From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Rapoport Subject: Re: [RFC PATCH] userfaultfd: Add feature to request for a signal delivery Date: Wed, 28 Jun 2017 16:18:07 +0300 Message-ID: <20170628131806.GD10091@rapoport-lnx> References: <9363561f-a9cd-7ab6-9c11-ab9a99dc89f1@oracle.com> <20170627070643.GA28078@dhcp22.suse.cz> <20170627153557.GB10091@rapoport-lnx> <51508e99-d2dd-894f-8d8a-678e3747c1ee@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <51508e99-d2dd-894f-8d8a-678e3747c1ee-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Prakash Sangappa Cc: Michal Hocko , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Andrea Arcangeli , Mike Kravetz , Dave Hansen , Christoph Hellwig , linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-api@vger.kernel.org On Tue, Jun 27, 2017 at 09:01:20AM -0700, Prakash Sangappa wrote: > On 6/27/17 8:35 AM, Mike Rapoport wrote: > > >On Tue, Jun 27, 2017 at 09:06:43AM +0200, Michal Hocko wrote: > >>This is an user visible API so let's CC linux-api mailing list. > >> > >>On Mon 26-06-17 12:46:13, Prakash Sangappa wrote: > >>>In some cases, userfaultfd mechanism should just deliver a SIGBUS signal > >>>to the faulting process, instead of the page-fault event. Dealing with > >>>page-fault event using a monitor thread can be an overhead in these > >>>cases. For example applications like the database could use the signaling > >>>mechanism for robustness purpose. > >>this is rather confusing. What is the reason that the monitor would be > >>slower than signal delivery and handling? > >> > >>>Database uses hugetlbfs for performance reason. Files on hugetlbfs > >>>filesystem are created and huge pages allocated using fallocate() API. > >>>Pages are deallocated/freed using fallocate() hole punching support. > >>>These files are mmapped and accessed by many processes as shared memory. > >>>The database keeps track of which offsets in the hugetlbfs file have > >>>pages allocated. > >>> > >>>Any access to mapped address over holes in the file, which can occur due > >>>to bugs in the application, is considered invalid and expect the process > >>>to simply receive a SIGBUS. However, currently when a hole in the file is > >>>accessed via the mapped address, kernel/mm attempts to automatically > >>>allocate a page at page fault time, resulting in implicitly filling the > >>>hole in the file. This may not be the desired behavior for applications > >>>like the database that want to explicitly manage page allocations of > >>>hugetlbfs files. > >>So you register UFFD_FEATURE_SIGBUS on each region tha you are unmapping > >>and than just let those offenders die? > >If I understand correctly, the database will create the mapping, then it'll > >open userfaultfd and register those mappings with the userfault. > >Afterwards, when the application accesses a hole userfault will cause > >SIGBUS and the application will process it in whatever way it likes, e.g. > >just die. > > Yes. > > >What I don't understand is why won't you use userfault monitor process that > >will take care of the page fault events? > >It shouldn't be much overhead running it and it can keep track on all the > >userfault file descriptors for you and it will allow more versatile error > >handling that SIGBUS. > > > > Co-ordination with the external monitor process by all the database > processes > to send their userfaultfd is still an overhead. You are planning to register in userfaultfd only the holes you punch to deallocate pages, am I right? And the co-ordination of the userfault file descriptor with the monitor would have been added after calls to fallocate() and userfaultfd_register()? I've just been thinking that maybe it would be possible to use UFFD_EVENT_REMOVE for this case. We anyway need to implement the generation of UFFD_EVENT_REMOVE for the case of hole punching in hugetlbfs for non-cooperative userfaultfd. It could be that it will solve your issue as well. > >>>Using userfaultfd mechanism, with this support to get a signal, database > >>>application can prevent pages from being allocated implicitly when > >>>processes access mapped address over holes in the file. > >>> > >>>This patch adds the feature to request for a SIGBUS signal to userfaultfd > >>>mechanism. > >>> > >>>See following for previous discussion about the database requirement > >>>leading to this proposal as suggested by Andrea. > >>> > >>>http://www.spinics.net/lists/linux-mm/msg129224.html > >>Please make those requirements part of the changelog. > >> > >>>Signed-off-by: Prakash > >>>--- > >>> fs/userfaultfd.c | 5 +++++ > >>> include/uapi/linux/userfaultfd.h | 10 +++++++++- > >>> 2 files changed, 14 insertions(+), 1 deletion(-) > >>> > >>>diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > >>>index 1d622f2..5686d6d2 100644 > >>>--- a/fs/userfaultfd.c > >>>+++ b/fs/userfaultfd.c > >>>@@ -371,6 +371,11 @@ int handle_userfault(struct vm_fault *vmf, unsigned > >>>long reason) > >>> VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP)); > >>> VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP)); > >>> > >>>+ if (ctx->features & UFFD_FEATURE_SIGBUS) { > >>>+ goto out; > >>>+ } > >>>+ > >>> /* > >>> * If it's already released don't get it. This avoids to loop > >>> * in __get_user_pages if userfaultfd_release waits on the > >>>diff --git a/include/uapi/linux/userfaultfd.h > >>>b/include/uapi/linux/userfaultfd.h > >>>index 3b05953..d39d5db 100644 > >>>--- a/include/uapi/linux/userfaultfd.h > >>>+++ b/include/uapi/linux/userfaultfd.h > >>>@@ -23,7 +23,8 @@ > >>> UFFD_FEATURE_EVENT_REMOVE | \ > >>> UFFD_FEATURE_EVENT_UNMAP | \ > >>> UFFD_FEATURE_MISSING_HUGETLBFS | \ > >>>- UFFD_FEATURE_MISSING_SHMEM) > >>>+ UFFD_FEATURE_MISSING_SHMEM | \ > >>>+ UFFD_FEATURE_SIGBUS) > >>> #define UFFD_API_IOCTLS \ > >>> ((__u64)1 << _UFFDIO_REGISTER | \ > >>> (__u64)1 << _UFFDIO_UNREGISTER | \ > >>>@@ -153,6 +154,12 @@ struct uffdio_api { > >>> * UFFD_FEATURE_MISSING_SHMEM works the same as > >>> * UFFD_FEATURE_MISSING_HUGETLBFS, but it applies to shmem > >>> * (i.e. tmpfs and other shmem based APIs). > >>>+ * > >>>+ * UFFD_FEATURE_SIGBUS feature means no page-fault > >>>+ * (UFFD_EVENT_PAGEFAULT) event will be delivered, instead > >>>+ * a SIGBUS signal will be sent to the faulting process. > >>>+ * The application process can enable this behavior by adding > >>>+ * it to uffdio_api.features. > >>> */ > >>> #define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0) > >>> #define UFFD_FEATURE_EVENT_FORK (1<<1) > >>>@@ -161,6 +168,7 @@ struct uffdio_api { > >>> #define UFFD_FEATURE_MISSING_HUGETLBFS (1<<4) > >>> #define UFFD_FEATURE_MISSING_SHMEM (1<<5) > >>> #define UFFD_FEATURE_EVENT_UNMAP (1<<6) > >>>+#define UFFD_FEATURE_SIGBUS (1<<7) > >>> __u64 features; > >>> > >>> __u64 ioctls; > >>>-- > >>>2.7.4 > >>> > >>-- > >>Michal Hocko > >>SUSE Labs > >> > >-- > >Sincerely yours, > >Mike. > > >