Re: [PATCH mm-unstable v2] mm/hugetlb_vmemmap: fix memory loads ordering

[Will Deacon <will@kernel.org>]

On Wed, Jan 08, 2025 at 12:48:21AM -0700, Yu Zhao wrote:
> Using x86_64 as an example, for a 32KB struct page[] area describing a
> 2MB hugeTLB, HVO reduces the area to 4KB by the following steps:
> 1. Split the (r/w vmemmap) PMD mapping the area into 512 (r/w) PTEs;
> 2. For the 8 PTEs mapping the area, remap PTE 1-7 to the page mapped
>    by PTE 0, and at the same time change the permission from r/w to
>    r/o;
> 3. Free the pages PTE 1-7 used to map, hence the reduction from 32KB
>    to 4KB.
> 
> However, the following race can happen due to improperly memory loads
> ordering:
>   CPU 1 (HVO)                     CPU 2 (speculative PFN walker)
> 
>   page_ref_freeze()
>   synchronize_rcu()
>                                   rcu_read_lock()
>                                   page_is_fake_head() is false
>   vmemmap_remap_pte()
>   XXX: struct page[] becomes r/o
> 
>   page_ref_unfreeze()
>                                   page_ref_count() is not zero
> 
>                                   atomic_add_unless(&page->_refcount)
>                                   XXX: try to modify r/o struct page[]
> 
> Specifically, page_is_fake_head() must be ordered after
> page_ref_count() on CPU 2 so that it can only return true for this
> case, to avoid the later attempt to modify r/o struct page[].
> 
> This patch adds the missing memory barrier and makes the tests on
> page_is_fake_head() and page_ref_count() done in the proper order.
> 
> Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
> Reported-by: Will Deacon <will@kernel.org>
> Closes: https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  include/linux/page-flags.h | 37 +++++++++++++++++++++++++++++++++++++
>  include/linux/page_ref.h   |  2 +-
>  2 files changed, 38 insertions(+), 1 deletion(-)

Sorry for the very late reply, but I finally found time to sit down and
go through this. I think it resolves the problem I pointed out, so:

Acked-by: Will Deacon <will@kernel.org>

Thanks!

Will