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Subject: Re: [PATCH v6 17/17] mm: Distinguish VMalloc pages
References: <20180518194519.3820-1-willy@infradead.org>
 <20180518194519.3820-18-willy@infradead.org>
From: Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <74e9bf39-ae17-cc00-8fca-c34b75675d49@virtuozzo.com>
Date: Tue, 22 May 2018 19:10:52 +0300
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To: Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org
Cc: Matthew Wilcox <mawilcox@microsoft.com>, Andrew Morton <akpm@linux-foundation.org>, "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, Christoph Lameter <cl@linux.com>, Lai Jiangshan <jiangshanlai@gmail.com>, Pekka Enberg <penberg@kernel.org>, Vlastimil Babka <vbabka@suse.cz>, Dave Hansen <dave.hansen@linux.intel.com>, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>



On 05/18/2018 10:45 PM, Matthew Wilcox wrote:
> From: Matthew Wilcox <mawilcox@microsoft.com>
> 
> For diagnosing various performance and memory-leak problems, it is helpful
> to be able to distinguish pages which are in use as VMalloc pages.
> Unfortunately, we cannot use the page_type field in struct page, as
> this is in use for mapcount by some drivers which map vmalloced pages
> to userspace.
> 
> Use a special page->mapping value to distinguish VMalloc pages from
> other kinds of pages.  Also record a pointer to the vm_struct and the
> offset within the area in struct page to help reconstruct exactly what
> this page is being used for.
> 


This seems useless. page->vm_area and page->vm_offset are never used.
There are no follow up patches which use this new information 'For diagnosing various performance and memory-leak problems',
and no explanation how is it can be used in current form.

Also, this patch breaks code like this:
	if (mapping = page_mapping(page))
		// access mapping