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Date:   Mon, 22 Mar 2021 11:40:23 +0800
From:   Aili Yao <yaoaili@kingsoft.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     David Hildenbrand <david@redhat.com>, <akpm@linux-foundation.org>,
        <naoya.horiguchi@nec.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <yangfeng1@kingsoft.com>,
        <sunhao2@kingsoft.com>, Oscar Salvador <osalvador@suse.de>,
        Mike Kravetz <mike.kravetz@oracle.com>, <yaoaili@kingsoft.com>
Subject: Re: [PATCH v3] mm/gup: check page posion status for coredump.
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On Sat, 20 Mar 2021 00:35:16 +0000
Matthew Wilcox <willy@infradead.org> wrote:

> On Fri, Mar 19, 2021 at 10:44:37AM +0800, Aili Yao wrote:
> > +++ b/mm/gup.c
> > @@ -1536,6 +1536,10 @@ struct page *get_dump_page(unsigned long addr)
> >  				      FOLL_FORCE | FOLL_DUMP | FOLL_GET);
> >  	if (locked)
> >  		mmap_read_unlock(mm);
> > +
> > +	if (ret == 1 && is_page_poisoned(page))
> > +		return NULL;
> > +
> >  	return (ret == 1) ? page : NULL;
> >  }
> >  #endif /* CONFIG_ELF_CORE */
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 25d2b2439..902d993 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -97,6 +97,27 @@ static inline void set_page_refcounted(struct page *page)
> >  	set_page_count(page, 1);
> >  }
> >  
> > +/*
> > + * When kernel touch the user page, the user page may be have been marked
> > + * poison but still mapped in user space, if without this page, the kernel
> > + * can guarantee the data integrity and operation success, the kernel is
> > + * better to check the posion status and avoid touching it, be good not to
> > + * panic, coredump for process fatal signal is a sample case matching this
> > + * scenario. Or if kernel can't guarantee the data integrity, it's better
> > + * not to call this function, let kernel touch the poison page and get to
> > + * panic.
> > + */
> > +static inline bool is_page_poisoned(struct page *page)
> > +{
> > +	if (page != NULL) {  
> 
> Why are you checking page for NULL here?  How can it possibly be NULL?

For this get_dump_page() case, it can't be NULL, I thougt may other place
will call this function and may not guarantee this, But yes, kernel is a more
safer place and checking page NULL is not a common behavior.

Better to remove it, Thanks you very much for pointing this!

> > +		if (PageHWPoison(page))
> > +			return true;
> > +		else if (PageHuge(page) && PageHWPoison(compound_head(page)))
> > +			return true;
> > +	}
> > +	return 0;
> > +}
> > +
> >  extern unsigned long highest_memmap_pfn;
> >  
> >  /*
> > -- 
> > 1.8.3.1
> > 
> >   
-- 
Thanks!
Aili Yao