Re: [PATCH v3] mm/page_alloc: add a helper function to check page before alloc/free

[Yaowei Bai <bywxiaobai@163.com>]

On Tue, Sep 08, 2015 at 09:19:07PM +0200, Vlastimil Babka wrote:
> On 08/27/2015 02:51 PM, Yaowei Bai wrote:
> > The major portion of check_new_page() and free_pages_check() are same,
> > introduce a helper function check_one_page() for simplification.
> > 
> > Change in v3:
> > 	- add the missed __PG_HWPOISON check per Michal Hocko
> > Change in v2:
> > 	- use bad_flags as parameter directly per Michal Hocko
> > 
> > Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
> > ---
> >  mm/page_alloc.c | 54 +++++++++++++++++++++++-------------------------------
> >  1 file changed, 23 insertions(+), 31 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 5b5240b..0c9c82a 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -707,10 +707,9 @@ out:
> >  	zone->free_area[order].nr_free++;
> >  }
> >  
> > -static inline int free_pages_check(struct page *page)
> > +static inline int check_one_page(struct page *page, unsigned long bad_flags)
> >  {
> >  	const char *bad_reason = NULL;
> > -	unsigned long bad_flags = 0;
> >  
> >  	if (unlikely(page_mapcount(page)))
> >  		bad_reason = "nonzero mapcount";
> > @@ -718,9 +717,16 @@ static inline int free_pages_check(struct page *page)
> >  		bad_reason = "non-NULL mapping";
> >  	if (unlikely(atomic_read(&page->_count) != 0))
> >  		bad_reason = "nonzero _count";
> > -	if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_FREE)) {
> > -		bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag(s) set";
> > -		bad_flags = PAGE_FLAGS_CHECK_AT_FREE;
> > +	if (bad_flags == PAGE_FLAGS_CHECK_AT_PREP) {
> > +		if (unlikely(page->flags & bad_flags))
> > +			bad_reason = "PAGE_FLAGS_CHECK_AT_PREP flag set";
> > +		if (unlikely(page->flags & __PG_HWPOISON)) {
> > +			bad_reason = "HWPoisoned (hardware-corrupted)";
> > +			bad_flags = __PG_HWPOISON;
> > +		}
> 
> Before, HWPOISON was checked first, which means that it had lower priority than
> PAGE_FLAGS_CHECK_AT_PREP (counter-intuitively). I can see why you switched that
> though. You could fix that by changing the second nested "if" to "else if", but
> I guess it doesn't matter. The "priorities" don't seem to be carefuly sorted anyway.

OK, so let's leave as it is and see other guys' ideas. If there is any objection i will
resend with fixing the priority.

> 
> bloat-o-meter looks favorably with my gcc, although there shouldn't be a real
> reason for it, as the inlining didn't change:
> 
> add/remove: 1/1 grow/shrink: 1/1 up/down: 285/-336 (-51)
> function                                     old     new   delta
> bad_page                                       -     276    +276
> get_page_from_freelist                      2521    2530      +9
> free_pages_prepare                           745     667     -78
> bad_page.part                                258       -    -258
> 
> With that,
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks.

> 
> > +	} else if (bad_flags == PAGE_FLAGS_CHECK_AT_FREE) {
> > +		if (unlikely(page->flags & bad_flags))
> > +			bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag set";
> >  	}
> >  #ifdef CONFIG_MEMCG
> >  	if (unlikely(page->mem_cgroup))
> > @@ -730,6 +736,17 @@ static inline int free_pages_check(struct page *page)
> >  		bad_page(page, bad_reason, bad_flags);
> >  		return 1;
> >  	}
> > +	return 0;
> > +}
> > +
> > +static inline int free_pages_check(struct page *page)
> > +{
> > +	int ret = 0;
> > +
> > +	ret = check_one_page(page, PAGE_FLAGS_CHECK_AT_FREE);
> > +	if (ret)
> > +		return ret;
> > +
> >  	page_cpupid_reset_last(page);
> >  	if (page->flags & PAGE_FLAGS_CHECK_AT_PREP)
> >  		page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
> > @@ -1287,32 +1304,7 @@ static inline void expand(struct zone *zone, struct page *page,
> >   */
> >  static inline int check_new_page(struct page *page)
> >  {
> > -	const char *bad_reason = NULL;
> > -	unsigned long bad_flags = 0;
> > -
> > -	if (unlikely(page_mapcount(page)))
> > -		bad_reason = "nonzero mapcount";
> > -	if (unlikely(page->mapping != NULL))
> > -		bad_reason = "non-NULL mapping";
> > -	if (unlikely(atomic_read(&page->_count) != 0))
> > -		bad_reason = "nonzero _count";
> > -	if (unlikely(page->flags & __PG_HWPOISON)) {
> > -		bad_reason = "HWPoisoned (hardware-corrupted)";
> > -		bad_flags = __PG_HWPOISON;
> > -	}
> > -	if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_PREP)) {
> > -		bad_reason = "PAGE_FLAGS_CHECK_AT_PREP flag set";
> > -		bad_flags = PAGE_FLAGS_CHECK_AT_PREP;
> > -	}
> > -#ifdef CONFIG_MEMCG
> > -	if (unlikely(page->mem_cgroup))
> > -		bad_reason = "page still charged to cgroup";
> > -#endif
> > -	if (unlikely(bad_reason)) {
> > -		bad_page(page, bad_reason, bad_flags);
> > -		return 1;
> > -	}
> > -	return 0;
> > +	return check_one_page(page, PAGE_FLAGS_CHECK_AT_PREP);
> >  }
> >  
> >  static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
> > 

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