linux-kernel - Re: [PATCH v2] mm/page_alloc: add a helper function to check page before alloc/free

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Date:	Wed, 26 Aug 2015 22:50:46 +0800
From:	Yaowei Bai <bywxiaobai@....com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	akpm@...ux-foundation.org, mgorman@...e.de, vbabka@...e.cz,
	js1304@...il.com, hannes@...xchg.org, alexander.h.duyck@...hat.com,
	sasha.levin@...cle.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/page_alloc: add a helper function to check page
 before alloc/free

On Tue, Aug 25, 2015 at 04:03:22PM +0200, Michal Hocko wrote:
> On Tue 25-08-15 21:26:30, Yaowei Bai wrote:
> [...]
> >  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;
> > -	}
> 
> You have removed this check AFAICS. Now looking at 39ad4f19671d ("mm:

I missed that check mistakenly, it should be there.

> check __PG_HWPOISON separately from PAGE_FLAGS_CHECK_AT_*") I am not
> sure it is correct to check it in the free path as it was removed from
> the mask by this commit.

I investigated the commit you mentioned above. AFAICS, that commit assumes
that __PG_HWPOISON check should be performed in the alloc path, while in
the free path it doesn't need to. So there is a bit different in alloc/free
paths.
So Michal, do you think there is any obvious points to refactor these
two functions still? Anyway, appreciate your reviewing.

> 
> > -	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,
> > -- 
> > 1.9.1
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs

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