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

[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.
> 
> 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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