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

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

Minchan Kim <minchan@kernel.org> writes:

> Hi Huang,
>
> On Tue, Oct 31, 2017 at 01:32:32PM +0800, Huang, Ying wrote:
>> Hi, Minchan,
>> 
>> Minchan Kim <minchan@kernel.org> writes:
>> 
>> > Hi Huang,
>> >
>> > On Fri, Oct 27, 2017 at 01:53:27PM +0800, Huang, Ying wrote:
>> >> From: Huang Ying <huang.ying.caritas@gmail.com>
>> >> 
>> >> 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().
>> >> 
>> >> Cc: Tim Chen <tim.c.chen@linux.intel.com>
>> >> Cc: Minchan Kim <minchan@kernel.org>
>> >> Cc: Michal Hocko <mhocko@suse.com>
>> >> Cc: <stable@vger.kernel.org> # 4.11-4.13
>> >> Reported-by: Christian Kujau <lists@nerdbynature.de>
>> >> Fixes: e8c26ab60598 ("mm/swap: skip readahead for unreferenced swap slots")
>> >> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> >> ---
>> >>  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
>> >
>> > 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)];
>> > +}
>> > +
>> 
>> Why not just use swp_swap_info()?
>
> Yeap, I forgot the one that even I added the function. :(
>
>> 
>> >  /*
>> >   * 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);
>> >  	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;
>> > +
>> 
>> I don't think two implementation has much difference.  Both look OK for
>> me.
>
> The point is *readbility*. We always try to make code simple/understandable.
> Even, it's more important for stable candidate.
>
> We don't need to check the validation in loop so reviewer don't need to
> take care of it once it passes boundary check in above logic.
>
> As you say it's okay, please resend it.

OK.

Best Regards,
Huang, Ying

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