linux-kernel - Re: [PATCH] HUGETLBFS: Align memory request to multiple of huge page size to avoid underallocating.

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Message-Id: <20120227190432.55d7399a.akpm@linux-foundation.org>
Date:	Mon, 27 Feb 2012 19:04:32 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Truelove <steven.truelove@...ronto.ca>
Cc:	wli@...omorphy.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] HUGETLBFS: Align memory request to multiple of huge
 page size to avoid underallocating.

On Mon, 27 Feb 2012 21:55:17 -0500 Steven Truelove <steven.truelove@...ronto.ca> wrote:

> > A few things...
> >
> > - sys_mmap_pgoff() does the rounding up prior to calling
> >    hugetlb_file_setup().  ipc/shm.c:newseg() does not.
> >
> >    We should be consistent here: do it in the caller or the callee,
> >    not both (or neither!).  I guess doing it in the callee would be
> >    best.
> >
> > - The above code could/should have used ALIGN().  Or round_up(): the
> >    difference presently escapes me, even though it was so obvious that
> >    we left all these things undocumented.
> >
> > - What's the point in aligning the length if we don't also look at
> >    the start address?  If that isn't a multiple of huge_page_size(), we
> >    will need an additional page.
> >
> 
> Since mmap has an address to check and shmget does not, if the address 
> is going to be checked it will need to be in the caller.

Or pass a value of 0 from shmget.

>  If you like, I 
> will leave the size check in hugetlb_file_setup() and remove the size 
> check from mmap_pgoff, but replace it with a check of the address.  That 
> will centralize the common check (size of buffer), and let mmap_pgoff 
> check the part that is unique to it.  Patch shortly.
> 
> Steven Truelove
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