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From: Michal Hocko <mhocko@kernel.org>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: "Matthew Wilcox" <willy@infradead.org>,
	linux-mm@kvack.org, "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>,
	"Jérôme Glisse" <jglisse@redhat.com>
Subject: Re: [PATCH v6 17/17] mm: Distinguish VMalloc pages
Date: Wed, 23 May 2018 08:34:39 +0200
Message-ID: <20180523063439.GD20441@dhcp22.suse.cz> (raw)
In-Reply-To: <e8d8fd85-89a2-8e4f-24bf-b930b705bc49@virtuozzo.com>

On Tue 22-05-18 22:57:34, 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()

Do you have any specific example? Why would anybody map vmalloc pages to
the userspace? flush_dcache_page on a vmalloc page sounds quite
unexpected to me as well.

-- 
Michal Hocko
SUSE Labs

  parent 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
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 [this message]
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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