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    SubjectRe: [PATCH v4] mm: fix is_pinnable_page against on cma page
    On Tue, May 24, 2022 at 09:37:28AM -0700, Paul E. McKenney wrote:
    > On Tue, May 24, 2022 at 12:48:31PM -0300, Jason Gunthorpe wrote:
    > > 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.
    >
    > This is in fact the intent, except...
    >
    > And as that passage goes on to state, there really are compilers (such
    > as GCC) that tear stores of constants to machine aligned/sized locations.
    >
    > In short, use of the _ONCE() macros can save you a lot of pain.

    Thanks for the correction, Jason and Paul

    >
    > > > 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.

    Seems like there was some misunderstanding here.

    I didn't mean not to use READ_ONCE for the bitmap but wanted to have
    more concrete comment. Since you guys corrected "even though word-alinged
    access could be wrong without READ_ONCE", I would keep the comment John
    suggested.

    >
    > I am with Jason on this one.
    >
    > In fact, I believe that any naked C-language access to mutable shared
    > variables should have a comment stating why the compiler cannot mangle
    > that access.

    Agreed.

    Thanks!

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