[Xen-devel] [PATCH 1 of 4] Prevent low values of max_pages for domains doing sharing or paging

[Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>]

 xen/common/domctl.c |  8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)


Apparently, setting d->max_pages to something lower than d->tot_pages is
used as a mechanism for controling a domain's footprint. It will result
in all new page allocations failing.

This is a really bad idea with paging or sharing, as regular guest memory
accesses may need to be satisfied by allocating new memory (either to
page in or to unshare).

Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>

diff -r 62b1fe67b8d1 -r 11fd4e0a1e1a xen/common/domctl.c
--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -813,8 +813,14 @@ long do_domctl(XEN_GUEST_HANDLE(xen_domc
          * NB. We removed a check that new_max >= current tot_pages; this means
          * that the domain will now be allowed to "ratchet" down to new_max. In
          * the meantime, while tot > max, all new allocations are disallowed.
+         *
+         * Except that this is not a great idea for domains doing sharing or 
+         * paging, as they need to perform new allocations on the fly.
          */
-        d->max_pages = new_max;
+        if ( (new_max > d->max_pages) ||
+             !((d->mem_event->paging.ring_page != NULL) ||
+                d->arch.hvm_domain.mem_sharing_enabled) )
+            d->max_pages = new_max;
         ret = 0;
         spin_unlock(&d->page_alloc_lock);
 

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