From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f70.google.com (mail-pl0-f70.google.com [209.85.160.70]) by kanga.kvack.org (Postfix) with ESMTP id 6DED66B0003 for ; Tue, 22 May 2018 16:48:41 -0400 (EDT) Received: by mail-pl0-f70.google.com with SMTP id q16-v6so12844775pls.15 for ; Tue, 22 May 2018 13:48:41 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id d15-v6si18233767pln.533.2018.05.22.13.48.40 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 22 May 2018 13:48:40 -0700 (PDT) Date: Tue, 22 May 2018 13:48:38 -0700 From: Andrew Morton Subject: Re: [PATCH v6 17/17] mm: Distinguish VMalloc pages Message-Id: <20180522134838.fe59b6e4a405fa9af9fc0487@linux-foundation.org> In-Reply-To: <20180522201958.GC1237@bombadil.infradead.org> References: <20180518194519.3820-1-willy@infradead.org> <20180518194519.3820-18-willy@infradead.org> <74e9bf39-ae17-cc00-8fca-c34b75675d49@virtuozzo.com> <20180522175836.GB1237@bombadil.infradead.org> <20180522201958.GC1237@bombadil.infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: Andrey Ryabinin , linux-mm@kvack.org, Matthew Wilcox , "Kirill A . Shutemov" , Christoph Lameter , Lai Jiangshan , Pekka Enberg , Vlastimil Babka , Dave Hansen , =?ISO-8859-1?Q?J=E9r=F4me?= Glisse On Tue, 22 May 2018 13:19:58 -0700 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 > > >>> > > >>> 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) I give the -fix patches a single-line summary in the final rollup. > 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); > } -ENOCOMMENT ;) --- a/mm/util.c~mm-distinguish-vmalloc-pages-fix-fix +++ a/mm/util.c @@ -512,6 +512,8 @@ struct address_space *page_mapping(struc mapping = page->mapping; if ((unsigned long)mapping & PAGE_MAPPING_ANON) return NULL; + + /* Don't trip over a vmalloc page's MAPPING_VMalloc cookie */ if ((unsigned long)mapping < PAGE_SIZE) return NULL; It's a bit sad to put even more stuff into page_mapping() just for page_types diddling. Is this really justified? How many people will use it, and get significant benefit from it?