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. > Why would anybody map vmalloc pages to the userspace? To have shared memory between usespace and the kernel. > 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()
