linux-kernel - Re: [PATCH REBASED] hugetlbfs: fix potential over/underflow setting node specific nr

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Date:   Fri, 29 Mar 2019 15:42:01 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Alex Ghiti <alex@...ti.fr>,
        Jing Xiangfeng <jingxiangfeng@...wei.com>
Subject: Re: [PATCH REBASED] hugetlbfs: fix potential over/underflow setting
 node specific nr_hugepages

On Thu, Mar 28, 2019 at 03:05:33PM -0700, Mike Kravetz wrote:
> The number of node specific huge pages can be set via a file such as:
> /sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages
> When a node specific value is specified, the global number of huge
> pages must also be adjusted.  This adjustment is calculated as the
> specified node specific value + (global value - current node value).
> If the node specific value provided by the user is large enough, this
> calculation could overflow an unsigned long leading to a smaller
> than expected number of huge pages.
> 
> To fix, check the calculation for overflow.  If overflow is detected,
> use ULONG_MAX as the requested value.  This is inline with the user
> request to allocate as many huge pages as possible.
> 
> It was also noticed that the above calculation was done outside the
> hugetlb_lock.  Therefore, the values could be inconsistent and result
> in underflow.  To fix, the calculation is moved within the routine
> set_max_huge_pages() where the lock is held.
> 
> In addition, the code in __nr_hugepages_store_common() which tries to
> handle the case of not being able to allocate a node mask would likely
> result in incorrect behavior.  Luckily, it is very unlikely we will
> ever take this path.  If we do, simply return ENOMEM.
> 
> Reported-by: Jing Xiangfeng <jingxiangfeng@...wei.com>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

-- 
Oscar Salvador
SUSE L3