Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

[Minchan Kim <minchan@kernel.org>]

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!