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authorAl Viro <viro@zeniv.linux.org.uk>2019-06-29 18:31:24 -0400
committerAl Viro <viro@zeniv.linux.org.uk>2019-07-10 07:32:22 -0400
commit9bdebc2bd1c4abfbf44dc154cc152ec333e004de (patch)
tree9438b3e202e8adbb374ca2bbb607b3ca37026dbe /fs/internal.h
parente4e59906cfe4e215a4360962bedb3d6e5cca5158 (diff)
Teach shrink_dcache_parent() to cope with mixed-filesystem shrink lists
Currently, running into a shrink list that contains dentries from different filesystems can cause several unpleasant things for shrink_dcache_parent() and for umount(2). The first problem is that there's a window during shrink_dentry_list() between __dentry_kill() takes a victim out and dropping reference to its parent. During that window the parent looks like a genuine busy dentry. shrink_dcache_parent() (or, worse yet, shrink_dcache_for_umount()) coming at that time will see no eviction candidates and no indication that it needs to wait for some shrink_dentry_list() to proceed further. That applies for any shrink list that might intersect with the subtree we are trying to shrink; the only reason it does not blow on umount(2) in the mainline is that we unregister the memory shrinker before hitting shrink_dcache_for_umount(). Another problem happens if something in a mixed-filesystem shrink list gets be stuck in e.g. iput(), getting umount of unrelated fs to spin waiting for the stuck shrinker to get around to our dentries. Solution: 1) have shrink_dentry_list() decrement the parent's refcount and make sure it's on a shrink list (ours unless it already had been on some other) before calling __dentry_kill(). That eliminates the window when shrink_dcache_parent() would've blown past the entire subtree without noticing anything with zero refcount not on shrink lists. 2) when shrink_dcache_parent() has found no eviction candidates, but some dentries are still sitting on shrink lists, rather than repeating the scan in hope that shrinkers have progressed, scan looking for something on shrink lists with zero refcount. If such a thing is found, grab rcu_read_lock() and stop the scan, with caller locking it for eviction, dropping out of RCU and doing __dentry_kill(), with the same treatment for parent as shrink_dentry_list() would do. Note that right now mixed-filesystem shrink lists do not occur, so this is not a mainline bug. Howevere, there's a bunch of uses for such beasts (e.g. the "try and evict everything we can out of given page" patches; there are potential uses in mount-related code, considerably simplifying the life in fs/namespace.c, etc.) Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs/internal.h')
-rw-r--r--fs/internal.h2
1 files changed, 2 insertions, 0 deletions
diff --git a/fs/internal.h b/fs/internal.h
index a48ef81be37d..dc317abe31b5 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -156,6 +156,8 @@ extern int d_set_mounted(struct dentry *dentry);
extern long prune_dcache_sb(struct super_block *sb, struct shrink_control *sc);
extern struct dentry *d_alloc_cursor(struct dentry *);
extern struct dentry * d_alloc_pseudo(struct super_block *, const struct qstr *);
+extern void dput_to_list(struct dentry *, struct list_head *);
+extern void shrink_dentry_list(struct list_head *);
/*
* read_write.c