On 12/09/2017 09:19 AM, Pavel Machek wrote:
> On Thu 2017-12-07 15:02:21, Michal Hocko wrote:
>> On Thu 07-12-17 13:58:05, Cyril Hrubis wrote:
>>> Hi!
>>>>>> (It does seem unfortunate that the man page cannot help the programmer
>>>>>> actually write correct code here. He or she is forced to read the kernel
>>>>>> implementation, in order to figure out the true alignment rules. I was
>>>>>> hoping we could avoid that.)
>>>>>
>>>>> It would be nice if we had this information exported somehere so that we
>>>>> do not have to rely on per-architecture ifdefs.
>>>>>
>>>>> What about adding MapAligment or something similar to the /proc/meminfo?
>>>>>
>>>>
>>>> What's the use case you envision for that? I don't see how that would be
>>>> better than using SHMLBA, which is available at compiler time. Because
>>>> unless someone expects to be able to run an app that was compiled for
>>>> Arch X, on Arch Y (surely that's not requirement here?), I don't see how
>>>> the run-time check is any better.
>>>
>>> I guess that some kind of compile time constant in uapi headers will do
>>> as well, I'm really open to any solution that would expose this constant
>>> as some kind of official API.
>>
>> I am not sure this is really feasible. It is not only a simple alignment
>> thing. Look at ppc for example (slice_get_unmapped_area). Other
>> architectures might have even more complicated rules e.g. arm and its
>> cache_is_vipt_aliasing. Also this applies only on MAP_SHARED || file
>> backed mappings.
>>
>> I would really leave dogs sleeping... Trying to document all this in the
>> man page has chances to confuse more people than it has chances to help
>> those who already know all these nasty details.
>
> You don't have to provide all the details, but warning that there's arch-
> specific magic would be nice...
Hi Pavel,
In version 4 of this patch (which oddly enough, I have trouble finding via
google, it only seems to show up in patchwork.kernel.org [1]), I phrased it
like this:
Don't interpret addr as a hint: place the mapping at exactly that
address. addr must be suitably aligned: for most architectures a
multiple of page size is sufficient; however, some architectures
may impose additional restrictions.
...which is basically what Cyril was asking for, in his early feedback.
Does that work for you?
(Maybe I need to repost that patch. In any case the CC's need updating,
at least.)
[1] https://patchwork.kernel.org/patch/10094905/
thanks,
--
John Hubbard
NVIDIA
> Pavel
>
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
>
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