vfs: Add a sysctl for automated deletion of dentry

Commit 681ce8623567 ("vfs: Delete the associated dentry when deleting a
file") introduced an unconditional deletion of the associated dentry when a
file is removed. However, this led to performance regressions in specific
benchmarks, such as ilebench.sum_operations/s [0], prompting a revert in
commit 4a4be1ad3a6e ("Revert "vfs: Delete the associated dentry when
deleting a file"").

This patch seeks to reintroduce the concept conditionally, where the
associated dentry is deleted only when the user explicitly opts for it
during file removal. A new sysctl fs.automated_deletion_of_dentry is
added for this purpose. Its default value is set to 0.

There are practical use cases for this proactive dentry reclamation.
Besides the Elasticsearch use case mentioned in commit 681ce8623567,
additional examples have surfaced in our production environment. For
instance, in video rendering services that continuously generate temporary
files, upload them to persistent storage servers, and then delete them, a
large number of negative dentries—serving no useful purpose—accumulate.
Users in such cases would benefit from proactively reclaiming these
negative dentries.

Link: https://lore.kernel.org/linux-fsdevel/202405291318.4dfbb352-oliver.sang@intel.com
Link: https://lore.kernel.org/all/20240912-programm-umgibt-a1145fa73bb6@brauner/
Suggested-by: Christian Brauner <brauner@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20240929122831.92515-1-laoar.shao@gmail.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 47499a1742bd49e7c84fbc5309d4d6f76f094aa3..30c61474dec519fe860b9ccaccb85bb56b9d278e 100644 (file)
--- a/Documentation/admin-guide/sysctl/fs.rst
+++ b/Documentation/admin-guide/sysctl/fs.rst
@@ -38,6 +38,11 @@ requests.  ``aio-max-nr`` allows you to change the maximum value
 ``aio-max-nr`` does not result in the
 pre-allocation or re-sizing of any kernel data structures.
 
+dentry-negative
+----------------------------
+
+Policy for negative dentries. Set to 1 to to always delete the dentry when a
+file is removed, and 0 to disable it. By default, this behavior is disabled.
 
 dentry-state
 ------------

diff --git a/fs/dcache.c b/fs/dcache.c
index 0f6b16ba30d082b7613b084a23f6a01ed6436b1f..0d7d1bfa23bdf5137bdf60ea74209b4615ea88d6 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -135,6 +135,7 @@ struct dentry_stat_t {
 static DEFINE_PER_CPU(long, nr_dentry);
 static DEFINE_PER_CPU(long, nr_dentry_unused);
 static DEFINE_PER_CPU(long, nr_dentry_negative);
+static int dentry_negative_policy;
 
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
 /* Statistics gathering. */
@@ -199,6 +200,15 @@ static struct ctl_table fs_dcache_sysctls[] = {
                .mode           = 0444,
                .proc_handler   = proc_nr_dentry,
        },
+       {
+               .procname       = "dentry-negative",
+               .data           = &dentry_negative_policy,
+               .maxlen         = sizeof(dentry_negative_policy),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = SYSCTL_ZERO,
+               .extra2         = SYSCTL_ONE,
+       },
 };
 
 static int __init init_fs_dcache_sysctls(void)
@@ -2401,6 +2411,8 @@ void d_delete(struct dentry * dentry)
         * Are we the only user?
         */
        if (dentry->d_lockref.count == 1) {
+               if (dentry_negative_policy)
+                       __d_drop(dentry);
                dentry->d_flags &= ~DCACHE_CANT_MOUNT;
                dentry_unlink_inode(dentry);
        } else {