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ovl: skip overlayfs superblocks at global sync

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[ Upstream commit 32b1924b210a70dcacdf65abd687c5ef86a67541 ]

Stacked filesystems like overlayfs has no own writeback, but they have to
forward syncfs() requests to backend for keeping data integrity.

During global sync() each overlayfs instance calls method ->sync_fs() for
backend although it itself is in global list of superblocks too.  As a
result one syscall sync() could write one superblock several times and send
multiple disk barriers.

This patch adds flag SB_I_SKIP_SYNC into sb->sb_iflags to avoid that.

Reported-by: Dmitry Monakhov <[email protected]>
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Reviewed-by: Amir Goldstein <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>
Stable-dep-of: b836c4d29f27 ("ima: detect changes to the backing overlay file")
Signed-off-by: Sasha Levin <[email protected]>
This commit is contained in:
Konstantin Khlebnikov 2020-04-09 11:29:47 +03:00 • committed by Greg Kroah-Hartman
commit 8c85e455f7
3 changed files with 7 additions and 3 deletions
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5
fs/overlayfs/super.c
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@ -263,8 +263,8 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
return 0;
/*
* If this is a sync(2) call or an emergency sync, all the super blocks
* will be iterated, including upper_sb, so no need to do anything.
* Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC).
* All the super blocks will be iterated, including upper_sb.
*
* If this is a syncfs(2) call, then we do need to call
* sync_filesystem() on upper_sb, but enough if we do it when being
@ -1710,6 +1710,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
sb->s_xattr = ovl_xattr_handlers;
sb->s_fs_info = ofs;
sb->s_flags |= SB_POSIXACL;
sb->s_iflags |= SB_I_SKIP_SYNC;
err = -ENOMEM;
root_dentry = d_make_root(ovl_new_inode(sb, S_IFDIR, 0));