On 01/06/2011 11:49 AM, Andrea Arcangeli wrote:
On Wed, Jan 05, 2011 at 03:27:37PM -0600, Michael Roth wrote:
On 01/05/2011 02:35 PM, Andrea Arcangeli wrote:
On Wed, Jan 05, 2011 at 02:26:19PM -0600, Michael Roth wrote:
Yah you're right, but I've seen several discussions about using mempath
for tmpfs/ram-backed files for things like numa/zram/etc so tend to
think of it as something potentially more than just a hook for
hugetlbfs, which is becoming less and less useful. But the MADV_DONTFORK
stuff should still be immediately applicable.
Yes, MADV_DONTFORK should be set all on all guest physical memory
without options so I hope the new patch I just posted is fine to stop
the spurious -ENOMEM failures in fork.
The patch in this thread? A couple paths still aren't covered when using
-mem-path. Something like this should get them all:
Well the reason of MADV_DONTFORK is to avoid accounting issues with
anonymous memory, mem-path don't have that issue as hugetlbfs skips
the accounting (it has to because hugetlbfs are not always taken from
the regular page allocator). It could be however considered a minor
performance optimization.
That's one case, but there's also a wonky fallback in that path that
defaults to the normal qemu_vmalloc():
if (mem_path) {
#if defined (__linux__) && !defined(TARGET_S390X)
new_block->host = file_ram_alloc(new_block, size, mem_path);
if (!new_block->host) {
new_block->host = qemu_vmalloc(size);
qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
}
May make sense to only add coverage for that specific case though. If
file_ram_alloc() is generalized we could deal with it then.
Now you mention that you want to use -mem-path for other things too,
so maybe that's why you need it there too. BTW, if you ever need it
for more than hugetlbfs, I'm afraid this MAP_PRIVATE I see when
mem_prealloc isn't set, is going to screw any other potential useful
usage besides hugetlbfs, not exactly sure why it makes any sense to
use MAP_PRIVATE there and not only MAP_SHARED.
Not sure either...but if it's another hugetlbfs-ism it shouldn't matter
since using mem-path for something other than hugetlbfs would ideally be
configurable with a -mem path=/dev/shm/vm0.mem,hugetlbfs=off or
something along that line, which wouldn't necessarily set MAP_PRIVATE.
