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Re: [Xen-devel] [PATCH] xen/tmem: Don't use map_domain_page for long-life-time pages.

On 13/06/13 13:50, Konrad Rzeszutek Wilk wrote:
When using tmem with Xen 4.3 (and debug build) we end up with:

(XEN) Xen BUG at domain_page.c:143
(XEN) ----[ Xen-4.3-unstable  x86_64  debug=y  Not tainted ]----
(XEN) CPU:    3
(XEN) RIP:    e008:[<ffff82c4c01606a7>] map_domain_page+0x61d/0x6e1
(XEN) Xen call trace:
(XEN)    [<ffff82c4c01606a7>] map_domain_page+0x61d/0x6e1
(XEN)    [<ffff82c4c01373de>] cli_get_page+0x15e/0x17b
(XEN)    [<ffff82c4c01377c4>] tmh_copy_from_client+0x150/0x284
(XEN)    [<ffff82c4c0135929>] do_tmem_put+0x323/0x5c4
(XEN)    [<ffff82c4c0136510>] do_tmem_op+0x5a0/0xbd0
(XEN)    [<ffff82c4c022391b>] syscall_enter+0xeb/0x145

A bit of debugging revealed that the map_domain_page and unmap_domain_page
are meant for short life-time mappings. And that those mappings are finite.
In the 2 VCPU guest we only have 32 entries and once we have exhausted those
we trigger the BUG_ON condition.

The two functions - tmh_persistent_pool_page_[get,put] are used by the xmem_pool
when xmem_pool_[alloc,free] are called. These xmem_pool_* function are wrapped
in macro and functions - the entry points are via: tmem_malloc
and tmem_page_alloc. In both cases the users are in the hypervisor and they
do not seem to suffer from using the hypervisor virtual addresses.

CC: Bob Liu <bob.liu@xxxxxxxxxx>
CC: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
  xen/common/tmem_xen.c |    5 ++---
  1 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/xen/common/tmem_xen.c b/xen/common/tmem_xen.c
index 3a1f3c9..736a8c3 100644
--- a/xen/common/tmem_xen.c
+++ b/xen/common/tmem_xen.c
@@ -385,7 +385,7 @@ static void *tmh_persistent_pool_page_get(unsigned long 
      if ( (pi = _tmh_alloc_page_thispool(d)) == NULL )
          return NULL;
-    return __map_domain_page(pi);
+    return page_to_virt(pi);

Did I understand correctly that the map_domain_page() was required on >5TiB systems, presumably because of limited virtual address space? In which case this code will fail on those systems?

If that is the case, then we need to have a way to make sure tmem cannot be enabled on such systems.


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