Re: [PATCH v6 17/17] mm: Distinguish VMalloc pages

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 11:25:15 +0200	[thread overview]
Message-ID: <20180523092515.GL20441@dhcp22.suse.cz> (raw)
In-Reply-To: <e76d4238-9cfe-1f0f-0a52-cfaf476380a8@virtuozzo.com>

On Wed 23-05-18 12:14:10, Andrey Ryabinin wrote:
> 
> 
> On 05/23/2018 09:34 AM, Michal Hocko wrote:
> > 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?
> 
> git grep -e remap_vmalloc_range -e vmalloc_user
> 
> But that's not all, vmalloc*() + vmalloc_to_page() + vm_insert_page() are another candidates.

Thanks for the pointer. I was not aware of remap_vmalloc_range.
> 
> > Why would anybody map vmalloc pages to the userspace?
> 
> To have shared memory between usespace and the kernel.

OK, so the point seems to be to share large physically contiguous memory
with userspace.

> > flush_dcache_page on a vmalloc page sounds quite
> > unexpected to me as well.
> > 
> 
> remap_vmalloc_range()->vm_insret_page()->insert_page()->flush_dcache_page()

Thanks!
-- 
Michal Hocko
SUSE Labs