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authorMiklos Szeredi <mszeredi@redhat.com>2020-06-04 11:48:19 +0300
committerMiklos Szeredi <mszeredi@redhat.com>2020-06-04 11:48:19 +0300
commitdf820f8de4e481222b17f9bcee7b909ae8167529 (patch)
tree8410725a7cd85f540f59120710b8617c18348670 /Documentation/filesystems/porting.rst
parentb8e42a651bdee06202ebdd96cff64fdeabd5b1d6 (diff)
downloadlinux-df820f8de4e481222b17f9bcee7b909ae8167529.tar.xz
ovl: make private mounts longterm
Overlayfs is using clone_private_mount() to create internal mounts for underlying layers. These are used for operations requiring a path, such as dentry_open(). Since these private mounts are not in any namespace they are treated as short term, "detached" mounts and mntput() involves taking the global mount_lock, which can result in serious cacheline pingpong. Make these private mounts longterm instead, which trade the penalty on mntput() for a slightly longer shutdown time due to an added RCU grace period when putting these mounts. Introduce a new helper kern_unmount_many() that can take care of multiple longterm mounts with a single RCU grace period. Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
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diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index 26c093969573..867036aa90b8 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -858,3 +858,10 @@ be misspelled d_alloc_anon().
[should've been added in 2016] stale comment in finish_open() nonwithstanding,
failure exits in ->atomic_open() instances should *NOT* fput() the file,
no matter what. Everything is handled by the caller.
+
+---
+
+**mandatory**
+
+clone_private_mount() returns a longterm mount now, so the proper destructor of
+its result is kern_unmount() or kern_unmount_array().