[RFC PATCH -mm] provide estimated available memory in /proc/meminfo

[Rik van Riel]

Many load balancing and workload placing programs check /proc/meminfo
to estimate how much free memory is available. They generally do this
by adding up "free" and "cached", which was fine ten years ago, but
is pretty much guaranteed to be wrong today.

It is wrong because Cached includes memory that is not freeable as
page cache, for example shared memory segments, tmpfs, and ramfs,
and it does not include reclaimable slab memory, which can take up
a large fraction of system memory on mostly idle systems with lots
of files.

Currently, the amount of memory that is available for a new workload,
without pushing the system into swap, can be estimated from MemFree,
Active(file), Inactive(file), and SReclaimable, as well as the "low"
watermarks from /proc/zoneinfo.

However, this may change in the future, and user space really should
not be expected to know kernel internals to come up with an estimate
for the amount of free memory.

It is more convenient to provide such an estimate in /proc/meminfo,
if things change in the future, we only have to change it in one
place.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Reported-by: Erik Mouw <erik.mouw_2@xxxxxxx>
---
 fs/proc/meminfo.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index 5aa847a..1c43db5 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -25,9 +25,12 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	struct sysinfo i;
 	unsigned long committed;
 	unsigned long allowed;
+	long available;
+	unsigned long pagecache, wmark_low = 0;
 	struct vmalloc_info vmi;
 	long cached;
 	unsigned long pages[NR_LRU_LISTS];
+	struct zone *zone;
 	int lru;
 
 /*
@@ -50,12 +53,44 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)
 		pages[lru] = global_page_state(NR_LRU_BASE + lru);
 
+	for_each_zone(zone)
+		wmark_low += zone->watermark[WMARK_LOW];
+
+	/*
+	 * Estimate the amount of memory available for userspace allocations,
+	 * without causing swapping.
+	 *
+	 * Free memory cannot be taken below the low watermark, before the
+	 * system starts swapping.
+	 */
+	available = i.freeram - wmark_low;
+
+	/*
+	 * Not all the page cache can be freed, otherwise the system will
+	 * start swapping. Assume at least half of the page cache, or the
+	 * low watermark worth of cache, needs to stay.
+	 */
+	pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
+	pagecache -= min(pagecache / 2, wmark_low);
+	available += pagecache;
+
+	/*
+	 * Part of the reclaimable swap consists of items that are in use,
+	 * and cannot be freed. Cap this estimate at the low watermark.
+	 */
+	available += global_page_state(NR_SLAB_RECLAIMABLE) -
+		     min(global_page_state(NR_SLAB_RECLAIMABLE) / 2, wmark_low);
+
+	if (available < 0)
+		available = 0;
+
 	/*
 	 * Tagged format, for easy grepping and expansion.
 	 */
 	seq_printf(m,
 		"MemTotal:       %8lu kB\n"
 		"MemFree:        %8lu kB\n"
+		"MemAvailable:   %8lu kB\n"
 		"Buffers:        %8lu kB\n"
 		"Cached:         %8lu kB\n"
 		"SwapCached:     %8lu kB\n"
@@ -108,6 +143,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 		,
 		K(i.totalram),
 		K(i.freeram),
+		K(available),
 		K(i.bufferram),
 		K(cached),
 		K(total_swapcache_pages()),
--
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