Xen devel - Nov 2012 - [RFC/PATCH v6 1/2] hypervisor: XENMEM_claim_pages (subop of existing) hypercall

This is patch 1of2 of a sixth cut of the patch of the proposed
XENMEM_claim_pages hypercall/subop, taking into account review
feedback from Jan and Keir and IanC and Matthew Daley, plus some
fixes found via runtime debugging (using printk and privcmd only).

v5->v6:
- Fix post-inc problem [mattjd]

v4->v5:
- Split tools part into separate patch [JBeulich]
- Minor coding style fixups [JBeulich]
- Use rcu_lock_domain_by_id, not _target version [JBeulich]

v3->v4: (please ignore v3)
- Process error, sent stale patch, sorry [djm]

v2->v3:
- Add per-domain and global "get" for unclaimed info [JBeulich]
- New hypercall(s) should fail for unpriv callers [IanC]
- Non-zero extent_order disallowed [JBeulich]
- Remove bonehead ASSERTs [JBeulich]
- Need not hold heaplock for decrease case too [JBeulich]
- More descriptive failure return values [JBeulich]
- Don''t allow a claim to exceed max_pages [IanC]
- Subops must be in correct ifdef block in memory.c [keir]

v1->v2:
- Add reset-to-zero page claim in domain_kill [JBeulich]
- Proper handling of struct passed to hypercall [JBeulich]
- Fix alloc_heap_pages when a domain has a claim [JBeulich]
- Need not hold heap_lock if !d->unclaimed_pages [keir]
- Fix missed tot_pages call in donate_page [djm]
- Remove domain_reset_unclaimed_pages; use set with zero [djm]
- Bugfixes found through testing in set_unclaimed [djm]
- More comments in code [djm]
- Code formatting fixes [djm]

==
Motivation:

The goal of this hypercall is to attempt to atomically and very
quickly determine if there are sufficient pages available in the
system and, if so, "set aside" that quantity of pages for future
allocations by that domain.  Unlike an existing hypercall such as
increase_reservation or populate_physmap, specific physical
pageframes are not assigned to the domain because this
cannot be done sufficiently quickly (especially for very large
allocations in an arbitrarily fragmented system) and so the
existing mechanisms result in classic time-of-check-time-of-use
(TOCTOU) races.  One can think of claiming as similar to a
"lazy" allocation, but subsequent hypercalls are required
to do the actual physical pageframe allocation.

In order for a toolstack to "get" information about whether
a domain has a claim and, if so, how large, and also for
the toolstack to measure the total system-wide claim, a
second subop has been added and exposed through domctl
and libxl.

Built on top of this in the toolstack, I envision
a "xl create --claim" option to maximize backwards
compatibility while minimizing impact on existing toolstacks;
and perhaps an "xl free --claim" for the system-wide info.

It has been noted that this claim mechanism solves the
underlying problem (slow failure of domain creation) for
a large class of domains not not all, specifically not
handling (but also not making the problem worse for) PV
domains that specify the "superpages" flag, and 32-bit PV
domains on large RAM systems.  These may be addressed at a
later time.

Code overview:

Though the hypercall simply does arithmetic within locks,
some of the semantics in the code may be a bit subtle
so let me explain a bit.

The key variables (d->unclaimed_pages and total_unclaimed_pages)
start at zero if no claim has yet been staked for any domain.
(Perhaps a better name is "claimed_but_not_yet_possessed" but
that''s
a bit unwieldy.)  If no claim hypercalls are executed, there
should be no impact on existing usage.

When a claim is successfully staked by a domain, it is like a
watermark but there is no record kept of the size of the claim.
Instead, d->unclaimed_pages is set to the difference between
d->tot_pages and the claim.  When d->tot_pages increases or decreases,
d->unclaimed_pages atomically decreases or increases.  Once
d->unclaimed_pages reaches zero, the claim is satisfied and
d->unclaimed pages stays at zero -- unless a new claim is
subsequently staked.

The systemwide variable total_unclaimed_pages is always the sum
of d->unclaimed_pages, across all domains.  A non-domain-
specific heap allocation will fail if total_unclaimed_pages
exceeds free (plus, on tmem enabled systems, freeable) pages.

Claim semantics may be modified by flags.  The initial
implementation has one flag, which discerns whether the
caller would like tmem freeable pages to be considered
in determining whether or not the claim can be successfully
staked. Future flags may, for example, specify that the
caller would like the claim to be successful only if there
are sufficient pages available on a single node (per Dario''s
suggestion).

A claim can be cancelled by requesting a claim of zero pages.

A second subop returns the total outstanding claimed pages
systemwide.

Note: Save/restore/migrate may need to be modified,
else it can be documented that all claims are cancelled.

Thanks for any feedback!

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>

 arch/x86/mm.c             |    2 
 arch/x86/mm/mem_sharing.c |    4 -
 common/domain.c           |    1 
 common/domctl.c           |    1 
 common/grant_table.c      |    2 
 common/memory.c           |   33 ++++++++++++
 common/page_alloc.c       |  118 +++++++++++++++++++++++++++++++++++++++++++++-
 include/public/domctl.h   |    3 -
 include/public/memory.h   |   39 +++++++++++++++
 include/xen/mm.h          |    7 ++
 include/xen/sched.h       |    1 
 11 files changed, 203 insertions(+), 8 deletions(-)

diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index f2409ec..118f24a 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -36,7 +36,7 @@
 #include "grant_table.h"
 #include "hvm/save.h"
 
-#define XEN_DOMCTL_INTERFACE_VERSION 0x00000008
+#define XEN_DOMCTL_INTERFACE_VERSION 0x00000009
 
 /*
  * NB. xen_domctl.domain is an IN/OUT parameter for this operation.
@@ -95,6 +95,7 @@ struct xen_domctl_getdomaininfo {
     uint32_t flags;              /* XEN_DOMINF_* */
     uint64_aligned_t tot_pages;
     uint64_aligned_t max_pages;
+    uint64_aligned_t unclaimed_pages;
     uint64_aligned_t shr_pages;
     uint64_aligned_t paged_pages;
     uint64_aligned_t shared_info_frame; /* GMFN of shared_info struct */
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index fad3d33..7e55908 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3841,7 +3841,7 @@ int donate_page(
     {
         if ( d->tot_pages >= d->max_pages )
             goto fail;
-        d->tot_pages++;
+        domain_increase_tot_pages(d, 1);
     }
 
     page->count_info = PGC_allocated | 1;
diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index 5103285..943a3b5 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -639,7 +639,7 @@ static int page_make_sharable(struct domain *d,
     }
 
     page_set_owner(page, dom_cow);
-    d->tot_pages--;
+    domain_decrease_tot_pages(d, 1);
     drop_dom_ref = (d->tot_pages == 0);
     page_list_del(page, &d->page_list);
     spin_unlock(&d->page_alloc_lock);
@@ -680,7 +680,7 @@ static int page_make_private(struct domain *d, struct
page_info *page)
     ASSERT(page_get_owner(page) == dom_cow);
     page_set_owner(page, d);
 
-    if ( d->tot_pages++ == 0 )
+    if ( domain_increase_tot_pages(d, 1) == 1 )
         get_domain(d);
     page_list_add_tail(page, &d->page_list);
     spin_unlock(&d->page_alloc_lock);
diff --git a/xen/common/domain.c b/xen/common/domain.c
index 0e3e36a..95509e2 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -492,6 +492,7 @@ int domain_kill(struct domain *d)
         evtchn_destroy(d);
         gnttab_release_mappings(d);
         tmem_destroy(d->tmem);
+        domain_set_unclaimed_pages(d, 0, 0);
         d->tmem = NULL;
         /* fallthrough */
     case DOMDYING_dying:
diff --git a/xen/common/domctl.c b/xen/common/domctl.c
index e153cb4..19e3930 100644
--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -154,6 +154,7 @@ void getdomaininfo(struct domain *d, struct
xen_domctl_getdomaininfo *info)
 
     info->tot_pages         = d->tot_pages;
     info->max_pages         = d->max_pages;
+    info->unclaimed_pages   = d->unclaimed_pages;
     info->shr_pages         = atomic_read(&d->shr_pages);
     info->paged_pages       = atomic_read(&d->paged_pages);
     info->shared_info_frame = mfn_to_gmfn(d,
__pa(d->shared_info)>>PAGE_SHIFT);
diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 7912769..10ce78f 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1656,7 +1656,7 @@ gnttab_transfer(
         }
 
         /* Okay, add the page to ''e''. */
-        if ( unlikely(e->tot_pages++ == 0) )
+        if ( unlikely(domain_increase_tot_pages(e, 1) == 1) )
             get_knownalive_domain(e);
         page_list_add_tail(page, &e->page_list);
         page_set_owner(page, e);
diff --git a/xen/common/memory.c b/xen/common/memory.c
index 83e2666..adb7581 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -454,7 +454,7 @@ static long
memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
                              (j * (1UL << exch.out.extent_order)));
 
                 spin_lock(&d->page_alloc_lock);
-                d->tot_pages -= dec_count;
+                domain_decrease_tot_pages(d, dec_count);
                 drop_dom_ref = (dec_count && !d->tot_pages);
                 spin_unlock(&d->page_alloc_lock);
 
@@ -685,6 +685,37 @@ long do_memory_op(unsigned long cmd,
XEN_GUEST_HANDLE_PARAM(void) arg)
         break;
     }
 
+    case XENMEM_claim_pages:
+        if ( !IS_PRIV(current->domain) )
+            return -EPERM;
+
+        if ( copy_from_guest(&reservation, arg, 1) )
+            return -EFAULT;
+
+        if ( !guest_handle_is_null(reservation.extent_start) )
+            return -EINVAL;
+
+        if ( reservation.extent_order != 0 )
+            return -EINVAL;
+
+        d = rcu_lock_domain_by_id(reservation.domid);
+        if ( d == NULL )
+            return -EINVAL;
+
+        rc = domain_set_unclaimed_pages(d, reservation.nr_extents,
+                                        reservation.mem_flags);
+
+        rcu_unlock_domain(d);
+
+        break;
+
+    case XENMEM_get_unclaimed_pages:
+        if ( !IS_PRIV(current->domain) )
+            return -EPERM;
+
+        rc = get_total_unclaimed_pages();
+        break;
+
     default:
         rc = arch_memory_op(op, arg);
         break;
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 15ebc66..3d9ef54 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -238,6 +238,111 @@ static long midsize_alloc_zone_pages;
 #define MIDSIZE_ALLOC_FRAC 128
 
 static DEFINE_SPINLOCK(heap_lock);
+static long total_unclaimed_pages; /* total outstanding claims by all domains
*/
+
+unsigned long domain_increase_tot_pages(struct domain *d, unsigned long pages)
+{
+    long dom_before, dom_after, dom_claimed, sys_before, sys_after;
+
+    ASSERT(spin_is_locked(&d->page_alloc_lock));
+    if ( !d->unclaimed_pages )
+        return d->tot_pages += pages;
+    spin_lock(&heap_lock);
+    dom_before = d->unclaimed_pages;
+    dom_after = dom_before - pages;
+    if ( (dom_before > 0) && (dom_after < 0) )
+        dom_claimed = 0;
+    else
+        dom_claimed = dom_after;
+    sys_before = total_unclaimed_pages;
+    sys_after = sys_before - (dom_before - dom_claimed);
+    BUG_ON( (sys_before > 0) && (sys_after < 0) );
+    total_unclaimed_pages = sys_after;
+    d->unclaimed_pages = dom_claimed;
+    spin_unlock(&heap_lock);
+    return d->tot_pages;
+}
+
+unsigned long domain_decrease_tot_pages(struct domain *d, unsigned long pages)
+{
+    ASSERT(spin_is_locked(&d->page_alloc_lock));
+    if ( !d->unclaimed_pages )
+        return d->tot_pages -= pages;
+
+    spin_lock(&heap_lock);
+    d->tot_pages -= pages;
+    d->unclaimed_pages += pages;
+    total_unclaimed_pages += pages;
+    spin_unlock(&heap_lock);
+    return d->tot_pages;
+}
+
+int domain_set_unclaimed_pages(struct domain *d, unsigned long pages,
+                                unsigned long flags)
+{
+    int ret = -ENOMEM;
+    unsigned long claim, avail_pages;
+
+    /*
+     * take the domain''s page_alloc_lock, else all increases/decreases
+     * must always take the global heap_lock rather than only in the much
+     * rarer case that d->unclaimed_pages is non-zero
+     */
+    spin_lock(&d->page_alloc_lock);
+    spin_lock(&heap_lock);
+
+    /* pages==0 means "unset" the claim (and flags is ignored) */
+    if ( pages == 0 )
+    {
+        total_unclaimed_pages -= d->unclaimed_pages;
+        d->unclaimed_pages = 0;
+        ret = 0;
+        goto out;
+    }
+
+    /* only one active claim per domain please */
+    if ( d->unclaimed_pages )
+    {
+        ret = -EINVAL;
+        goto out;
+    }
+
+    /* disallow a claim not exceeding current tot_pages or above max_pages */
+    if ( (pages <= d->tot_pages) || (pages > d->max_pages) )
+    {
+        ret = -EINVAL;
+        goto out;
+    }
+
+    /* how much memory is available? */
+    avail_pages = total_avail_pages;
+    if ( !(flags & XENMEM_CLAIMF_free_only) )
+        avail_pages += tmem_freeable_pages();
+    avail_pages -= total_unclaimed_pages;
+
+    /*
+     * note, if domain has already allocated memory before making a claim 
+     * then the claim must take tot_pages into account
+     */
+    claim = pages - d->tot_pages;
+    if ( claim > avail_pages )
+        goto out;
+
+    /* yay, claim fits in available memory, stake the claim, success! */
+    d->unclaimed_pages = claim;
+    total_unclaimed_pages += d->unclaimed_pages;
+    ret = 0;
+
+out:
+    spin_unlock(&heap_lock);
+    spin_unlock(&d->page_alloc_lock);
+    return ret;
+}
+
+long get_total_unclaimed_pages(void)
+{
+    return total_unclaimed_pages;
+}
 
 static unsigned long init_node_heap(int node, unsigned long mfn,
                                     unsigned long nr, bool_t *use_tail)
@@ -443,6 +548,15 @@ static struct page_info *alloc_heap_pages(
     spin_lock(&heap_lock);
 
     /*
+     * Claimed memory is considered unavailable unless the request
+     * is made by a domain with sufficient unclaimed pages.
+     */
+    if ( (total_unclaimed_pages + request >
+          total_avail_pages + tmem_freeable_pages()) &&
+          (d == NULL || d->unclaimed_pages < request) )
+        goto not_found;
+
+    /*
      * TMEM: When available memory is scarce due to tmem absorbing it, allow
      * only mid-size allocations to avoid worst of fragmentation issues.
      * Others try tmem pools then fail.  This is a workaround until all
@@ -1291,7 +1405,7 @@ int assign_pages(
         if ( unlikely(d->tot_pages == 0) )
             get_knownalive_domain(d);
 
-        d->tot_pages += 1 << order;
+        domain_increase_tot_pages(d, 1 << order);
     }
 
     for ( i = 0; i < (1 << order); i++ )
@@ -1375,7 +1489,7 @@ void free_domheap_pages(struct page_info *pg, unsigned int
order)
             page_list_del2(&pg[i], &d->page_list,
&d->arch.relmem_list);
         }
 
-        d->tot_pages -= 1 << order;
+        domain_decrease_tot_pages(d, 1 << order);
         drop_dom_ref = (d->tot_pages == 0);
 
         spin_unlock_recursive(&d->page_alloc_lock);
diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
index f1ddbc0..15d6c72 100644
--- a/xen/include/public/memory.h
+++ b/xen/include/public/memory.h
@@ -68,6 +68,8 @@ struct xen_memory_reservation {
      *   IN:  GPFN bases of extents to populate with memory
      *   OUT: GMFN bases of extents that were allocated
      *   (NB. This command also updates the mach_to_phys translation table)
+     * XENMEM_claim_pages:
+     *   IN: must be zero
      */
     XEN_GUEST_HANDLE(xen_pfn_t) extent_start;
 
@@ -421,6 +423,43 @@ struct xen_mem_sharing_op {
 typedef struct xen_mem_sharing_op xen_mem_sharing_op_t;
 DEFINE_XEN_GUEST_HANDLE(xen_mem_sharing_op_t);
 
+/*
+ * Attempt to stake a claim for a domain on a quantity of pages
+ * of system RAM, but _not_ assign specific pageframes.  Only
+ * arithmetic is performed so the hypercall is very fast and need
+ * not be preemptible, thus sidestepping time-of-check-time-of-use
+ * races for memory allocation.  Returns 0 if the hypervisor page
+ * allocator has atomically and successfully claimed the requested
+ * number of pages, else non-zero.
+ *
+ * Any domain may have only one active claim.  When sufficient memory
+ * has been allocated to resolve the claim, the claim silently expires.
+ * Claiming zero pages effectively resets any outstanding claim and
+ * is always successful.
+ *
+ * Note that a valid claim may be staked even after memory has been
+ * allocated for a domain.  In this case, the claim is not incremental,
+ * i.e. if the domain''s tot_pages is 3, and a claim is staked for 10,
+ * only 7 additional pages are claimed.
+ *
+ * Caller must be privileged or the hypercall fails.
+ */
+#define XENMEM_claim_pages                  24
+/*
+ * XENMEM_claim_pages flags:
+ *  free_only: claim is successful only if sufficient free pages
+ *    are available.  If not set and tmem is enabled, hypervisor
+ *    may also consider tmem "freeable" pages to satisfy the claim.
+ */
+#define _XENMEM_CLAIMF_free_only            0
+#define XENMEM_CLAIMF_free_only            
(1U<<_XENMEM_CLAIMF_free_only)
+/*
+ * Get the number of pages currently claimed (but not yet
"possessed")
+ * across all domains.  The caller must be privileged but otherwise
+ * the call never fails. 
+ */
+#define XENMEM_get_unclaimed_pages            25
+
 #endif /* defined(__XEN__) || defined(__XEN_TOOLS__) */
 
 #endif /* __XEN_PUBLIC_MEMORY_H__ */
diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
index 64a0cc1..b4dee92 100644
--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -48,6 +48,13 @@ void free_xenheap_pages(void *v, unsigned int order);
 #define alloc_xenheap_page() (alloc_xenheap_pages(0,0))
 #define free_xenheap_page(v) (free_xenheap_pages(v,0))
 
+/* Claim handling */
+unsigned long domain_increase_tot_pages(struct domain *d, unsigned long pages);
+unsigned long domain_decrease_tot_pages(struct domain *d, unsigned long pages);
+int domain_set_unclaimed_pages(
+    struct domain *d, unsigned long pages, unsigned long flags);
+long get_total_unclaimed_pages(void);
+
 /* Domain suballocator. These functions are *not* interrupt-safe.*/
 void init_domheap_pages(paddr_t ps, paddr_t pe);
 struct page_info *alloc_domheap_pages(
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 6c55039..480ef39 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -242,6 +242,7 @@ struct domain
     struct page_list_head page_list;  /* linked list */
     struct page_list_head xenpage_list; /* linked list (size xenheap_pages) */
     unsigned int     tot_pages;       /* number of pages currently possesed */
+    unsigned int     unclaimed_pages; /* pages claimed but not possessed    */
     unsigned int     max_pages;       /* maximum value for tot_pages        */
     atomic_t         shr_pages;       /* number of shared pages             */
     atomic_t         paged_pages;     /* number of paged-out pages          */


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>>> On 20.11.12 at 18:04, Dan Magenheimer
<dan.magenheimer@oracle.com> wrote:
> This is patch 1of2 of a sixth cut of the patch of the proposed
> XENMEM_claim_pages hypercall/subop, taking into account review
> feedback from Jan and Keir and IanC and Matthew Daley, plus some
> fixes found via runtime debugging (using printk and privcmd only).
> 
> v5->v6:
> - Fix post-inc problem [mattjd]
I continue to miss an expiry mechanism for claims; as said before
I don''t think these should be held indefinitely.

Jan

> From: Jan Beulich [mailto:JBeulich@suse.com]
> Subject: Re: [RFC/PATCH v6 1/2] hypervisor: XENMEM_claim_pages (subop of
existing) hypercall
> 
> >>> On 20.11.12 at 18:04, Dan Magenheimer
<dan.magenheimer@oracle.com> wrote:
> > This is patch 1of2 of a sixth cut of the patch of the proposed
> > XENMEM_claim_pages hypercall/subop, taking into account review
> > feedback from Jan and Keir and IanC and Matthew Daley, plus some
> > fixes found via runtime debugging (using printk and privcmd only).
> >
> > v5->v6:
> > - Fix post-inc problem [mattjd]
> 
> I continue to miss an expiry mechanism for claims; as said before
> I don''t think these should be held indefinitely.
Do you mean some kind of time-based expiry?

The simple use model I have in mind results in toolstack
changes as follows:

Current:
- call populate_physmap repeatedly to achieve mem=N memory
- if any populate_physmap call fails, report -ENOMEM up the stack
- memory is held until domain dies or the toolstack decreases it

Proposed:
- call claim for mem=N amount of memory
- if claim succeeds:
    call populate_physmap repeatedly to achieve mem=N memory (failsafe)
  else
    report -ENOMEM up the stack
- claim is held until mem=N is achieved or the domain dies or
   the toolstack changes it to 0
- memory is held until domain dies or the toolstack decreases it

In both cases, the memory is held "indefinitely" so I''m
not sure I see why a claim would need a time-based expiry
when a populate_physmap doesn''t.  A claim is really just
a "soft" version of a populate_physmap where the quantity
of pages are (atomically) assigned to the domain but the specific
physical pageframes are (non-atomically) assigned later.

Does that make sense?

Thanks,
Dan

>>> On 21.11.12 at 17:16, Dan Magenheimer
<dan.magenheimer@oracle.com> wrote:
>>  From: Jan Beulich [mailto:JBeulich@suse.com]
>> Subject: Re: [RFC/PATCH v6 1/2] hypervisor: XENMEM_claim_pages (subop
of
> existing) hypercall
>> 
>> >>> On 20.11.12 at 18:04, Dan Magenheimer
<dan.magenheimer@oracle.com> wrote:
>> > This is patch 1of2 of a sixth cut of the patch of the proposed
>> > XENMEM_claim_pages hypercall/subop, taking into account review
>> > feedback from Jan and Keir and IanC and Matthew Daley, plus some
>> > fixes found via runtime debugging (using printk and privcmd only).
>> >
>> > v5->v6:
>> > - Fix post-inc problem [mattjd]
>> 
>> I continue to miss an expiry mechanism for claims; as said before
>> I don''t think these should be held indefinitely.
> 
> Do you mean some kind of time-based expiry?
> 
> The simple use model I have in mind results in toolstack
> changes as follows:
> 
> Current:
> - call populate_physmap repeatedly to achieve mem=N memory
> - if any populate_physmap call fails, report -ENOMEM up the stack
> - memory is held until domain dies or the toolstack decreases it
> 
> Proposed:
> - call claim for mem=N amount of memory
> - if claim succeeds:
>     call populate_physmap repeatedly to achieve mem=N memory (failsafe)
>   else
>     report -ENOMEM up the stack
> - claim is held until mem=N is achieved or the domain dies or
>    the toolstack changes it to 0
> - memory is held until domain dies or the toolstack decreases it
> 
> In both cases, the memory is held "indefinitely" so I''m
> not sure I see why a claim would need a time-based expiry
> when a populate_physmap doesn''t.  A claim is really just
> a "soft" version of a populate_physmap where the quantity
> of pages are (atomically) assigned to the domain but the specific
> physical pageframes are (non-atomically) assigned later.
> 
> Does that make sense?
I get your point, but it seems wasteful to me to set aside perhaps
large quantities of memory for e.g. a zombie domain cleanup of
which didn''t get to releasing the claim, yet.

Jan