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

[Minchan Kim <minchan@kernel.org>]

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

> 
> > 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?

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?

If the load tearing would still happens in the bitmap, it would be
better to keep last suggestion from John.

+       /*
+        * This races, without locks, with set_pfnblock_flags_mask(). Ensure
+        * a consistent read of the memory array, so that results, even though
+        *  racy, are not corrupted.
+        */