> On Jan 8, 2025, at 15:48, Yu Zhao <yuzhao@xxxxxxxxxx> 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@xxxxxxxxxx>
> Closes: https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/
> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>
Thanks.
