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

[Josh Zhao <joshsystem@xxxxxxxxx>]

2013/6/13 Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>:
> On Thu, Jun 13, 2013 at 02:24:11PM +0100, George Dunlap wrote:
>> 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
>> >(XEN)
>> >
>> >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 
>> >size)
>> >      if ( (pi = _tmh_alloc_page_thispool(d)) == NULL )
>> >          return NULL;
>> >      ASSERT(IS_VALID_PAGE(pi));
>> >-    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?
>
> Correct.


I don't understand why the map_domain_page() was required on >5TiB ?



>>
>> If that is the case, then we need to have a way to make sure tmem
>> cannot be enabled on such systems.
>
> Which Jan has had already done when he posted the 5TB patches.
>
>>
>>  -George
>
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