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()
