[PATCH v2 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping

[Muchun Song]

On Sun, May 08, 2022 at 05:36:41PM +0800, Baolin Wang wrote:
> On some architectures (like ARM64), it can support CONT-PTE/PMD size
> hugetlb, which means it can support not only PMD/PUD size hugetlb:
> 2M and 1G, but also CONT-PTE/PMD size: 64K and 32M if a 4K page
> size specified.
> 
> When unmapping a hugetlb page, we will get the relevant page table
> entry by huge_pte_offset() only once to nuke it. This is correct
> for PMD or PUD size hugetlb, since they always contain only one
> pmd entry or pud entry in the page table.
> 
> However this is incorrect for CONT-PTE and CONT-PMD size hugetlb,
> since they can contain several continuous pte or pmd entry with
> same page table attributes, so we will nuke only one pte or pmd
> entry for this CONT-PTE/PMD size hugetlb page.
> 
> And now try_to_unmap() is only passed a hugetlb page in the case
> where the hugetlb page is poisoned. Which means now we will unmap
> only one pte entry for a CONT-PTE or CONT-PMD size poisoned hugetlb
> page, and we can still access other subpages of a CONT-PTE or CONT-PMD
> size poisoned hugetlb page, which will cause serious issues possibly.
> 
> So we should change to use huge_ptep_clear_flush() to nuke the
> hugetlb page table to fix this issue, which already considered
> CONT-PTE and CONT-PMD size hugetlb.
> 
> We've already used set_huge_swap_pte_at() to set a poisoned
> swap entry for a poisoned hugetlb page. Meanwhile adding a VM_BUG_ON()
> to make sure the passed hugetlb page is poisoned in try_to_unmap().
> 
> Signed-off-by: Baolin Wang <baolin.wang at linux.alibaba.com>

Reviewed-by: Muchun Song <songmuchun at bytedance.com>

Thanks.