linux-kernel - Re: [PATCH 2/2] mm: don't hide potentially null memmap pointer in sparse_remove

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Message-ID: <befab2a0a9f160f8af8c1a412068060636a7a64c.camel@d-silva.org>
Date:   Tue, 27 Aug 2019 17:00:16 +1000
From:   "Alastair D'Silva" <alastair@...ilva.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Wei Yang <richardw.yang@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>, Qian Cai <cai@....pw>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: don't hide potentially null memmap pointer in
 sparse_remove_section

On Tue, 2019-08-27 at 08:24 +0200, Michal Hocko wrote:
> On Tue 27-08-19 15:36:55, Alastair D'Silva wrote:
> > From: Alastair D'Silva <alastair@...ilva.org>
> > 
> > By adding offset to memmap before passing it in to
> > clear_hwpoisoned_pages,
> > we hide a theoretically null memmap from the null check inside
> > clear_hwpoisoned_pages.
> 
> Isn't that other way around? Calculating the offset struct page
> pointer
> will actually make the null check effective. Besides that I cannot
> really see how pfn_to_page would return NULL. I have to confess that
> I
> cannot really see how offset could lead to a NULL struct page either
> and
> I strongly suspect that the NULL check is not really needed. Maybe it
> used to be in the past.
> 

You're probably right, but I didn't feel confident in removing the NULL
check. 

While the NULL check remains though, I can't see how adding the offset
would turn a non-NULL pointer into a NULL unless the pointer is invalid
in the first place, and if this is the case, we should have a comment
explaining this.

The NULL check was added in commit:
95a4774d055c ("memory-hotplug: update mce_bad_pages when removing the
memory")
where memmap was originally inited to NULL, and only conditionally
given a value.

With this in mind, since that situation is no longer true, I think we
could instead drop the NULL check.

-- 
Alastair D'Silva           mob: 0423 762 819
skype: alastair_dsilva    
Twitter: @EvilDeece
blog: http://alastair.d-silva.org