linux-kernel - Re: [PATCH v5 3/5] mm: introduce mmap3 for safely defining new mmap flags

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Date:   Wed, 16 Aug 2017 09:35:11 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v5 3/5] mm: introduce mmap3 for safely defining new mmap flags

On Wed, Aug 16, 2017 at 4:15 AM, Kirill A. Shutemov
<kirill@...temov.name> wrote:
> On Wed, Aug 16, 2017 at 12:44:22AM -0700, Dan Williams wrote:
>> diff --git a/include/linux/mman.h b/include/linux/mman.h
>> index c8367041fafd..0e1de42c836f 100644
>> --- a/include/linux/mman.h
>> +++ b/include/linux/mman.h
>> @@ -7,6 +7,40 @@
>>  #include <linux/atomic.h>
>>  #include <uapi/linux/mman.h>
>>
>> +#ifndef MAP_32BIT
>> +#define MAP_32BIT 0
>> +#endif
>> +#ifndef MAP_HUGE_2MB
>> +#define MAP_HUGE_2MB 0
>> +#endif
>> +#ifndef MAP_HUGE_1GB
>> +#define MAP_HUGE_1GB 0
>> +#endif
>> +
>> +/*
>> + * The historical set of flags that all mmap implementations implicitly
>> + * support when file_operations.mmap_supported_mask is zero.
>> + */
>> +#define LEGACY_MAP_SUPPORTED_MASK (MAP_SHARED \
>> +             | MAP_PRIVATE \
>> +             | MAP_FIXED \
>> +             | MAP_ANONYMOUS \
>> +             | MAP_UNINITIALIZED \
>> +             | MAP_GROWSDOWN \
>> +             | MAP_DENYWRITE \
>> +             | MAP_EXECUTABLE \
>> +             | MAP_LOCKED \
>> +             | MAP_NORESERVE \
>> +             | MAP_POPULATE \
>> +             | MAP_NONBLOCK \
>> +             | MAP_STACK \
>> +             | MAP_HUGETLB \
>> +             | MAP_32BIT \
>> +             | MAP_HUGE_2MB \
>> +             | MAP_HUGE_1GB)
>> +
>> +#define      MAP_SUPPORTED_MASK (LEGACY_MAP_SUPPORTED_MASK)
>> +
>>  extern int sysctl_overcommit_memory;
>>  extern int sysctl_overcommit_ratio;
>>  extern unsigned long sysctl_overcommit_kbytes;
>
> Since we looking into mmap(2) ABI, maybe we should consider re-defining
> MAP_DENYWRITE and MAP_EXECUTABLE as 0 in hope that we would be able to
> re-use these bits in the future? These flags are ignored now anyway.

Yes, we can make these -EOPNOTSUPP in the new syscall.