/22 09:33, Minchan Kim wrote:
> > > > ...
> > > > > > So then:
> > > > > >
> > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > > > index 0e42038382c1..b404f87e2682 100644
> > > > > > +++ b/mm/page_alloc.c
> > > > > > @@ -482,7 +482,12 @@ unsigned long __get_pfnblock_flags_mask(const struct page *page,
> > > > > > word_bitidx = bitidx / BITS_PER_LONG;
> > > > > > bitidx &= (BITS_PER_LONG-1);
> > > > > >
> > > > > > - word = bitmap[word_bitidx];
> > > > > > + /*
> > > > > > + * This races, without locks, with set_pageblock_migratetype(). Ensure
> > > > > set_pfnblock_flags_mask would be better?
> > > > > > + * a consistent (non-tearing) read of the memory array, so that results,
> > > > >
> > > > > Thanks for proceeding and suggestion, John.
> > > > >
> > > > > IIUC, the load tearing wouldn't be an issue since [1] fixed the issue.
> > > >
> > > > Did it? [1] fixed something, but I'm not sure we can claim that that
> > > > code is now safe against tearing in all possible cases, especially given
> > > > the recent discussion here. Specifically, having this code do a read,
> > > > then follow that up with calculations, seems correct. Anything else is
> > >
> > > The load tearing you are trying to explain in the comment would be
> > > solved by [1] since the bits will always align on a word and accessing
> > > word size based on word aligned address is always atomic so there is
> > > no load tearing problem IIUC.
> >
> > That is not technically true. It is exactly the sort of thing
> > READ_ONCE is intended to guard against.
>
> Oh, does word access based on the aligned address still happen
> load tearing?
>
> I just referred to
> https://elixir.bootlin.com/linux/latest/source/Documentation/memory-barriers.txt#L1759
I read that as saying load tearing is technically allowed but doesn't
happen in gcc, and so must use the _ONCE macros.
> I didn't say it doesn't refetch the value without the READ_ONCE.
>
> What I am saying is READ_ONCE(bitmap_word_bitidx] prevents "refetching"
> issue rather than "tearing" issue in specific __get_pfnblock_flags_mask
> context because I though there is no load-tearing issue there since
> bitmap is word-aligned/accessed. No?
It does both. AFAIK our memory model has no guarentees on what naked C
statements will do. Tearing, multi-load, etc - it is all technically
permitted. Use the proper accessors.
Jason
next prev parent reply other threads:[~2022-05-24 15:48 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-10 21:17 [PATCH v4] mm: fix is_pinnable_page against on cma page Minchan Kim
2022-05-10 22:56 ` John Hubbard
2022-05-10 23:31 ` Minchan Kim
2022-05-10 23:58 ` John Hubbard
2022-05-11 0:09 ` Minchan Kim
2022-05-11 4:32 ` John Hubbard
2022-05-11 21:46 ` Minchan Kim
2022-05-11 22:25 ` John Hubbard
2022-05-11 22:37 ` Minchan Kim
2022-05-11 22:49 ` John Hubbard
2022-05-11 23:08 ` Minchan Kim
2022-05-11 23:13 ` John Hubbard
2022-05-11 23:15 ` Minchan Kim
2022-05-11 23:28 ` Minchan Kim
2022-05-11 23:33 ` John Hubbard
2022-05-11 23:45 ` Paul E. McKenney
2022-05-11 23:57 ` John Hubbard
2022-05-12 0:12 ` Paul E. McKenney
2022-05-12 0:12 ` John Hubbard
2022-05-12 0:22 ` Paul E. McKenney
2022-05-12 0:26 ` Minchan Kim
2022-05-12 0:34 ` John Hubbard
2022-05-12 0:49 ` Paul E. McKenney
2022-05-12 1:02 ` John Hubbard
2022-05-12 1:03 ` Minchan Kim
2022-05-12 1:08 ` John Hubbard
2022-05-12 2:18 ` John Hubbard
2022-05-12 3:44 ` Minchan Kim
2022-05-12 4:47 ` John Hubbard
2022-05-17 14:00 ` Jason Gunthorpe
2022-05-17 18:12 ` John Hubbard
2022-05-17 19:28 ` Jason Gunthorpe
2022-05-17 20:12 ` John Hubbard
2022-05-17 20:21 ` Paul E. McKenney
2022-05-23 16:33 ` Minchan Kim
2022-05-24 2:55 ` John Hubbard
2022-05-24 5:16 ` Minchan Kim
2022-05-24 6:22 ` John Hubbard
2022-05-24 14:19 ` Jason Gunthorpe
2022-05-24 15:43 ` Minchan Kim
2022-05-24 15:48 ` Jason Gunthorpe [this message]
2022-05-24 16:37 ` Paul E. McKenney
2022-05-24 16:59 ` Minchan Kim
2022-05-12 3:57 ` Paul E. McKenney
2022-05-12 1:03 ` Minchan Kim
2022-05-12 0:35 ` Paul E. McKenney
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