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 19:47:17 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dan Williams <dan.j.williams@...el.com>
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 09:35:11AM -0700, Dan Williams wrote:
> 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.

You cannot detect them, if we would redefine them as 0. :)

-- 
 Kirill A. Shutemov