On Wed, Feb 14, 2024 at 04:16:31PM -0800, Eric Biggers wrote: > On Mon, Feb 12, 2024 at 09:13:15PM -0500, Gabriel Krisman Bertazi wrote: > > From fscrypt perspective, once the key is available, the dentry will > > remain valid until evicted for other reasons, since keyed dentries don't > > require revalidation and, if the key is removed, the dentry is > > forcefully evicted. Therefore, we don't need to keep revalidating them > > repeatedly. > > > > Obviously, we can only do this if fscrypt is the only thing requiring > > revalidation for a dentry. For this reason, we only disable > > d_revalidate if the .d_revalidate hook is fscrypt_d_revalidate itself. > > > > It is safe to do it here because when moving the dentry to the > > plain-text version, we are holding the d_lock. We might race with a > > concurrent RCU lookup but this is harmless because, at worst, we will > > get an extra d_revalidate on the keyed dentry, which is will find the > > dentry is valid. > > > > Finally, now that we do more than just clear the DCACHE_NOKEY_NAME in > > fscrypt_handle_d_move, skip it entirely for plaintext dentries, to avoid > > extra costs. > > > > Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxx> > > I think this explanation misses an important point, which is that it's only > *directories* where a no-key dentry can become the regular dentry. The VFS does > the move because it only allows one dentry to exist per directory. > > For nondirectories, the dentries don't get reused and this patch is irrelevant. > > (Of course, there's no point in making fscrypt_handle_d_move() check whether the > dentry is a directory, since checking DCACHE_NOKEY_NAME is sufficient.) > > The diff itself looks good -- thanks. > Also, do I understand correctly that this patch is a performance optimization, not preventing a performance regression? The similar patch that precedes this one, "fscrypt: Drop d_revalidate for valid dentries during lookup", is about preventing a performance regression on dentries that aren't no-key. This patch looks deceptively similar, but it only affects no-key directory dentries, which we were already doing the fscrypt_d_revalidate for, even after the move to the plaintext name. It's probably still a worthwhile optimization to stop doing the fscrypt_d_revalidate when a directory dentry gets moved like that. But I want to make sure I'm correctly understanding each patch. - Eric
