linux-kernel - Re: [patch] mmap.2: document the munmap exception for underlying page size

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Date:	Fri, 24 Jul 2015 08:57:34 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Mike Kravetz <mike.kravetz@...cle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Eric B Munson <emunson@...mai.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-man <linux-man@...r.kernel.org>
Subject: Re: [patch] mmap.2: document the munmap exception for underlying page size

Hello David,

On 23 July 2015 at 22:52, David Rientjes <rientjes@...gle.com> wrote:
> On Thu, 23 Jul 2015, Michael Kerrisk (man-pages) wrote:
>
>> >> Should we also add a similar comment for the mmap offset?  Currently
>> >> the man page says:
>> >>
>> >> "offset must be a multiple of the page size as returned by
>> >>  sysconf(_SC_PAGE_SIZE)."
>> >>
>> >> For hugetlbfs, I beieve the offset must be a multiple of the
>> >> hugetlb page size.  A similar comment/exception about using
>> >> the "underlying page size" would apply here as well.
>> >>
>> >
>> > Yes, that makes sense, thanks.  We should also explicitly say that mmap(2)
>> > automatically aligns length to be hugepage aligned if backed by hugetlbfs.
>>
>> And, surely, it also does something similar for mmap()'s 'addr'
>> argument?
>>
>> I suggest we add a subsection to describe the HugeTLB differences. How
>> about something like:
>>
>>    Huge page (Huge TLB) mappings
>>        For  mappings  that  employ  huge pages, the requirements for the
>>        arguments  of  mmap()  and  munmap()  differ  somewhat  from  the
>>        requirements for mappings that use the native system page size.
>>
>>        For mmap(), offset must be a multiple of the underlying huge page
>>        size.  The system automatically aligns length to be a multiple of
>>        the underlying huge page size.
>>
>>        For  munmap(),  addr  and  length  must both be a multiple of the
>>        underlying huge page size.
>> ?
>>
>
> Looks good, please add my acked-by.

Done. Thanks for checking the text.

> The commit that expanded on the
> documentation of this behavior was
> 80d6b94bd69a7a49b52bf503ef6a841f43cf5bbb.
>
> Answering from your other email, no, this behavior in the kernel has not
> changed recently but we found it wasn't properly documented so we wanted
> to fix that both in the kernel tree and in the man-pages to make it
> explicit.

Okay -- thanks for the clarification.

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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