On 05/23/2018 12:25 PM, Michal Hocko wrote: > 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. > Not physically, but virtually contiguous.
