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@xxxxxxxxxxxxx> > >>>>> > >>>>> 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
