Re: [Qemu-devel] [PATCH] os: don't corrupt pre-existing memory-backend data with prealloc

[Rik van Riel]

On Mon, 2017-02-27 at 11:10 +0000, Stefan Hajnoczi wrote:
> On Thu, Feb 23, 2017 at 10:59:22AM +0000, Daniel P. Berrange wrote:
> > When using a memory-backend object with prealloc turned on, QEMU
> > will memset() the first byte in every memory page to zero. While
> > this might have been acceptable for memory backends associated
> > with RAM, this corrupts application data for NVDIMMs.
> > 
> > Instead of setting every page to zero, read the current byte
> > value and then just write that same value back, so we are not
> > corrupting the original data.
> > 
> > Signed-off-by: Daniel P. Berrange <address@hidden>
> > ---
> > 
> > I'm unclear if this is actually still safe in practice ? Is the
> > compiler permitted to optimize away the read+write since it doesn't
> > change the memory value. I'd hope not, but I've been surprised
> > before...
> > 
> > IMHO this is another factor in favour of requesting an API from
> > the kernel to provide the prealloc behaviour we want.
> > 
> >  util/oslib-posix.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> > index 35012b9..8f5b656 100644
> > --- a/util/oslib-posix.c
> > +++ b/util/oslib-posix.c
> > @@ -355,7 +355,8 @@ void os_mem_prealloc(int fd, char *area, size_t
> > memory, Error **errp)
> >  
> >          /* MAP_POPULATE silently ignores failures */
> >          for (i = 0; i < numpages; i++) {
> > -            memset(area + (hpagesize * i), 0, 1);
> > +            char val = *(area + (hpagesize * i));
> > +            memset(area + (hpagesize * i), 0, val);
> 
> Please include a comment in the final patch explaining why we want to
> preserve memory contents.
> 
> In the case of NVDIMM I'm not sure if the memset is needed at
> all.  The
> memory already exists - no new pages need to be allocated by the
> kernel.
> We just want the page table entries to be populated for the NVDIMM
> when
> -mem-prealloc is used.
> 
> Perhaps Andrea or Rik have ideas on improving the kernel interface
> and
> whether mmap(MAP_POPULATE) should be used with NVDIMM instead of this
> userspace "touch every page" workaround?

Why do we need the page table entries to be populated
in advance at all?

The high cost of the page fault for regular memory
is zeroing out the memory pages before we give them
to userspace.

Simply faulting in the NVDIMM memory as it is touched
may make more sense than treating it like DRAM,
especially given that with DAX, NVDIMM areas may be
orders of magnitude larger than RAM, and we really
do not want to set up all the page tables for every
part of the guest DAX "disk".