linux-kernel - Re: [PATCH 1/2] Report the pagesize backing a VMA in /proc/pid/smaps

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Message-Id: <20081009104014.DEBD.KOSAKI.MOTOHIRO@jp.fujitsu.com>
Date:	Thu,  9 Oct 2008 11:16:58 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com, Mel Gorman <mel@....ul.ie>,
	akpm@...ux-foundation.org, dave@...ux.vnet.ibm.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Report the pagesize backing a VMA in /proc/pid/smaps

Hi

> > It is useful to verify a hugepage-aware application is using the expected
> > pagesizes for its memory regions. This patch creates an entry called
> > KernelPageSize in /proc/pid/smaps that is the size of page used by the
> > kernel to back a VMA. The entry is not called PageSize as it is possible
> > the MMU uses a different size. This extension should not break any sensible
> > parser that skips lines containing unrecognised information.
> 
> > +		   "KernelPageSize: %8lu kB\n",
> 
> > +unsigned long vma_kernel_pagesize(struct vm_area_struct *vma)
> > +{
> > +	struct hstate *hstate;
> > +
> > +	if (!is_vm_hugetlb_page(vma))
> > +		return PAGE_SIZE;
> > +
> > +	hstate = hstate_vma(vma);
> > +	VM_BUG_ON(!hstate);
> > +
> > +	return 1UL << (hstate->order + PAGE_SHIFT);
> 			    ^^^^
> VM_BUG_ON is unneeded because kernel will oops here if hstate is NULL.

yup.


> Also, in /proc/*/maps it's printed only for hugetlb vmas and called
> hpagesize, in smaps it's printed for every vma and called
> KernelPageSize. All of this is inconsistent.

Is this a problem?
/proc/*/maps and /proc/*/smaps are different purpose file.

/proc/*/maps:  summary & suppressed information & easy readable
/proc/*/smaps: verbose output

Already some information output only smaps.


> And app will verify once that hugepages are of right size, so Pss cost
> argument for changing /proc/*/maps seems weak to me.

sorry, I don't understand yet.
Why pss cost changed?



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