Re: [PATCH] mm/damon/vaddr: Safely walk page table

["Boehme, Markus" <markubo@amazon.de>]

On Fri, 2021-08-27 at 15:04 +0000, SeongJae Park wrote:
> 
> From: SeongJae Park <sjpark@amazon.de>
> 
> Commit d7f647622761 ("mm/damon: implement primitives for the virtual
> memory address spaces") of linux-mm[1] tries to find PTE or PMD for
> arbitrary virtual address using 'follow_invalidate_pte()' without proper
> locking[2].  This commit fixes the issue by using another page table
> walk function for more general use case under proper locking.
> 
> [1] https://github.com/hnaz/linux-mm/commit/d7f647622761
> [2] https://lore.kernel.org/linux-mm/3b094493-9c1e-6024-bfd5-7eca66399b7e@redhat.com
> 
> Fixes: d7f647622761 ("mm/damon: implement primitives for the virtual memory address spaces")
> Reported-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>  mm/damon/vaddr.c | 81 +++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 74 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index 230db7413278..b3677f2ef54b 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c
> @@ -8,10 +8,12 @@
>  #define pr_fmt(fmt) "damon-va: " fmt
> 
>  #include <linux/damon.h>
> +#include <linux/hugetlb.h>
>  #include <linux/mm.h>
>  #include <linux/mmu_notifier.h>
>  #include <linux/highmem.h>
>  #include <linux/page_idle.h>
> +#include <linux/pagewalk.h>
>  #include <linux/random.h>
>  #include <linux/sched/mm.h>
>  #include <linux/slab.h>
> @@ -446,14 +448,69 @@ static void damon_pmdp_mkold(pmd_t *pmd, struct mm_struct *mm,
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  }
> 
> +struct damon_walk_private {
> +       pmd_t *pmd;
> +       pte_t *pte;
> +       spinlock_t *ptl;
> +};
> +
> +static int damon_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
> +               struct mm_walk *walk)
> +{
> +       struct damon_walk_private *priv = walk->private;
> +
> +       if (pmd_huge(*pmd)) {
> +               priv->ptl = pmd_lock(walk->mm, pmd);
> +               if (pmd_huge(*pmd)) {
> +                       priv->pmd = pmd;
> +                       return 0;
> +               }
> +               spin_unlock(priv->ptl);
> +       }
> +
> +       if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
> +               return -EINVAL;
> +       priv->pte = pte_offset_map_lock(walk->mm, pmd, addr, &priv->ptl);
> +       if (!pte_present(*priv->pte)) {
> +               pte_unmap_unlock(priv->pte, priv->ptl);
> +               priv->pte = NULL;
> +               return -EINVAL;
> +       }
> +       return 0;
> +}
> +
> +static struct mm_walk_ops damon_walk_ops = {
> +       .pmd_entry = damon_pmd_entry,
> +};
> +
> +int damon_follow_pte_pmd(struct mm_struct *mm, unsigned long addr,
> +               struct damon_walk_private *private)
> +{
> +       int rc;
> +
> +       private->pte = NULL;
> +       private->pmd = NULL;
> +       rc = walk_page_range(mm, addr, addr + 1, &damon_walk_ops, private);
> +       if (!rc && !private->pte && !private->pmd)
> +               return -EINVAL;
> +       return rc;
> +}
> +
>  static void damon_va_mkold(struct mm_struct *mm, unsigned long addr)
>  {
> -       pte_t *pte = NULL;
> -       pmd_t *pmd = NULL;
> +       struct damon_walk_private walk_result;
> +       pte_t *pte;
> +       pmd_t *pmd;
>         spinlock_t *ptl;
> 
> -       if (follow_invalidate_pte(mm, addr, NULL, &pte, &pmd, &ptl))
> +       mmap_write_lock(mm);
> +       if (damon_follow_pte_pmd(mm, addr, &walk_result)) {
> +               mmap_write_unlock(mm);
>                 return;
> +       }
> +       pte = walk_result.pte;
> +       pmd = walk_result.pmd;
> +       ptl = walk_result.ptl;
> 
>         if (pte) {
>                 damon_ptep_mkold(pte, mm, addr);
> @@ -462,6 +519,7 @@ static void damon_va_mkold(struct mm_struct *mm, unsigned long addr)
>                 damon_pmdp_mkold(pmd, mm, addr);
>                 spin_unlock(ptl);
>         }
> +       mmap_write_unlock(mm);
>  }
> 
>  /*
> @@ -495,14 +553,21 @@ void damon_va_prepare_access_checks(struct damon_ctx *ctx)
>  static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
>                         unsigned long *page_sz)
>  {
> -       pte_t *pte = NULL;
> -       pmd_t *pmd = NULL;
> +       struct damon_walk_private walk_result;
> +       pte_t *pte;
> +       pmd_t *pmd;
>         spinlock_t *ptl;
>         struct page *page;
>         bool young = false;
> 
> -       if (follow_invalidate_pte(mm, addr, NULL, &pte, &pmd, &ptl))
> +       mmap_write_lock(mm);
> +       if (damon_follow_pte_pmd(mm, addr, &walk_result)) {
> +               mmap_write_unlock(mm);
>                 return false;
> +       }
> +       pte = walk_result.pte;
> +       pmd = walk_result.pmd;
> +       ptl = walk_result.ptl;
> 
>         *page_sz = PAGE_SIZE;
>         if (pte) {
> @@ -513,7 +578,7 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
>                 if (page)
>                         put_page(page);
>                 pte_unmap_unlock(pte, ptl);
> -               return young;
> +               goto out;
>         }
> 
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> @@ -528,6 +593,8 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
>         *page_sz = ((1UL) << HPAGE_PMD_SHIFT);
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> 
> +out:
> +       mmap_write_unlock(mm);
>         return young;
>  }
> 
> --
> 2.17.1
> 

Reviewed-by: Markus Boehme <markubo@amazon.de>



Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879