linux-kernel - Re: [PATCH v4] mm: fix is_pinnable

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Date:   Tue, 24 May 2022 12:48:31 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Minchan Kim <minchan@...nel.org>
Cc:     John Hubbard <jhubbard@...dia.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Dias <joaodias@...gle.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

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