Re: [PATCH] thp: move preallocated PTE page table on move_huge_pmd()

[Andrey Wagin <avagin@gmail.com>]

2013/12/4 Kirill A. Shutemov <kirill.shutemov@linux.intel.com>:
> Andrey Wagin reported crash on VM_BUG_ON() in pgtable_pmd_page_dtor()
> with fallowing backtrace:
>
>   [<ffffffff8119427f>] free_pgd_range+0x2bf/0x410
>   [<ffffffff8119449e>] free_pgtables+0xce/0x120
>   [<ffffffff8119b900>] unmap_region+0xe0/0x120
>   [<ffffffff811a0036>] ? move_page_tables+0x526/0x6b0
>   [<ffffffff8119d6a9>] do_munmap+0x249/0x360
>   [<ffffffff811a0304>] move_vma+0x144/0x270
>   [<ffffffff811a07e9>] SyS_mremap+0x3b9/0x510
>   [<ffffffff8172d512>] system_call_fastpath+0x16/0x1b
>
> The crash can be reproduce with this test case:
>
>   #define _GNU_SOURCE
>   #include <sys/mman.h>
>   #include <stdio.h>
>   #include <unistd.h>
>
>   #define MB (1024 * 1024UL)
>   #define GB (1024 * MB)
>
>   int main(int argc, char **argv)
>   {
>         char *p;
>         int i;
>
>         p = mmap((void *) GB, 10 * MB, PROT_READ | PROT_WRITE,
>                         MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
>         for (i = 0; i < 10 * MB; i += 4096)
>                 p[i] = 1;
>         mremap(p, 10 * MB, 10 * MB, MREMAP_FIXED | MREMAP_MAYMOVE, 2 * GB);
>         return 0;
>   }
>
> Due to split PMD lock, we now store preallocated PTE tables for THP
> pages per-PMD table.  It means we need to move them to other PMD table
> if huge PMD moved there.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reported-by: Andrey Vagin <avagin@openvz.org>

My tests were working for the night without any problem.  Thanks for
the quick response.

Tested-by: Andrey Vagin <avagin@openvz.org>

> ---
>  mm/huge_memory.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bccd5a628ea6..33a5dc492810 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1481,8 +1481,18 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
>                 pmd = pmdp_get_and_clear(mm, old_addr, old_pmd);
>                 VM_BUG_ON(!pmd_none(*new_pmd));
>                 set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
> -               if (new_ptl != old_ptl)
> +               if (new_ptl != old_ptl) {
> +                       pgtable_t pgtable;
> +
> +                       /*
> +                        * Move preallocated PTE page table if new_pmd is on
> +                        * different PMD page table.
> +                        */
> +                       pgtable = pgtable_trans_huge_withdraw(mm, old_pmd);
> +                       pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
> +
>                         spin_unlock(new_ptl);
> +               }
>                 spin_unlock(old_ptl);
>         }
>  out:
> --
> 1.8.4.4
>

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