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

[Minchan Kim <minchan@kernel.org>]

On Mon, Oct 30, 2017 at 09:02:30AM +0100, Michal Hocko wrote:
> On Mon 30-10-17 08:57:13, Minchan Kim wrote:
> [...]
> > Although it's better than old, we can make it simple, still.
> > 
> > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > index 291c4b534658..f50d5a48f03a 100644
> > --- a/include/linux/swapops.h
> > +++ b/include/linux/swapops.h
> > @@ -41,6 +41,13 @@ static inline unsigned swp_type(swp_entry_t entry)
> >  	return (entry.val >> SWP_TYPE_SHIFT(entry));
> >  }
> >  
> > +extern struct swap_info_struct *swap_info[];
> > +
> > +static inline struct swap_info_struct *swp_si(swp_entry_t entry)
> > +{
> > +	return swap_info[swp_type(entry)];
> > +}
> > +
> >  /*
> >   * Extract the `offset' field from a swp_entry_t.  The swp_entry_t is in
> >   * arch-independent format
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 378262d3a197..a0fe2d54ad09 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -554,6 +554,7 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >  			struct vm_area_struct *vma, unsigned long addr)
> >  {
> >  	struct page *page;
> > +	struct swap_info_struct *si = swp_si(entry);
> 
> Aren't you accessing beyond the array here?

I couldn't understand what you intend. Could you explain what case does it accesses
beyond the arrary?

Thanks.

> 
> >  	unsigned long entry_offset = swp_offset(entry);
> >  	unsigned long offset = entry_offset;
> >  	unsigned long start_offset, end_offset;
> > @@ -572,6 +573,9 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >  	if (!start_offset)	/* First page is swap header. */
> >  		start_offset++;
> >  
> > +	if (end_offset >= si->max)
> > +		end_offset = si->max - 1;
> > +
> >  	blk_start_plug(&plug);
> >  	for (offset = start_offset; offset <= end_offset ; offset++) {
> >  		/* Ok, do the async read-ahead now */
> 
> -- 
> Michal Hocko
> SUSE Labs
> 
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