linux-kernel - Re: [PATCH] mm/gup: check page posion status for coredump.

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Message-ID: <20210318044600.GJ3420@casper.infradead.org>
Date:   Thu, 18 Mar 2021 04:46:00 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Aili Yao <yaoaili@...gsoft.com>, akpm@...ux-foundation.org,
        naoya.horiguchi@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, yangfeng1@...gsoft.com,
        sunhao2@...gsoft.com, Oscar Salvador <osalvador@...e.de>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH] mm/gup: check page posion status for coredump.

On Wed, Mar 17, 2021 at 10:12:02AM +0100, David Hildenbrand wrote:
> > +	if (IS_ENABLED(CONFIG_MEMORY_FAILURE) && ret == 1) {
> > +		if (unlikely(PageHuge(page) && PageHWPoison(compound_head(page))))
> > +			ret = 0;
> > +		else if (unlikely(PageHWPoison(page)))
> > +			ret = 0;
> > +	}
> 
> I wonder if a simple
> 
> if (PageHWPoison(compound_head(page)))
> 	ret = 0;
> 
> won't suffice. But I guess the "issue" is compound pages that are not huge
> pages or transparent huge pages.

THPs don't set the HWPoison bit on the head page.

https://lore.kernel.org/linux-mm/20210316140947.GA3420@casper.infradead.org/

(and PAGEFLAG(HWPoison, hwpoison, PF_ANY))

By the way,

#ifdef CONFIG_MEMORY_FAILURE
PAGEFLAG(HWPoison, hwpoison, PF_ANY)
TESTSCFLAG(HWPoison, hwpoison, PF_ANY)
#define __PG_HWPOISON (1UL << PG_hwpoison)
extern bool take_page_off_buddy(struct page *page);
#else
PAGEFLAG_FALSE(HWPoison)
#define __PG_HWPOISON 0
#endif

so there's no need for this 
	if (IS_ENABLED(CONFIG_MEMORY_FAILURE)
check, as it simply turns into

	if (PageHuge(page) && 0)
	else if (0)

and the compiler can optimise it all away.