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b98072af60
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.
Link: https://lkml.kernel.org/r/20250108074822.722696-1-yuzhao@google.com
Fixes:
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