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From: Andrew Morton <akpm@linux-foundation.org>
To: Matthew Wilcox <willy@infradead.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>,
	"Jérôme Glisse" <jglisse@redhat.com>
Subject: Re: [PATCH v6 17/17] mm: Distinguish VMalloc pages
Date: Tue, 22 May 2018 13:48:38 -0700
Message-ID: <20180522134838.fe59b6e4a405fa9af9fc0487@linux-foundation.org> (raw)
In-Reply-To: <20180522201958.GC1237@bombadil.infradead.org>

On Tue, 22 May 2018 13:19:58 -0700 Matthew Wilcox <willy@infradead.org> wrote:

> On Tue, May 22, 2018 at 10:57:34PM +0300, Andrey Ryabinin wrote:
> > On 05/22/2018 08:58 PM, Matthew Wilcox wrote:
> > > On Tue, May 22, 2018 at 07:10:52PM +0300, Andrey Ryabinin wrote:
> > >> 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.
> > > 
> > > Right now, it's by-hand.  tools/vm/page-types.c will tell you which pages
> > > are allocated to VMalloc.  Many people use kernel debuggers, crashdumps
> > > and similar to examine the kernel's memory.  Leaving these breadcrumbs
> > > is helpful, and those fields simply weren't in use before.
> > > 
> > >> Also, this patch breaks code like this:
> > >> 	if (mapping = page_mapping(page))
> > >> 		// access mapping
> > > 
> > > Example of broken code, please?  Pages allocated from the page allocator
> > > with alloc_page() come with page->mapping == NULL.  This code snippet
> > > would not have granted access to vmalloc pages before.
> > > 
> > 
> > Some implementation of the flush_dcache_page(), also set_page_dirty() can be called
> > on userspace-mapped vmalloc pages during unmap - zap_pte_range() -> set_page_dirty()
> 
> Ah, good catch!  I'm anticipating we'll have other special values for
> page->mapping in the future. so how about this?
> 
> (no changelog because I assume Andrew will add this as a -fix patch)

I give the -fix patches a single-line summary in the final rollup.

> diff --git a/mm/util.c b/mm/util.c
> index 10ca6f1d5c75..be81c9052ef7 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -561,6 +561,8 @@ struct address_space *page_mapping(struct page *page)
>  	mapping = page->mapping;
>  	if ((unsigned long)mapping & PAGE_MAPPING_ANON)
>  		return NULL;
> +	if ((unsigned long)mapping < PAGE_SIZE)
> +		return NULL;
>  
>  	return (void *)((unsigned long)mapping & ~PAGE_MAPPING_FLAGS);
>  }

-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?

  reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-18 19:45 [PATCH v6 00/17] Rearrange struct page Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 01/17] s390: Use _refcount for pgtables Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 02/17] mm: Split page_type out from _mapcount Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 03/17] mm: Mark pages in use for page tables Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 04/17] mm: Switch s_mem and slab_cache in struct page Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 05/17] mm: Move 'private' union within " Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 06/17] mm: Move _refcount out of struct page union Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 07/17] mm: Combine first three unions in struct page Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 08/17] mm: Use page->deferred_list Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 09/17] mm: Move lru union within struct page Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 10/17] mm: Combine LRU and main union in " Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 11/17] mm: Improve struct page documentation Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 12/17] mm: Add pt_mm to struct page Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 13/17] mm: Add hmm_data " Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 14/17] slab,slub: Remove rcu_head size checks Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 15/17] slub: Remove kmem_cache->reserved Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 16/17] slub: Remove 'reserved' file from sysfs Matthew Wilcox
2018-05-18 19:45 ` [PATCH v6 17/17] mm: Distinguish VMalloc pages Matthew Wilcox
2018-05-22 16:10   ` Andrey Ryabinin
2018-05-22 17:58     ` Matthew Wilcox
2018-05-22 19:49       ` Andrew Morton
2018-05-22 19:57       ` Andrey Ryabinin
2018-05-22 20:19         ` Matthew Wilcox
2018-05-22 20:48           ` Andrew Morton [this message]
2018-05-22 21:45             ` Matthew Wilcox
2018-05-22 23:02               ` Andrew Morton
2018-05-23  6:36           ` Michal Hocko
2018-05-23  6:34         ` Michal Hocko
2018-05-23  9:14           ` Andrey Ryabinin
2018-05-23  9:25             ` Michal Hocko
2018-05-23  9:28               ` Andrey Ryabinin
2018-05-23  9:52                 ` Michal Hocko

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