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Tsirkin" , "Dr . David Alan Gilbert" , qemu-devel@nongnu.org, Pankaj Gupta , Igor Mammedov , Paolo Bonzini Subject: Re: [PATCH v3 6/7] util/oslib-posix: Support concurrent os_mem_prealloc() invocation Message-ID: References: <20210816094739.21970-1-david@redhat.com> <20210816094739.21970-7-david@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, Sep 28, 2021 at 05:56:41PM +0100, Daniel P. Berrangé wrote: > On Mon, Aug 16, 2021 at 11:47:38AM +0200, David Hildenbrand wrote: > > Add a mutex to protect the SIGBUS case, as we cannot mess concurrently > > with the sigbus handler and we have to manage the global variable > > sigbus_memset_context. The MADV_POPULATE_WRITE path can run > > concurrently. > > > > Note that page_mutex and page_cond are shared between concurrent > > invocations, which shouldn't be a problem. > > > > This is a preparation for future virtio-mem prealloc code, which will call > > os_mem_prealloc() asynchronously from an iothread when handling guest > > requests. > > > > Add a comment that messing with the SIGBUS handler is frowned upon and > > can result in problems we fortunately haven't seen so far. Note that > > forwarding signals to the already installed SIGBUS handler isn't clean > > either, as that one might modify the SIGBUS handler again. > > Even with the mutex, messing with SIGBUS post-startup still isn't safe > as we're clashing with SIGBUS usage in softmmu/cpus.c > > IIUC, the virtio-mem prealloc code is something new that we've not > shipped yet. With this in mind, how about we simply enforce that > usage of this new feature is dependant on kernel support for > MADV_POPULATE_WRITE ? If users want this feature they'll simply > need to update to a modern kernel. This shouldn't break any existing > deployed QEMU guests IIUC Oh, I should have read ahead to the next patch where you address this issue. With that in mind I'm ok adding Reviewed-by: Daniel P. Berrangé > > > > > Reviewed-by: Pankaj Gupta > > Signed-off-by: David Hildenbrand > > --- > > util/oslib-posix.c | 9 +++++++++ > > 1 file changed, 9 insertions(+) > > > > diff --git a/util/oslib-posix.c b/util/oslib-posix.c > > index efa4f96d56..9829149e4b 100644 > > --- a/util/oslib-posix.c > > +++ b/util/oslib-posix.c > > @@ -95,6 +95,7 @@ typedef struct MemsetThread MemsetThread; > > > > /* used by sigbus_handler() */ > > static MemsetContext *sigbus_memset_context; > > +static QemuMutex sigbus_mutex; > > > > static QemuMutex page_mutex; > b> static QemuCond page_cond; > > @@ -625,6 +626,7 @@ static bool madv_populate_write_possible(char *area, size_t pagesize) > > void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus, > > Error **errp) > > { > > + static gsize initialized; > > int ret; > > struct sigaction act, oldact; > > size_t hpagesize = qemu_fd_getpagesize(fd); > > @@ -638,6 +640,12 @@ void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus, > > use_madv_populate_write = madv_populate_write_possible(area, hpagesize); > > > > if (!use_madv_populate_write) { > > + if (g_once_init_enter(&initialized)) { > > + qemu_mutex_init(&sigbus_mutex); > > + g_once_init_leave(&initialized, 1); > > + } > > + > > + qemu_mutex_lock(&sigbus_mutex); > > memset(&act, 0, sizeof(act)); > > act.sa_handler = &sigbus_handler; > > act.sa_flags = 0; > > @@ -665,6 +673,7 @@ void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus, > > perror("os_mem_prealloc: failed to reinstall signal handler"); > > exit(1); > > } > > + qemu_mutex_unlock(&sigbus_mutex); > > } > > } > > > > -- > > 2.31.1 > > > > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| > > Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|