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

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Date:   Tue, 24 May 2022 11:19:37 -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 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.

> Instead of the tearing problem, what we are trying to solve with
> READ_ONCE is to prevent refetching when the function would be
> inlined in the future.

It is the same problem, who is to say it doesn't refetch while doing
the maths?

Jason