From 07e5bfe651f8595ca6a777d989016aa8d8217924 Mon Sep 17 00:00:00 2001
From: Chinwen Chang <chinwen.chang@mediatek.com>
Date: Tue, 13 Oct 2020 16:53:39 -0700
Subject: [PATCH 1/3] mmap locking API: add mmap_lock_is_contended()
Git-commit: 07e5bfe651f8595ca6a777d989016aa8d8217924
Patch-mainline: v5.10-rc1
References: bsc#1201990

Patch series "Try to release mmap_lock temporarily in smaps_rollup", v4.

Recently, we have observed some janky issues caused by unpleasantly long
contention on mmap_lock which is held by smaps_rollup when probing large
processes.  To address the problem, we let smaps_rollup detect if anyone
wants to acquire mmap_lock for write attempts.  If yes, just release the
lock temporarily to ease the contention.

smaps_rollup is a procfs interface which allows users to summarize the
process's memory usage without the overhead of seq_* calls.  Android uses
it to sample the memory usage of various processes to balance its memory
pool sizes.  If no one wants to take the lock for write requests,
smaps_rollup with this patch will behave like the original one.

Although there are on-going mmap_lock optimizations like range-based
locks, the lock applied to smaps_rollup would be the coarse one, which is
hard to avoid the occurrence of aforementioned issues.  So the detection
and temporary release for write attempts on mmap_lock in smaps_rollup is
still necessary.

This patch (of 3):

Add new API to query if someone wants to acquire mmap_lock for write
attempts.

Using this instead of rwsem_is_contended makes it more tolerant of future
changes to the lock type.

Signed-off-by: Chinwen Chang <chinwen.chang@mediatek.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Michel Lespinasse <walken@google.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Daniel Kiss <daniel.kiss@arm.com>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jimmy Assarsson <jimmyassarsson@gmail.com>
Cc: Laurent Dufour <ldufour@linux.ibm.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: http://lkml.kernel.org/r/1597715898-3854-1-git-send-email-chinwen.chang@mediatek.com
Link: http://lkml.kernel.org/r/1597715898-3854-2-git-send-email-chinwen.chang@mediatek.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>

---
 include/linux/mmap_lock.h |    5 +++++
 1 file changed, 5 insertions(+)

--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -54,4 +54,9 @@ static inline void mmap_read_unlock(stru
 	up_read(&mm->mmap_sem);
 }
 
+static inline int mmap_sem_is_contended(struct mm_struct *mm)
+{
+	return rwsem_is_contended(&mm->mmap_sem);
+}
+
 #endif /* _LINUX_MMAP_LOCK_H */