[Xen-devel] [PATCH v5] xen: Fix selfballooning and ensure it doesn't go too far

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

Note: This patch is also now in a git tree at:
git://oss.oracle.com/git/djm/tmem.git#selfballoon-fix-v5

xen: Fix selfballooning and ensure it doesn't go too far

The balloon driver's "current_pages" is very different from
totalram_pages.  Self-ballooning needs to be driven by
the latter.  Also, Committed_AS doesn't account for pages
used by the kernel so:
1) Add totalreserve_pages to Committed_AS for the normal target.
2) Enforce a floor for when there are little or no user-space threads
   using memory (e.g. single-user mode) to avoid OOMs.  The floor
   function includes a "min_usable_mb" tuneable in case we discover
   later that the floor function is still too aggressive in some
   workloads, though likely it will not be needed.

Changes since version 4:
- change floor calculation so that it is not as aggressive; this version
  uses a piecewise linear function similar to minimum_target in the 2.6.18
  balloon driver, but modified to add to totalreserve_pages instead of
  subtract from max_pfn, the 2.6.18 version causes OOMs on recent kernels
  because the kernel has bloated over time
- change safety_margin to min_usable_mb and comment on its use
- since committed_as does NOT include kernel space (and other reserved
  pages), totalreserve_pages is now added to committed_as.  The result is
  less aggressive self-ballooning, but theoretically more appropriate.
Changes since version 3:
- missing include causes compile problem when CONFIG_FRONTSWAP is disabled
- add comments after includes
Changes since version 2:
- missing include causes compile problem only on 32-bit
Changes since version 1:
- tuneable safety margin added
    
[v5: avi.miller@xxxxxxxxxx: still too aggressive, seeing some OOMs]
[v4: konrad.wilk@xxxxxxxxxx: fix compile when CONFIG_FRONTSWAP is disabled]
[v3: guru.anbalagane@xxxxxxxxxx: fix 32-bit compile]
[v2: konrad.wilk@xxxxxxxxxx: make safety margin tuneable]

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

diff --git a/drivers/xen/xen-selfballoon.c b/drivers/xen/xen-selfballoon.c
index 6ea852e..8bb07b6 100644
--- a/drivers/xen/xen-selfballoon.c
+++ b/drivers/xen/xen-selfballoon.c
@@ -68,6 +68,7 @@
  */
 
 #include <linux/kernel.h>
+#include <linux/bootmem.h>
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/module.h>
@@ -93,6 +94,15 @@ static unsigned int selfballoon_uphysteresis __read_mostly = 
1;
 /* In HZ, controls frequency of worker invocation. */
 static unsigned int selfballoon_interval __read_mostly = 5;
 
+/*
+ * Minimum usable RAM in MB for selfballooning target for balloon.
+ * If non-zero, it is added to totalreserve_pages and self-ballooning
+ * will not balloon below the sum.  If zero, a piecewise linear function
+ * is calculated as a minimum and added to totalreserve_pages.  Note that
+ * setting this value indiscriminately may cause OOMs and crashes.
+ */
+static unsigned int selfballoon_min_usable_mb;
+
 static void selfballoon_process(struct work_struct *work);
 static DECLARE_DELAYED_WORK(selfballoon_worker, selfballoon_process);
 
@@ -189,20 +199,23 @@ static int __init xen_selfballooning_setup(char *s)
 __setup("selfballooning", xen_selfballooning_setup);
 #endif /* CONFIG_FRONTSWAP */
 
+#define MB2PAGES(mb)   ((mb) << (20 - PAGE_SHIFT))
+
 /*
  * Use current balloon size, the goal (vm_committed_as), and hysteresis
  * parameters to set a new target balloon size
  */
 static void selfballoon_process(struct work_struct *work)
 {
-       unsigned long cur_pages, goal_pages, tgt_pages;
+       unsigned long cur_pages, goal_pages, tgt_pages, floor_pages;
+       unsigned long useful_pages;
        bool reset_timer = false;
 
        if (xen_selfballooning_enabled) {
-               cur_pages = balloon_stats.current_pages;
+               cur_pages = totalram_pages;
                tgt_pages = cur_pages; /* default is no change */
                goal_pages = percpu_counter_read_positive(&vm_committed_as) +
-                       balloon_stats.current_pages - totalram_pages;
+                               totalreserve_pages;
 #ifdef CONFIG_FRONTSWAP
                /* allow space for frontswap pages to be repatriated */
                if (frontswap_selfshrinking && frontswap_enabled)
@@ -217,7 +230,26 @@ static void selfballoon_process(struct work_struct *work)
                                ((goal_pages - cur_pages) /
                                  selfballoon_uphysteresis);
                /* else if cur_pages == goal_pages, no change */
-               balloon_set_new_target(tgt_pages);
+               useful_pages = max_pfn - totalreserve_pages;
+               if (selfballoon_min_usable_mb != 0)
+                       floor_pages = totalreserve_pages +
+                                       MB2PAGES(selfballoon_min_usable_mb);
+               /* piecewise linear function ending in ~3% slope */
+               else if (useful_pages < MB2PAGES(16))
+                       floor_pages = max_pfn; /* not worth ballooning */
+               else if (useful_pages < MB2PAGES(64))
+                       floor_pages = totalreserve_pages + MB2PAGES(16) +
+                                       ((useful_pages - MB2PAGES(16)) >> 1);
+               else if (useful_pages < MB2PAGES(512))
+                       floor_pages = totalreserve_pages + MB2PAGES(40) +
+                                       ((useful_pages - MB2PAGES(40)) >> 3);
+               else /* useful_pages >= MB2PAGES(512) */
+                       floor_pages = totalreserve_pages + MB2PAGES(99) +
+                                       ((useful_pages - MB2PAGES(99)) >> 5);
+               if (tgt_pages < floor_pages)
+                       tgt_pages = floor_pages;
+               balloon_set_new_target(tgt_pages +
+                       balloon_stats.current_pages - totalram_pages);
                reset_timer = true;
        }
 #ifdef CONFIG_FRONTSWAP
@@ -340,6 +372,31 @@ static ssize_t store_selfballoon_uphys(struct sys_device 
*dev,
 static SYSDEV_ATTR(selfballoon_uphysteresis, S_IRUGO | S_IWUSR,
                   show_selfballoon_uphys, store_selfballoon_uphys);
 
+SELFBALLOON_SHOW(selfballoon_min_usable_mb, "%d\n",
+                               selfballoon_min_usable_mb);
+
+static ssize_t store_selfballoon_min_usable_mb(struct sys_device *dev,
+                                              struct sysdev_attribute *attr,
+                                              const char *buf,
+                                              size_t count)
+{
+       unsigned long val;
+       int err;
+
+       if (!capable(CAP_SYS_ADMIN))
+               return -EPERM;
+       err = strict_strtoul(buf, 10, &val);
+       if (err || val == 0)
+               return -EINVAL;
+       selfballoon_min_usable_mb = val;
+       return count;
+}
+
+static SYSDEV_ATTR(selfballoon_min_usable_mb, S_IRUGO | S_IWUSR,
+                  show_selfballoon_min_usable_mb,
+                  store_selfballoon_min_usable_mb);
+
+
 #ifdef CONFIG_FRONTSWAP
 SELFBALLOON_SHOW(frontswap_selfshrinking, "%d\n", frontswap_selfshrinking);
 
@@ -421,6 +478,7 @@ static struct attribute *selfballoon_attrs[] = {
        &attr_selfballoon_interval.attr,
        &attr_selfballoon_downhysteresis.attr,
        &attr_selfballoon_uphysteresis.attr,
+       &attr_selfballoon_min_usable_mb.attr,
 #ifdef CONFIG_FRONTSWAP
        &attr_frontswap_selfshrinking.attr,
        &attr_frontswap_hysteresis.attr,

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