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Date: Tue, 22 May 2018 14:45:17 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v6 17/17] mm: Distinguish VMalloc pages
Message-ID: <20180522214517.GA30913@bombadil.infradead.org>
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>, linux-mm@kvack.org, Matthew Wilcox <mawilcox@microsoft.com>, "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>, =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>

On Tue, May 22, 2018 at 01:48:38PM -0700, Andrew Morton wrote:
> -ENOCOMMENT ;)
> 
> --- a/mm/util.c~mm-distinguish-vmalloc-pages-fix-fix
> +++ a/mm/util.c
> @@ -512,6 +512,8 @@ struct address_space *page_mapping(struc
>  	mapping = page->mapping;
>  	if ((unsigned long)mapping & PAGE_MAPPING_ANON)
>  		return NULL;
> +
> +	/* Don't trip over a vmalloc page's MAPPING_VMalloc cookie */
>  	if ((unsigned long)mapping < PAGE_SIZE)
>  		return NULL;
>  
> It's a bit sad to put even more stuff into page_mapping() just for
> page_types diddling.  Is this really justified?  How many people will
> use it, and get significant benefit from it?

We could leave page->mapping NULL for vmalloc pages.  We just need to
find a spot where we can put a unique identifier.  The first word of
the union looks like a string candidate; bit 0 is already reserved for
PageTail.  The other users are list_head.prev, a struct page *, and
struct dev_pagemap *, so that should work out OK.

If you want to just drop this patch, I'd be OK with that.  I can always
submit it to you again next merge window.