mm, oom: add comment for why oom_adj exists

/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@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 29595af328669543a70115a7613539a48f6e4226..bd3e9e68125b8933b2e32a4fea2e2282397dc1cd 100644 (file)
--- 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)
 {