Re: [PATCH] mm: Fix warning in move_normal_pmd()

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Jul 15, 2020 at 03:35:13PM +0300, Kirill A. Shutemov wrote:
> mremap(2) does not allow source and destination regions to overlap, but
> shift_arg_pages() calls move_page_tables() directly and in this case the
> source and destination overlap often. It confuses move_normal_pmd():
> 
>   WARNING: CPU: 3 PID: 27091 at mm/mremap.c:211 move_page_tables+0x6ef/0x720
> 
> move_normal_pmd() expects the destination PMD to be empty, but when
> ranges overlap nobody removes PTE page tables on source side.
> move_ptes() only removes PTE entries, leaving tables behind.
> When the source PMD becomes destination and alignment/size is right we
> step onto the warning.
> 
> The warning is harmless: kernel correctly fallbacks to handle entries on
> per-entry basis.

A link to the debugging effort could be added to the change log:
https://lore.kernel.org/lkml/20200713025354.GB3644504@google.com/

> The fix is to avoid move_normal_pmd() if we see that source and
> destination ranges overlap.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

And one thing that bothers me:

>  mm/mremap.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 5dd572d57ca9..e33fcee541fe 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -245,6 +245,18 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  	unsigned long extent, next, old_end;
>  	struct mmu_notifier_range range;
>  	pmd_t *old_pmd, *new_pmd;
> +	bool overlaps;
> +
> +	/*
> +	 * shift_arg_pages() can call move_page_tables() on overlapping ranges.
> +	 * In this case we cannot use move_normal_pmd() because destination pmd
> +	 * might be established page table: move_ptes() doesn't free page
> +	 * table.
> +	 */
> +	if (old_addr > new_addr)
> +		overlaps = old_addr - new_addr < len;
> +	else
> +		overlaps = new_addr - old_addr < len;

Does the code really work properly if old_addr < new_addr and overlaps ==
true ? If not, then we should add a warning here in the else IMHO:

	if (old_addr >= new_addr) {
		overlaps = old_addr - new_addr < len;
	} else {
		overlaps = new_addr - old_addr < len;
		WARN_ON(overlaps);
	}

(More so, since you have added code that detects overlaps for such a case).

thanks,

 - Joel

>  
>  	old_end = old_addr + len;
>  	flush_cache_range(vma, old_addr, old_end);
> @@ -282,7 +294,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  			split_huge_pmd(vma, old_pmd, old_addr);
>  			if (pmd_trans_unstable(old_pmd))
>  				continue;
> -		} else if (extent == PMD_SIZE) {
> +		} else if (!overlaps && extent == PMD_SIZE) {
>  #ifdef CONFIG_HAVE_MOVE_PMD
>  			/*
>  			 * If the extent is PMD-sized, try to speed the move by
> -- 
> 2.26.2
> 
>