From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Liu Subject: Re: [PATCH XEN v8 24/29] tools/libs/call: linux: touch newly allocated pages after madvise lockdown Date: Tue, 19 Jan 2016 13:24:20 +0000 Message-ID: <20160119132420.GZ1691@citrix.com> References: <1452864168.32341.97.camel@citrix.com> <1452864188-2417-1-git-send-email-ian.campbell@citrix.com> <1452864188-2417-25-git-send-email-ian.campbell@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1452864188-2417-25-git-send-email-ian.campbell@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Ian Campbell Cc: wei.liu2@citrix.com, Roger Pau =?iso-8859-1?Q?Monn=E9?= , ian.jackson@eu.citrix.com, xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org On Fri, Jan 15, 2016 at 01:23:03PM +0000, Ian Campbell wrote: > This avoids a potential issue with a fork after allocation but before > madvise. > > Signed-off-by: Ian Campbell > --- > v7: New, replacing "tools/libs/call: linux: avoid forking between mmap > and madvise". > --- > tools/libs/call/linux.c | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/tools/libs/call/linux.c b/tools/libs/call/linux.c > index 3641e41..651f380 100644 > --- a/tools/libs/call/linux.c > +++ b/tools/libs/call/linux.c I didn't notice you only handled this for Linux until now. I think FreeBSD and NetBSD need similar treatment, too? But then current BSD* code doesn't even support DONTFORK in madvise. Adding Roger for more input. The changes in this patch look fine to me. Wei. > @@ -88,7 +88,7 @@ void *osdep_alloc_pages(xencall_handle *xcall, size_t npages) > { > size_t size = npages * PAGE_SIZE; > void *p; > - int rc, saved_errno; > + int rc, i, saved_errno; > > /* Address returned by mmap is page aligned. */ > p = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_LOCKED, -1, 0); > @@ -107,6 +107,18 @@ void *osdep_alloc_pages(xencall_handle *xcall, size_t npages) > goto out; > } > > + /* > + * Touch each page in turn to force them to be un-CoWed, in case a > + * fork happened in another thread at an inopportune moment > + * above. The madvise() will prevent any subsequent fork calls from > + * causing the same problem. > + */ > + for ( i = 0; i < npages ; i++ ) > + { > + char *c = (char *)p + (i*PAGE_SIZE); > + *c = 0; > + } > + > return p; > > out: > -- > 2.1.4 >