Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation

On 12/09/2017 09:19 AM, Pavel Machek wrote:

On Thu 2017-12-07 15:02:21, Michal Hocko wrote:

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On Thu 07-12-17 13:58:05, Cyril Hrubis wrote:

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Hi!

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(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
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