Re: [PATCH v6 3/5] mm: introduce mmap3 for safely defining new mmap flags

[Dan Williams]

On Fri, Aug 25, 2017 at 9:19 AM, Helge Deller <deller@xxxxxx> wrote:
> On 25.08.2017 18:16, Kirill A. Shutemov wrote:
>> On Fri, Aug 25, 2017 at 09:02:36AM -0700, Christoph Hellwig wrote:
>>> On Fri, Aug 25, 2017 at 06:58:03PM +0300, Kirill A. Shutemov wrote:
>>>> Not all archs are ready for this:
>>>>
>>>> arch/parisc/include/uapi/asm/mman.h:#define MAP_TYPE    0x03            /* Mask for type of mapping */
>>>> arch/parisc/include/uapi/asm/mman.h:#define MAP_FIXED   0x04            /* Interpret addr exactly */
>>>
>>> I'd be happy to say that we should not care about parisc for
>>> persistent memory.  We'll just have to find a way to exclude
>>> parisc without making life too ugly.
>>
>> I don't think creapling mmap() interface for one arch is the right way to
>> go. I think the interface should be universal.
>>
>> I may imagine MAP_DIRECT can be useful not only for persistent memory.
>> For tmpfs instead of mlock()?
>
> On parisc we have
> #define MAP_SHARED      0x01            /* Share changes */
> #define MAP_PRIVATE     0x02            /* Changes are private */
> #define MAP_TYPE        0x03            /* Mask for type of mapping */
> #define MAP_FIXED       0x04            /* Interpret addr exactly */
> #define MAP_ANONYMOUS   0x10            /* don't use a file */
>
> So, if you need a MAP_DIRECT, wouldn't e.g.
> #define MAP_DIRECT      0x08
> be possible (for parisc, and others 0x04).
> And if MAP_TYPE needs to include this flag on parisc:
> #define MAP_TYPE        (0x03 | 0x08)  /* Mask for type of mapping */

The problem here is that to support new the mmap flags the arch needs
to find a flag that is guaranteed to fail on older kernels. Defining
MAP_DIRECT to 0x8 on parisc doesn't work because it will simply be
ignored on older parisc kernels.

However, it's already the case that several archs have their own
sys_mmap entry points. Those archs that can't follow the common scheme
(only parsic it seems) will need to add a new mmap syscall. I think
that's a reasonable tradeoff to allow every other architecture to add
this support with their existing mmap syscall paths.

That means MAP_DIRECT should be defined to MAP_TYPE on parisc until it
later defines an opt-in mechanism to a new syscall that honors
MAP_DIRECT as a valid flag.