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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [RFC/PATCH] XENMEM_claim_pages (subop of an existing) hypercall
This is a first cut of the hypervisor patch of the proposed
XENMEM_claim_pages hypercall/subop. 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.
This patch is (so far) only compile-tested.
I don't have a patch for the toolstack side, but I envision
a "xl create --claim" option to maximize backwards
compatibility while minimizing impact on existing toolstacks.
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 (d->tot_pages may normally be zero
but see Note 1). 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. See Note 2.
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).
Note 1: Tim: I'm thinking this may resolve your concern that
the claim mechanism must be more complicated to handle
restricted memory allocations and order>0 allocations.
The proposed claim mechanism only guarantees a quantity of
order==0 pages; if restricted allocations are required, these
are done first by the toolstack, and followed by the claim.
Order>0 allocations still work if memory is not fragmented,
but the claim mechanism doesn't guarantee anything but
a quantity of order==0 pages.
Note 2: Tim: This arithmetic also indirectly implements the
"claim auto-expire" discussed earlier. We certainly don't
want a negative claim. It's possible that the toolstack need
never call "unclaim" (and so such a hypercall/subop need
not even exist) as long as domain_reset_unclaimed_pages()
is called when a domain dies. This draft doesn't even
provide unclaim... though if a reason for its existence
is determined, it should be easy to add.
Thanks for any feedback!
Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Diffstat:
arch/x86/mm/mem_sharing.c | 4 -
common/grant_table.c | 2
common/memory.c | 15 ++++++-
common/page_alloc.c | 93 +++++++++++++++++++++++++++++++++++++++++++++-
include/public/memory.h | 19 +++++++++
include/xen/mm.h | 7 +++
include/xen/sched.h | 1
7 files changed, 135 insertions(+), 6 deletions(-)
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) == 0 )
get_domain(d);
page_list_add_tail(page, &d->page_list);
spin_unlock(&d->page_alloc_lock);
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) == 0) )
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..fc7b24c 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 +685,19 @@ long do_memory_op(unsigned long cmd,
XEN_GUEST_HANDLE_PARAM(void) arg)
break;
}
+ case XENMEM_claim_pages:
+
+ rc = rcu_lock_target_domain_by_id(domid, &d);
+ if ( rc )
+ return rc;
+
+ rc = domain_set_unclaimed_pages(d, reservation.nr_extents,
+ reservation.mem_flags);
+
+ rcu_unlock_domain(d);
+
+ 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..ddda9a1 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -238,6 +238,90 @@ 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));
+ ASSERT(!spin_is_locked(&heap_lock));
+ spin_lock(&heap_lock);
+ d->tot_pages += pages;
+ if ( d->unclaimed_pages )
+ {
+ 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));
+ ASSERT(!spin_is_locked(&heap_lock));
+ spin_lock(&heap_lock);
+ d->tot_pages -= pages;
+ if ( d->unclaimed_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 avail_pages;
+
+ ASSERT(!spin_is_locked(&d->page_alloc_lock));
+ ASSERT(!spin_is_locked(&heap_lock));
+ spin_lock(&d->page_alloc_lock);
+ spin_lock(&heap_lock);
+ /* only one active claim per domain please */
+ if ( d->unclaimed_pages )
+ goto out;
+ avail_pages = total_avail_pages;
+ if ( !(flags & XENMEM_CLAIMF_free_only) )
+ avail_pages += tmem_freeable_pages();
+ avail_pages -= total_unclaimed_pages;
+ if ( pages < avail_pages )
+ {
+ /* ok if domain has allocated some pages before making a claim */
+ d->unclaimed_pages = pages - d->tot_pages;
+ total_unclaimed_pages += total_unclaimed_pages;
+ ret = 0;
+ }
+out:
+ spin_unlock(&heap_lock);
+ spin_unlock(&d->page_alloc_lock);
+ return ret;
+}
+
+void domain_reset_unclaimed_pages(struct domain *d)
+{
+ ASSERT(!spin_is_locked(&d->page_alloc_lock));
+ ASSERT(!spin_is_locked(&heap_lock));
+ spin_lock(&d->page_alloc_lock);
+ spin_lock(&heap_lock);
+ total_unclaimed_pages -= d->unclaimed_pages;
+ d->unclaimed_pages = 0;
+ spin_unlock(&heap_lock);
+ spin_unlock(&d->page_alloc_lock);
+}
static unsigned long init_node_heap(int node, unsigned long mfn,
unsigned long nr, bool_t *use_tail)
@@ -442,6 +526,11 @@ static struct page_info *alloc_heap_pages(
spin_lock(&heap_lock);
+ /* Claimed memory is considered not available */
+ if ( total_unclaimed_pages + request >
+ total_avail_pages + tmem_freeable_pages() )
+ 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.
@@ -1291,7 +1380,7 @@ 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 +1464,7 @@ 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..d8c8b4c 100644
--- a/xen/include/public/memory.h
+++ b/xen/include/public/memory.h
@@ -421,6 +421,25 @@ 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 -ENOMEM.
+ */
+#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)
+
#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..120b93f 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);
+void domain_reset_unclaimed_pages(struct domain *d);
+
/* 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 */
Attachment:
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