From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f174.google.com (mail-io0-f174.google.com [209.85.223.174]) by kanga.kvack.org (Postfix) with ESMTP id 75B474402FE for ; Fri, 2 Oct 2015 17:21:51 -0400 (EDT) Received: by ioii196 with SMTP id i196so133540789ioi.3 for ; Fri, 02 Oct 2015 14:21:51 -0700 (PDT) Received: from ale.deltatee.com (ale.deltatee.com. [207.54.116.67]) by mx.google.com with ESMTPS id e14si10192906ioi.65.2015.10.02.14.21.48 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 02 Oct 2015 14:21:50 -0700 (PDT) Date: Fri, 2 Oct 2015 15:21:37 -0600 From: Logan Gunthorpe Subject: Re: [PATCH 14/15] mm, dax, pmem: introduce {get|put}_dev_pagemap() for dax-gup Message-ID: <20151002212137.GB30448@deltatee.com> References: <20150923043737.36490.70547.stgit@dwillia2-desk3.jf.intel.com> <20150923044227.36490.99741.stgit@dwillia2-desk3.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150923044227.36490.99741.stgit@dwillia2-desk3.jf.intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams Cc: akpm@linux-foundation.org, Dave Hansen , linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Alexander Viro , linux-fsdevel@vger.kernel.org, Matthew Wilcox , Ross Zwisler , Stephen Bates Hi Dan, We've been doing some experimenting and testing with this patchset. Specifically, we are trying to use you're ZONE_DEVICE work to enable peer to peer PCIe transfers. This is actually working pretty well (though we're still testing and working through some things). However, we've found a couple of issues: On Wed, Sep 23, 2015 at 12:42:27AM -0400, Dan Williams wrote: > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 3d6baa7d4534..20097e7b679a 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -49,12 +49,16 @@ struct page { > * updated asynchronously */ > union { > struct address_space *mapping; /* If low bit clear, points to > - * inode address_space, or NULL. > + * inode address_space, unless > + * the page is in ZONE_DEVICE > + * then it points to its parent > + * dev_pagemap, otherwise NULL. > * If page mapped as anonymous > * memory, low bit is set, and > * it points to anon_vma object: > * see PAGE_MAPPING_ANON below. > */ > + struct dev_pagemap *pgmap; > void *s_mem; /* slab first object */ > }; When you add to this union and overide the mapping value, we see bugs in calls to set_page_dirty when it tries to dereference mapping. I believe a change to page_mapping is required such as the patch that's at the end of this email. > diff --git a/mm/gup.c b/mm/gup.c > index a798293fc648..1064e9a489a4 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -98,7 +98,16 @@ retry: > } > > page = vm_normal_page(vma, address, pte); > - if (unlikely(!page)) { > + if (!page && pte_devmap(pte) && (flags & FOLL_GET)) { > + /* > + * Only return device mapping pages in the FOLL_GET case since > + * they are only valid while holding the pgmap reference. > + */ > + if (get_dev_pagemap(pte_pfn(pte), NULL)) > + page = pte_page(pte); > + else > + goto no_page; > + } else if (unlikely(!page)) { I've found that if a driver creates a ZONE_DEVICE mapping but doesn't create the pagemap (using devm_register_pagemap) then the get_user_pages code will go into an infinite loop. I'm not really sure if this as an issue or not but it seems a bit undesirable for a buggy driver to be able to cause this. My thoughts are that either devm_register_pagemap needs to be done by devm_memremap_pages so a driver cannot use one without the other, or the GUP code needs to return EFAULT if no pagemap was registered so it doesn't loop forever. Thanks! Logan diff --git a/mm/util.c b/mm/util.c index 68ff8a5..19af683 100644 --- a/mm/util.c +++ b/mm/util.c @@ -368,6 +368,9 @@ struct address_space *page_mapping(struct page *page) return swap_address_space(entry); } + if (unlikely(is_zone_device_page(page))) + return NULL; + mapping = (unsigned long)page->mapping; if (mapping & PAGE_MAPPING_FLAGS) return NULL; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org