linux-kernel - Re: [PATCH 1/2] mm/khugepaged: Fix an uninitialized variable bug

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Date:   Mon, 23 Oct 2023 07:57:40 +0300
From:   Dan Carpenter <dan.carpenter@...aro.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Zach O'Keefe <zokeefe@...gle.com>, Yang Shi <shy828301@...il.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/khugepaged: Fix an uninitialized variable bug

On Fri, Oct 20, 2023 at 09:34:07AM -0700, Andrew Morton wrote:
> On Fri, 20 Oct 2023 17:13:32 +0300 Dan Carpenter <dan.carpenter@...aro.org> wrote:
> 
> > Smatch complains that "hpage" can be used uninitialized:
> > 
> >     mm/khugepaged.c:1234 collapse_huge_page()
> >     error: uninitialized symbol 'hpage'.
> > 
> > Initialized it on this path.
> > 
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1062,8 +1062,10 @@ static int alloc_charge_hpage(struct page **hpage, struct mm_struct *mm,
> >  	int node = hpage_collapse_find_target_node(cc);
> >  	struct folio *folio;
> >  
> > -	if (!hpage_collapse_alloc_folio(&folio, gfp, node, &cc->alloc_nmask))
> > +	if (!hpage_collapse_alloc_folio(&folio, gfp, node, &cc->alloc_nmask)) {
> > +		*hpage = NULL;
> >  		return SCAN_ALLOC_HUGE_PAGE_FAIL;
> > +	}
> >  
> >  	if (unlikely(mem_cgroup_charge(folio, mm, gfp))) {
> >  		folio_put(folio);
> 
> Thanks.   Seems this was accidentally fixed by 
> 
> Author:     Peter Xu <peterx@...hat.com>
> AuthorDate: Wed Feb 22 14:52:47 2023 -0500
> Commit:     Andrew Morton <akpm@...ux-foundation.org>
> CommitDate: Tue Mar 28 16:20:06 2023 -0700
> 
>     mm/khugepaged: alloc_charge_hpage() take care of mem charge errors
> 
> 
> Which was quite a long time ago.  Are you scanning old kernel versions?

There are two error paths.  Peter's patch changed the second
SCAN_CGROUP_CHARGE_FAIL error path but left the first
SCAN_ALLOC_HUGE_PAGE_FAIL error path.

To be honest, it's probably a better idea to just add a *hpage = NULL
at the start of the function.

regards,
dan carpenter