[patch] mm, oom: add comment for why oom_adj exists

From: David Rientjes
Date: Wed Nov 04 2015 - 15:32:20 EST

/proc/pid/oom_adj exists solely to avoid breaking existing userspace
binaries that write to the tunable.

Add a comment in the only possible location within the kernel tree to
describe the situation and motivation for keeping it around.

Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
fs/proc/base.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1032,6 +1032,16 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
return simple_read_from_buffer(buf, count, ppos, buffer, len);

+ * /proc/pid/oom_adj exists solely for backwards compatibility with previous
+ * kernels. The effective policy is defined by oom_score_adj, which has a
+ * different scale: oom_adj grew exponentially and oom_score_adj grows linearly.
+ * Values written to oom_adj are simply mapped linearly to oom_score_adj.
+ * Processes that become oom disabled via oom_adj will still be oom disabled
+ * with this implementation.
+ *
+ * oom_adj cannot be removed since existing userspace binaries use it.
+ */
static ssize_t oom_adj_write(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
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