On Tue, May 22, 2018 at 10:57:34PM +0300, 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() Ah, good catch! I'm anticipating we'll have other special values for page->mapping in the future. so how about this? (no changelog because I assume Andrew will add this as a -fix patch) diff --git a/mm/util.c b/mm/util.c index 10ca6f1d5c75..be81c9052ef7 100644 --- a/mm/util.c +++ b/mm/util.c @@ -561,6 +561,8 @@ struct address_space *page_mapping(struct page *page) mapping = page->mapping; if ((unsigned long)mapping & PAGE_MAPPING_ANON) return NULL; + if ((unsigned long)mapping < PAGE_SIZE) + return NULL; return (void *)((unsigned long)mapping & ~PAGE_MAPPING_FLAGS); }
