Re: [PATCH -mm -V3] mm, swap: Fix false error message in __swp_swapcount()

[Minchan Kim]

On Thu, Nov 02, 2017 at 01:42:25PM +0800, 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
> swapin_readahead() to avoid to pass the out-of-bound swap entries and
> the swap entry reserved for the swap header to __swp_swapcount() and
> swapcache_prepare().
> 
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.11-4.13
> Fixes: e8c26ab60598 ("mm/swap: skip readahead for unreferenced swap slots")
> Reported-by: Christian Kujau <lists@xxxxxxxxxxxxxxx>
> Suggested-by: Minchan Kim <minchan@xxxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>