| Message ID | 4D24E249.2040501@linux.vnet.ibm.com |
|---|---|
| State | New |
| Headers | show |
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. 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.
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.
diff --git a/exec.c b/exec.c index 49c28b1..cbdcb16 100644 --- a/exec.c +++ b/exec.c @@ -2853,6 +2853,9 @@ ram_addr_t qemu_ram_alloc_from_ptr(DeviceState *dev, const char *name, #endif qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE); } + + /* no allocation failures during fork/exec for migrate/hotplug */ + qemu_madvise(new_block->host, size, QEMU_MADV_DONTFORK); }