Re: [PATCH] mremap: take anon_vma lock in shared mode

[Hugh Dickins <hughd@google.com>]

On Tue, 28 Oct 2014, Kirill A. Shutemov wrote:

> There's no modification to anon_vma interval tree. We only need to
> serialize against exclusive rmap walker who want s to catch all ptes the
> page is mapped with. Shared lock is enough for that.
> 
> Suggested-by: Davidlohr Bueso <dbueso@suse.de>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

NAK: please read Michel's comment on need_rmap_locks again, there is
no point in using read locks on anon_vma (and i_mmap) here, those will
not exclude the read locks on anon_vma (and i_mmap) in the rmap walk,
while we move ptes around.

Or am I confused?

Hugh

> ---
>  mm/mremap.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index c855922497a3..1e35ba664406 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -123,7 +123,7 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
>  		}
>  		if (vma->anon_vma) {
>  			anon_vma = vma->anon_vma;
> -			anon_vma_lock_write(anon_vma);
> +			anon_vma_lock_read(anon_vma);
>  		}
>  	}
>  
> @@ -154,7 +154,7 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
>  	pte_unmap(new_pte - 1);
>  	pte_unmap_unlock(old_pte - 1, old_ptl);
>  	if (anon_vma)
> -		anon_vma_unlock_write(anon_vma);
> +		anon_vma_unlock_read(anon_vma);
>  	if (mapping)
>  		mutex_unlock(&mapping->i_mmap_mutex);
>  }
> @@ -199,12 +199,12 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  					      vma);
>  				/* See comment in move_ptes() */
>  				if (need_rmap_locks)
> -					anon_vma_lock_write(vma->anon_vma);
> +					anon_vma_lock_read(vma->anon_vma);
>  				err = move_huge_pmd(vma, new_vma, old_addr,
>  						    new_addr, old_end,
>  						    old_pmd, new_pmd);
>  				if (need_rmap_locks)
> -					anon_vma_unlock_write(vma->anon_vma);
> +					anon_vma_unlock_read(vma->anon_vma);
>  			}
>  			if (err > 0) {
>  				need_flush = true;
> -- 
> 2.1.1

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