Re: [PATCH v3 2/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when migration

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 10 May 2022 11:45:59 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> 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 migrating a hugetlb page, we will get the relevant page table
> entry by huge_pte_offset() only once to nuke it and remap it with
> a migration pte entry. 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 or remap only one pte
> or pmd entry for this CONT-PTE/PMD size hugetlb page, which is
> not expected for hugetlb migration. The problem is we can still
> continue to modify the subpages' data of a hugetlb page during
> migrating a hugetlb page, which can cause a serious data consistent
> issue, since we did not nuke the page table entry and set a
> migration pte for the subpages of a hugetlb page.
> 
> To fix this issue, we should change to use huge_ptep_clear_flush()
> to nuke a hugetlb page table, and remap it with set_huge_pte_at()
> and set_huge_swap_pte_at() when migrating a hugetlb page, which
> already considered the CONT-PTE or CONT-PMD size hugetlb.
> 
> ...
>
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1093,6 +1093,17 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
>  					pte_t *ptep, pte_t pte, unsigned long sz)
>  {
>  }
> +
> +static inline pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
> +					  unsigned long addr, pte_t *ptep)
> +{
> +	return ptep_get(ptep);
> +}
> +
> +static inline void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
> +				   pte_t *ptep, pte_t pte)
> +{
> +}
>  #endif	/* CONFIG_HUGETLB_PAGE */
>  

This blows up nommu (arm allnoconfig):

In file included from fs/io_uring.c:71:
./include/linux/hugetlb.h: In function 'huge_ptep_clear_flush':
./include/linux/hugetlb.h:1100:16: error: implicit declaration of function 'ptep_get' [-Werror=implicit-function-declaration]
 1100 |         return ptep_get(ptep);
      |                ^~~~~~~~


huge_ptep_clear_flush() is only used in CONFIG_NOMMU=n files, so I simply
zapped this change.

--- a/include/linux/hugetlb.h~mm-rmap-fix-cont-pte-pmd-size-hugetlb-issue-when-migration-fix
+++ a/include/linux/hugetlb.h
@@ -1093,17 +1093,6 @@ static inline void set_huge_swap_pte_at(
 					pte_t *ptep, pte_t pte, unsigned long sz)
 {
 }
-
-static inline pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
-					  unsigned long addr, pte_t *ptep)
-{
-	return ptep_get(ptep);
-}
-
-static inline void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
-				   pte_t *ptep, pte_t pte)
-{
-}
 #endif	/* CONFIG_HUGETLB_PAGE */
 
 static inline spinlock_t *huge_pte_lock(struct hstate *h,
_