[PATCH v3 0/3] add support for drop_caches for individual filesystem

[PATCH v3 0/3] add support for drop_caches for individual filesystem

[Ye Bin]

From: Ye Bin <yebin10@huawei.com>

In order to better analyze the issue of file system uninstallation caused
by kernel module opening files, it is necessary to perform dentry recycling
on a single file system. But now, apart from global dentry recycling, it is
not supported to do dentry recycling on a single file system separately.
This feature has usage scenarios in problem localization scenarios.At the
same time, it also provides users with a slightly fine-grained
pagecache/entry recycling mechanism.
This patchset supports the recycling of pagecache/entry for individual file
systems.

Diff v3 vs v2
1. Introduce introduce drop_sb_dentry_inode() helper instead of
reclaim_dcache_sb()/reclaim_icache_sb() helper for reclaim dentry/inode.
2. Fixing compilation issues in specific architectures and configurations.

Diff v2 vs v1:
1. Fix possible live lock for shrink_icache_sb().
2. Introduce reclaim_dcache_sb() for reclaim dentry.
3. Fix potential deadlocks as follows:
https://lore.kernel.org/linux-fsdevel/00000000000098f75506153551a1@google.com/
After some consideration, it was decided that this feature would primarily
be used for debugging purposes. Instead of adding a new IOCTL command, the
task_work mechanism was employed to address potential deadlock issues.

Ye Bin (3):
  mm/vmscan: introduce drop_sb_dentry_inode() helper
  sysctl: add support for drop_caches for individual filesystem
  Documentation: add instructions for using 'drop_fs_caches sysctl'
    sysctl

 Documentation/admin-guide/sysctl/vm.rst |  44 +++++++++
 fs/drop_caches.c                        | 125 ++++++++++++++++++++++++
 include/linux/mm.h                      |   1 +
 mm/internal.h                           |   3 +
 mm/shrinker.c                           |   4 +-
 mm/vmscan.c                             |  50 ++++++++++
 6 files changed, 225 insertions(+), 2 deletions(-)

-- 
2.34.1

[PATCH v3 1/3] mm/vmscan: introduce drop_sb_dentry_inode() helper

[Ye Bin]

From: Ye Bin <yebin10@huawei.com>

This patch is prepare for support drop_caches for specify file system.

Signed-off-by: Ye Bin <yebin10@huawei.com>
---
 include/linux/mm.h |  1 +
 mm/internal.h      |  3 +++
 mm/shrinker.c      |  4 ++--
 mm/vmscan.c        | 50 ++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5be3d8a8f806..5bab9472a758 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4470,6 +4470,7 @@ static inline int in_gate_area(struct mm_struct *mm, unsigned long addr)
 bool process_shares_mm(const struct task_struct *p, const struct mm_struct *mm);
 
 void drop_slab(void);
+void drop_sb_dentry_inode(struct super_block *sb);
 
 #ifndef CONFIG_MMU
 #define randomize_va_space 0
diff --git a/mm/internal.h b/mm/internal.h
index cb0af847d7d9..4690a58c4820 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1662,6 +1662,9 @@ void __meminit __init_page_from_nid(unsigned long pfn, int nid);
 unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg,
 			  int priority);
 
+unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
+			     struct shrinker *shrinker, int priority);
+
 int shmem_add_to_page_cache(struct folio *folio,
 			    struct address_space *mapping,
 			    pgoff_t index, void *expected, gfp_t gfp);
diff --git a/mm/shrinker.c b/mm/shrinker.c
index 4a93fd433689..075e4393da9c 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -368,8 +368,8 @@ static long add_nr_deferred(long nr, struct shrinker *shrinker,
 
 #define SHRINK_BATCH 128
 
-static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
-				    struct shrinker *shrinker, int priority)
+unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
+			     struct shrinker *shrinker, int priority)
 {
 	unsigned long freed = 0;
 	unsigned long long delta;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 01d3364fe506..310bed25df78 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -455,6 +455,56 @@ void drop_slab(void)
 	} while ((freed >> shift++) > 1);
 }
 
+static unsigned long drop_shrinker_node(int nid, struct shrinker *shrinker)
+{
+	unsigned long freed = 0;
+	struct mem_cgroup *memcg = NULL;
+
+	memcg = mem_cgroup_iter(NULL, NULL, NULL);
+	do {
+		unsigned long ret;
+
+		struct shrink_control sc = {
+			.gfp_mask = GFP_KERNEL,
+			.nid = nid,
+			.memcg = memcg,
+		};
+
+		if (!mem_cgroup_disabled() &&
+		    !mem_cgroup_is_root(memcg) &&
+		    !mem_cgroup_online(memcg))
+			continue;
+
+		ret = do_shrink_slab(&sc, shrinker, 0);
+		if (ret == SHRINK_EMPTY)
+			ret = 0;
+		freed += ret;
+		cond_resched();
+	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
+
+	return freed;
+}
+
+void drop_sb_dentry_inode(struct super_block *sb)
+{
+	int nid;
+	int shift = 0;
+	unsigned long freed;
+
+	if (!sb || !sb->s_shrink)
+		return;
+
+	do {
+		freed = 0;
+
+		for_each_online_node(nid) {
+			if (fatal_signal_pending(current))
+				return;
+			freed += drop_shrinker_node(nid, sb->s_shrink);
+		}
+	} while ((freed >> shift++) > 1);
+}
+
 #define CHECK_RECLAIMER_OFFSET(type)					\
 	do {								\
 		BUILD_BUG_ON(PGSTEAL_##type - PGSTEAL_KSWAPD !=		\
-- 
2.34.1

[PATCH v3 2/3] sysctl: add support for drop_caches for individual filesystem

[Ye Bin]

From: Ye Bin <yebin10@huawei.com>

In order to better analyze the issue of file system uninstallation caused
by kernel module opening files, it is necessary to perform dentry recycling
on a single file system. But now, apart from global dentry recycling, it is
not supported to do dentry recycling on a single file system separately.
This feature has usage scenarios in problem localization scenarios.At the
same time, it also provides users with a slightly fine-grained
pagecache/entry recycling mechanism.
This patch supports the recycling of pagecache/entry for individual file
systems.

Signed-off-by: Ye Bin <yebin10@huawei.com>
---
 fs/drop_caches.c | 125 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)

diff --git a/fs/drop_caches.c b/fs/drop_caches.c
index 49f56a598ecb..0cd8ad9df07a 100644
--- a/fs/drop_caches.c
+++ b/fs/drop_caches.c
@@ -11,6 +11,8 @@
 #include <linux/sysctl.h>
 #include <linux/gfp.h>
 #include <linux/swap.h>
+#include <linux/task_work.h>
+#include <linux/namei.h>
 #include "internal.h"
 
 /* A global variable is a bit ugly, but it keeps the code simple */
@@ -78,6 +80,124 @@ static int drop_caches_sysctl_handler(const struct ctl_table *table, int write,
 	return 0;
 }
 
+struct drop_fs_caches_work {
+	struct callback_head task_work;
+	dev_t dev;
+	char *path;
+	unsigned int ctl;
+};
+
+static void drop_fs_caches(struct callback_head *twork)
+{
+	int ret;
+	struct super_block *sb;
+	static bool suppress;
+	struct drop_fs_caches_work *work = container_of(twork,
+			struct drop_fs_caches_work, task_work);
+	unsigned int ctl = work->ctl;
+	dev_t dev = work->dev;
+
+	if (work->path) {
+		struct path path;
+
+		ret = kern_path(work->path, LOOKUP_FOLLOW, &path);
+		if (ret) {
+			pr_err("%s (%d): %s: failed to get path(%s) %d\n",
+			       current->comm, task_pid_nr(current),
+			       __func__, work->path, ret);
+			goto out;
+		}
+		dev = path.dentry->d_sb->s_dev;
+		/* Make this file's dentry and inode recyclable */
+		path_put(&path);
+	}
+
+	sb = user_get_super(dev, false);
+	if (!sb) {
+		pr_err("%s (%d): %s: failed to get dev(%u:%u)'s sb\n",
+		       current->comm, task_pid_nr(current), __func__,
+		       MAJOR(dev), MINOR(dev));
+		goto out;
+	}
+
+	if (ctl & BIT(0)) {
+		lru_add_drain_all();
+		drop_pagecache_sb(sb, NULL);
+		count_vm_event(DROP_PAGECACHE);
+	}
+
+	if (ctl & BIT(1)) {
+		drop_sb_dentry_inode(sb);
+		count_vm_event(DROP_SLAB);
+	}
+
+	if (!READ_ONCE(suppress)) {
+		pr_info("%s (%d): %s: %d %u:%u\n", current->comm,
+			task_pid_nr(current), __func__, ctl,
+			MAJOR(sb->s_dev), MINOR(sb->s_dev));
+
+		if (ctl & BIT(2))
+			WRITE_ONCE(suppress, true);
+	}
+
+	drop_super(sb);
+out:
+	kfree(work->path);
+	kfree(work);
+}
+
+static int drop_fs_caches_sysctl_handler(const struct ctl_table *table,
+					 int write, void *buffer,
+					 size_t *length, loff_t *ppos)
+{
+	struct drop_fs_caches_work *work = NULL;
+	unsigned int major, minor;
+	unsigned int ctl;
+	int ret;
+	char *path = NULL;
+
+	if (!write)
+		return 0;
+
+	if (sscanf(buffer, "%u %u:%u", &ctl, &major, &minor) != 3) {
+		path = kstrdup(buffer, GFP_NOFS);
+		if (!path) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		if (sscanf(buffer, "%u %s", &ctl, path) != 2) {
+			ret = -EINVAL;
+			goto out;
+		}
+	}
+
+	if (ctl < 1 || ctl > 7) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	work = kzalloc(sizeof(*work), GFP_KERNEL);
+	if (!work) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	init_task_work(&work->task_work, drop_fs_caches);
+	if (!path)
+		work->dev = MKDEV(major, minor);
+	work->path = path;
+	work->ctl = ctl;
+	ret = task_work_add(current, &work->task_work, TWA_RESUME);
+out:
+	if (ret) {
+		kfree(path);
+		kfree(work);
+	}
+
+	return ret;
+}
+
 static const struct ctl_table drop_caches_table[] = {
 	{
 		.procname	= "drop_caches",
@@ -88,6 +208,11 @@ static const struct ctl_table drop_caches_table[] = {
 		.extra1		= SYSCTL_ONE,
 		.extra2		= SYSCTL_FOUR,
 	},
+	{
+		.procname	= "drop_fs_caches",
+		.mode		= 0200,
+		.proc_handler	= drop_fs_caches_sysctl_handler,
+	},
 };
 
 static int __init init_vm_drop_caches_sysctls(void)
-- 
2.34.1

[PATCH v3 3/3] Documentation: add instructions for using 'drop_fs_caches sysctl' sysctl

[Ye Bin]

From: Ye Bin <yebin10@huawei.com>

Add instructions for 'drop_fs_caches sysctl' sysctl in 'vm.rst'.

Signed-off-by: Ye Bin <yebin10@huawei.com>
---
 Documentation/admin-guide/sysctl/vm.rst | 44 +++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 97e12359775c..76545da53e20 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -37,6 +37,7 @@ Currently, these files are in /proc/sys/vm:
 - dirtytime_expire_seconds
 - dirty_writeback_centisecs
 - drop_caches
+- drop_fs_caches
 - enable_soft_offline
 - extfrag_threshold
 - highmem_is_dirtyable
@@ -286,6 +287,49 @@ used::
 These are informational only.  They do not mean that anything is wrong
 with your system.  To disable them, echo 4 (bit 2) into drop_caches.
 
+drop_fs_caches
+==============
+
+Writing to this will cause the kernel to drop clean for a specific file system
+caches, as well as reclaimable slab objects like dentries and inodes. Once
+dropped, their memory becomes free. Except for specifying the device number
+or file path for a specific file system, everything else is consistent with
+drop_caches. The device number can be viewed through "cat /proc/self/montinfo"
+or 'lsblk'.
+
+To free pagecache::
+
+	echo "1 MAJOR:MINOR" > /proc/sys/vm/drop_fs_caches
+Or
+	echo "1 /mnt/XX" > /proc/sys/vm/drop_fs_caches
+
+To free reclaimable slab objects (includes dentries and inodes)::
+
+	echo "2 MAJOR:MINOR" > /proc/sys/vm/drop_fs_caches
+Or
+	echo "2 /mnt/XX" > /proc/sys/vm/drop_fs_caches
+
+To free slab objects and pagecache::
+
+	echo "3 MAJOR:MINOR" > /proc/sys/vm/drop_fs_caches
+Or
+	echo "3 /mnt/XX" > /proc/sys/vm/drop_fs_caches
+
+You may see error messages in your kernel log when incorrect path or device
+number provided::
+
+	echo (1234): drop_fs_caches: failed to get path(/mnt/XX) ERRNO
+Or
+	echo (1234): drop_fs_caches: failed to get dev(MAJOR:MINOR)'s sb
+
+You may see informational messages in your kernel log when this file is
+used::
+
+	echo (1234): drop_fs_caches: 3 MAJOR:MINOR
+
+These are informational only. They do not mean that anything is wrong
+with your system. To disable them, echo 4 (bit 2) into drop_fs_caches.
+
 enable_soft_offline
 ===================
 Correctable memory errors are very common on servers. Soft-offline is kernel's
-- 
2.34.1

Re: [PATCH v3 0/3] add support for drop_caches for individual filesystem

[Muchun Song]

> On Feb 27, 2026, at 10:55, Ye Bin <yebin@huaweicloud.com> wrote:
> 
> From: Ye Bin <yebin10@huawei.com>
> 
> In order to better analyze the issue of file system uninstallation caused
> by kernel module opening files, it is necessary to perform dentry recycling
> on a single file system. But now, apart from global dentry recycling, it is
> not supported to do dentry recycling on a single file system separately.

Would shrinker-debugfs satisfy your needs (See Documentation/admin-guide/mm/shrinker_debugfs.rst)?

Thanks,
Muchun

> This feature has usage scenarios in problem localization scenarios.At the
> same time, it also provides users with a slightly fine-grained
> pagecache/entry recycling mechanism.
> This patchset supports the recycling of pagecache/entry for individual file
> systems.
> 
> Diff v3 vs v2
> 1. Introduce introduce drop_sb_dentry_inode() helper instead of
> reclaim_dcache_sb()/reclaim_icache_sb() helper for reclaim dentry/inode.
> 2. Fixing compilation issues in specific architectures and configurations.
> 
> Diff v2 vs v1:
> 1. Fix possible live lock for shrink_icache_sb().
> 2. Introduce reclaim_dcache_sb() for reclaim dentry.
> 3. Fix potential deadlocks as follows:
> https://lore.kernel.org/linux-fsdevel/00000000000098f75506153551a1@google.com/
> After some consideration, it was decided that this feature would primarily
> be used for debugging purposes. Instead of adding a new IOCTL command, the
> task_work mechanism was employed to address potential deadlock issues.
> 
> Ye Bin (3):
>  mm/vmscan: introduce drop_sb_dentry_inode() helper
>  sysctl: add support for drop_caches for individual filesystem
>  Documentation: add instructions for using 'drop_fs_caches sysctl'
>    sysctl
> 
> Documentation/admin-guide/sysctl/vm.rst |  44 +++++++++
> fs/drop_caches.c                        | 125 ++++++++++++++++++++++++
> include/linux/mm.h                      |   1 +
> mm/internal.h                           |   3 +
> mm/shrinker.c                           |   4 +-
> mm/vmscan.c                             |  50 ++++++++++
> 6 files changed, 225 insertions(+), 2 deletions(-)
> 
> -- 
> 2.34.1
>