On Tue, Dec 17, 2024 at 8:44 AM Paul Moore <paul@xxxxxxxxxxxxxx> wrote: > > On Mon, Dec 16, 2024 at 8:24 PM Song Liu <song@xxxxxxxxxx> wrote: > > On Mon, Dec 16, 2024 at 4:22 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote: > > > > > > On Mon, Dec 16, 2024 at 6:43 PM Song Liu <song@xxxxxxxxxx> wrote: > > > > > > > > inode->i_security needes to be freed from RCU callback. A rcu_head was > > > > added to i_security to call the RCU callback. However, since struct inode > > > > already has i_rcu, the extra rcu_head is wasteful. Specifically, when any > > > > LSM uses i_security, a rcu_head (two pointers) is allocated for each > > > > inode. > > > > > > > > Add security_inode_free_rcu() to i_callback to free i_security so that > > > > a rcu_head is saved for each inode. Special care are needed for file > > > > systems that provide a destroy_inode() callback, but not a free_inode() > > > > callback. Specifically, the following logic are added to handle such > > > > cases: > > > > > > > > - XFS recycles inode after destroy_inode. The inodes are freed from > > > > recycle logic. Let xfs_inode_free_callback() and xfs_inode_alloc() > > > > call security_inode_free_rcu() before freeing the inode. > > > > - Let pipe free inode from a RCU callback. > > > > - Let btrfs-test free inode from a RCU callback. > > > > > > If I recall correctly, historically the vfs devs have pushed back on > > > filesystem specific changes such as this, requiring LSM hooks to > > > operate at the VFS layer unless there was absolutely no other choice. > > > > > > From a LSM perspective I'm also a little concerned that this approach > > > is too reliant on individual filesystems doing the right thing with > > > respect to LSM hooks which I worry will result in some ugly bugs in > > > the future. > > > > Totally agree with the concerns. However, given the savings is quite > > significant (saving two pointers per inode), I think the it may justify > > the extra effort to maintain the logic. Note that, some LSMs are > > enabled in most systems and cannot be easily disabled, so I am > > assuming most systems will see the savings. > > I suggest trying to find a solution that is not as fragile in the face > of cross subsystem changes and ideally also limits the number of times > the LSM calls must be made in individual filesystems. There are three (groups of) subsystems here: VFS, file systems, and LSM. It is not really possible to do this without crossing subsystem boundaries. Specifically, since VFS allow a file system to have destroy_inode callback, but not free_inode callback, we will need such file systems to handle rcu callback. Does this make sense? Suggestions on how we can solve this better are always appreciated. Thanks, Song
