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Subject: Re: [PATCH 2/6] mm/mremap: For MREMAP_DONTUNMAP check
 security_vm_enough_memory_mm()
To:     Brian Geffon <bgeffon@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Hugh Dickins <hughd@google.com>,
        Ingo Molnar <mingo@redhat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Minchan Kim <minchan@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>, linux-aio@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
References: <20201013013416.390574-1-dima@arista.com>
 <20201013013416.390574-3-dima@arista.com>
 <CADyq12y4WAjT7O3_4E3FmBv4dr5fY6utQZod1UN0Xv8PhOAnQA@mail.gmail.com>
From:   Dmitry Safonov <dima@arista.com>
Message-ID: <d25ad10c-6f67-8c11-18c3-0193b8ea14c4@arista.com>
Date:   Mon, 28 Dec 2020 19:12:21 +0000
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On 12/28/20 6:21 PM, Brian Geffon wrote:
> This looks good to me with a small comment.
> 
>>         if (do_munmap(mm, old_addr, old_len, uf_unmap) < 0) {
>>                 /* OOM: unable to split vma, just get accounts right */
>> -               if (vm_flags & VM_ACCOUNT)
>> +               if (vm_flags & VM_ACCOUNT && !(flags & MREMAP_DONTUNMAP))
>>                         vm_acct_memory(new_len >> PAGE_SHIFT);
> 
> Checking MREMAP_DONTUNMAP in the do_munmap path is unnecessary as
> MREMAP_DONTUNMAP will have already returned by this point.

In this code it is also used as err-path. In case move_page_tables()
fails to move all page tables or .mremap() callback fails, the new VMA
is unmapped.

IOW, MREMAP_DONTUNMAP returns under:
:	if (unlikely(!err && (flags & MREMAP_DONTUNMAP))) {

-- 
          Dima