linux-kernel - Re: [PATCH v4] mm/hugetlb: Fix unsigned overflow in __nr_hugepages_store

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Date:   Mon, 4 Mar 2019 14:48:12 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Jing Xiangfeng <jingxiangfeng@...wei.com>, mhocko@...nel.org,
        hughd@...gle.com, linux-mm@...ck.org, n-horiguchi@...jp.nec.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm/hugetlb: Fix unsigned overflow in
 __nr_hugepages_store_common()

On Tue, Feb 26, 2019 at 04:03:23PM -0800, Mike Kravetz wrote:
> I was just going to update the comments and send you a new patch, but
> but your comment got me thinking about this situation.  I did not really
> change the way this code operates.  As a reminder, the original code is like:
> 
> NODEMASK_ALLOC(nodemask_t, nodes_allowed, GFP_KERNEL | __GFP_NORETRY);
> 
> if (nid == NUMA_NO_NODE) {
> 	/* do something */
> } else if (nodes_allowed) {
> 	/* do something else */
> } else {
> 	nodes_allowed = &node_states[N_MEMORY];
> }
> 
> So, the only way we get to that final else if if we can not allocate
> a node mask (kmalloc a few words).  Right?  I wonder why we should
> even try to continue in this case.  Why not just return right there?
> 
> The specified count value is either a request to increase number of
> huge pages or decrease.  If we can't allocate a few words, we certainly
> are not going to find memory to create huge pages.  There 'might' be
> surplus pages which can be converted to permanent pages.  But remember
> this is a 'node specific' request and we can't allocate a mask to pass
> down to the conversion routines.  So, chances are good we would operate
> on the wrong node.  The same goes for a request to 'free' huge pages.
> Since, we can't allocate a node mask we are likely to free them from
> the wrong node.
> 
> Unless my reasoning above is incorrect, I think that final else block
> in __nr_hugepages_store_common() is wrong.
> 
> Any additional thoughts?

Could not we kill the NODEMASK_ALLOC there?
__nr_hugepages_store_common() should be called from a rather shallow stack,
and unless I am wrong, the maximum value we can get for a nodemask_t is 128bytes
(1024 NUMA nodes).

-- 
Oscar Salvador
SUSE L3