Re: [PATCH -mm -V2] mm, swap: Fix false error message in __swp_swapcount()

[Michal Hocko]

On Fri 27-10-17 13:53:27, Huang, Ying wrote:
> From: Huang Ying <huang.ying.caritas@xxxxxxxxx>
> 
> When a page fault occurs for a swap entry, the physical swap readahead
> (not the VMA base swap readahead) may readahead several swap entries
> after the fault swap entry.  The readahead algorithm calculates some
> of the swap entries to readahead via increasing the offset of the
> fault swap entry without checking whether they are beyond the end of
> the swap device and it relys on the __swp_swapcount() and
> swapcache_prepare() to check it.  Although __swp_swapcount() checks
> for the swap entry passed in, it will complain with the error message
> as follow for the expected invalid swap entry.  This may make the end
> users confused.
> 
>   swap_info_get: Bad swap offset entry 0200f8a7
> 
> To fix the false error message, the swap entry checking is added in
> swap readahead to avoid to pass the out-bound swap entries and the
> swap entry reserved for the swap header to __swp_swapcount() and
> swapcache_prepare().

I have lost an overview in the swap code after recent changes so I
cannot really give you a responsible ack but this looks much better
than the previous attempt. So it looks good at first sight.

> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.11-4.13
> Reported-by: Christian Kujau <lists@xxxxxxxxxxxxxxx>
> Fixes: e8c26ab60598 ("mm/swap: skip readahead for unreferenced swap slots")
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> ---
>  include/linux/swap.h |  1 +
>  mm/swap_state.c      |  6 ++++--
>  mm/swapfile.c        | 21 +++++++++++++++++++++
>  3 files changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 84255b3da7c1..43b4b821c805 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -476,6 +476,7 @@ extern int page_swapcount(struct page *);
>  extern int __swap_count(struct swap_info_struct *si, swp_entry_t entry);
>  extern int __swp_swapcount(swp_entry_t entry);
>  extern int swp_swapcount(swp_entry_t entry);
> +extern bool swap_entry_check(swp_entry_t entry);
>  extern struct swap_info_struct *page_swap_info(struct page *);
>  extern struct swap_info_struct *swp_swap_info(swp_entry_t entry);
>  extern bool reuse_swap_page(struct page *, int *);
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 6c017ced11e6..7dd70e77058d 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -569,11 +569,13 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
>  	/* Read a page_cluster sized and aligned cluster around offset. */
>  	start_offset = offset & ~mask;
>  	end_offset = offset | mask;
> -	if (!start_offset)	/* First page is swap header. */
> -		start_offset++;
>  
>  	blk_start_plug(&plug);
>  	for (offset = start_offset; offset <= end_offset ; offset++) {
> +		swp_entry_t ent = swp_entry(swp_type(entry), offset);
> +
> +		if (!swap_entry_check(ent))
> +			continue;
>  		/* Ok, do the async read-ahead now */
>  		page = __read_swap_cache_async(
>  			swp_entry(swp_type(entry), offset),
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 3074b02eaa09..b04cec29c234 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1107,6 +1107,27 @@ static struct swap_info_struct *swap_info_get_cont(swp_entry_t entry,
>  	return p;
>  }
>  
> +bool swap_entry_check(swp_entry_t entry)
> +{
> +	struct swap_info_struct *p;
> +	unsigned long offset, type;
> +
> +	type = swp_type(entry);
> +	if (type >= nr_swapfiles)
> +		goto bad_file;
> +	p = swap_info[type];
> +	offset = swp_offset(entry);
> +	if (unlikely(!offset || offset >= p->max))
> +		goto out;
> +
> +	return true;
> +
> +bad_file:
> +	pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
> +out:
> +	return false;
> +}
> +
>  static unsigned char __swap_entry_free(struct swap_info_struct *p,
>  				       swp_entry_t entry, unsigned char usage)
>  {
> -- 
> 2.14.2
> 

-- 
Michal Hocko
SUSE Labs