Re: [PATCH] mm: Report bad PTEs in lookup_swap_cache()

["Huang\, Ying" <ying.huang@intel.com>]

Matthew Wilcox <willy@infradead.org> writes:

> From: Matthew Wilcox <mawilcox@microsoft.com>
>
> If we have garbage in the PTE, we can call the radix tree code with a
> NULL radix tree head which leads to an OOPS.  Detect the case where
> we've found a PTE that refers to a non-existent swap device and report
> the error correctly.

There is a patch in -mm tree which addresses this problem at least
partially as follow.  Please take a look at it.

https://www.spinics.net/lists/linux-mm/msg145242.html

[snip]

> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 39ae7cfad90f..5a7755ecbb03 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -328,14 +328,22 @@ void free_pages_and_swap_cache(struct page **pages, int nr)
>   * lock getting page table operations atomic even if we drop the page
>   * lock before returning.
>   */
> -struct page *lookup_swap_cache(swp_entry_t entry, struct vm_area_struct *vma,
> -			       unsigned long addr)
> +struct page *lookup_swap_cache(swp_entry_t entry, bool vma_ra,
> +			struct vm_fault *vmf)
>  {
>  	struct page *page;
> -	unsigned long ra_info;
> -	int win, hits, readahead;
> +	int readahead;
> +	struct address_space *swapper_space = swap_address_space(entry);
>  
> -	page = find_get_page(swap_address_space(entry), swp_offset(entry));
> +	if (!swapper_space) {
> +		if (vmf)
> +			pte_ERROR(vmf->orig_pte);
> +		else
> +			pr_err("Bad swp_entry: %lx\n", entry.val);

>From we get swap_entry from PTE to lookup_swap_cache(), nothing prevent
the swap device to be swapoff.  So, I think sometimes it is hard to
distinguish between garbage PTEs and PTEs which become deprecated
because of swapoff.

Best Regards,
Huang, Ying

> +		return NULL;
> +	}
> +
> +	page = find_get_page(swapper_space, swp_offset(entry));
>  
>  	INC_CACHE_INFO(find_total);
>  	if (page) {

[snip]

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