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

[Minchan Kim <minchan@kernel.org>]

On Tue, May 17, 2022 at 01:12:02PM -0700, John Hubbard wrote:
> On 5/17/22 12:28, Jason Gunthorpe wrote:
> > > If you compare this to the snippet above, you'll see that there is
> > > an extra mov statement, and that one dereferences a pointer from
> > > %rax:
> > > 
> > >      mov    (%rax),%rbx
> > 
> > That is the same move as:
> > 
> >     mov    0x8(%rdx,%rax,8),%rbx
> > 
> > Except that the EA calculation was done in advance and stored in rax.
> > 
> > lea isn't a memory reference, it is just computing the pointer value
> > that 0x8(%rdx,%rax,8) represents. ie the lea computes
> > 
> >    %rax = %rdx + %rax*8 + 6
> > 
> > Which is then fed into the mov. Maybe it is an optimization to allow
> > one pipe to do the shr and an other to the EA - IDK, seems like a
> > random thing for the compiler to do.
> 
> Apologies for getting that wrong, and thanks for walking me through the
> asm.
> 
> [...]
> > 
> > Paul can correct me, but I understand we do not have a list of allowed
> > operations that are exempted from the READ_ONCE() requirement. ie it
> > is not just conditional branching that requires READ_ONCE().
> > 
> > This is why READ_ONCE() must always be on the memory load, because the
> > point is to sanitize away the uncertainty that comes with an unlocked
> > read of unstable memory contents. READ_ONCE() samples the value in
> > memory, and removes all tearing, multiload, etc "instability" that may
> > effect down stream computations. In this way down stream compulations
> > become reliable.
> > 
> > Jason
> 
> So then:
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0e42038382c1..b404f87e2682 100644
> --- a/mm/page_alloc.c
> +++ 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. 

The concern in our dicussion was aggressive compiler(e.g., LTO) or code refactoring
to make the code inline in *future* could potentially cause forcing refetching(i.e.,
re-read) tie bitmap[word_bitidx].

If so, shouldn't the comment be the one you helped before?

/*
 * Defend against future compiler LTO features, or code refactoring
 * that inlines the above function, by forcing a single read. Because,
 * re-reads of bitmap[word_bitidx] by inlining could cause trouble
 * for whom believe they use a local variable for the value.
 */

[1] e58469bafd05, mm: page_alloc: use word-based accesses for get/set pageblock bitmaps

> +        * even though racy, are not corrupted.
> +        */
> +       word = READ_ONCE(bitmap[word_bitidx]);
>         return (word >> bitidx) & mask;
>  }
> 
> 
> thanks,
> -- 
> John Hubbard
> NVIDIA