Messages in this thread | | | Date | Fri, 24 Apr 2009 10:31:28 +0200 | From | Ingo Molnar <> | Subject | Re: [rfc 2/2] x86, bts: use physically non-contiguous trace buffer |
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* Andrew Morton <akpm@linux-foundation.org> wrote:
> On Fri, 24 Apr 2009 10:00:55 +0200 Markus Metzger <markus.t.metzger@intel.com> wrote: > > > Use vmalloc to allocate the branch trace buffer. > > > > Peter Zijlstra suggested to use vmalloc rather than kmalloc to > > allocate the potentially multi-page branch trace buffer. > > The changelog provides no reason for this change. It should do so. > > > Is there a way to have vmalloc allocate a physically non-contiguous > > buffer for test purposes? Ideally, the memory area would have big > > holes in it with sensitive data in between so I would know immediately > > when this is overwritten. > > I suppose you could allocate the pages by hand and then vmap() them. > Allocating 2* the number you need and then freeing every second one > should make them physically holey. > > > --- a/arch/x86/kernel/ptrace.c > > +++ b/arch/x86/kernel/ptrace.c > > @@ -22,6 +22,7 @@ > > #include <linux/seccomp.h> > > #include <linux/signal.h> > > #include <linux/workqueue.h> > > +#include <linux/vmalloc.h> > > > > #include <asm/uaccess.h> > > #include <asm/pgtable.h> > > @@ -626,7 +627,7 @@ static int alloc_bts_buffer(struct bts_c > > if (err < 0) > > return err; > > > > - buffer = kzalloc(size, GFP_KERNEL); > > + buffer = vmalloc(size); > > if (!buffer) > > goto out_refund; > > > > @@ -646,7 +647,7 @@ static inline void free_bts_buffer(struc > > if (!context->buffer) > > return; > > > > - kfree(context->buffer); > > + vfree(context->buffer); > > context->buffer = NULL; > > > > The patch looks like a regression to me. vmalloc memory is slower > to allocate, slower to free, slower to access and can exhaust or > fragment the vmalloc arena. Confused.
Performance does not matter here (this is really a slowpath), but fragmentation does matter, especially on 32-bit systems.
I'd not uglify the code via vmap() - and vmap has the same fundamental address space limitations on 32-bit as vmalloc().
The existing kmalloc() is fine. We do larger than PAGE_SIZE allocations elsewhere too (the kernel stack for example), and this is a debug facility, so failing the allocation is not a big problem even if it happens.
Ingo
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