On Tue 22-05-18 13:19:58, Matthew Wilcox wrote: > 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); > } Well, this would be quite unfortunate. We do not want to pay a branch price for something that doesn't have a _real_ user. Which is kinda sad because I found the explicit vmalloc page "flag" nice to have (if it was for free basically). -- Michal Hocko SUSE Labs
