linux-kernel - Re: [v2 PATCH 2/2 -mm] mm: brk: dwongrade mmap

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Date:   Thu, 27 Sep 2018 15:21:09 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     mhocko@...nel.org, willy@...radead.org, ldufour@...ux.vnet.ibm.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 2/2 -mm] mm: brk: dwongrade mmap_sem to read when
 shrinking

On 9/27/18 2:50 PM, Kirill A. Shutemov wrote:
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 017bcfa..0d2fae1 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -193,9 +193,11 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
>>  	unsigned long retval;
>>  	unsigned long newbrk, oldbrk;
>>  	struct mm_struct *mm = current->mm;
>> +	unsigned long origbrk = mm->brk;
> 
> Is it safe to read mm->brk outside the lock?

Good catch! I guess not, parallel brk()'s could then race.