vfs: Add sysctl vfs_cache_pressure_denom for bulk file operations

On our HDFS servers with 12 HDDs per server, a HDFS datanode[0] startup
involves scanning all files and caching their metadata (including dentries
and inodes) in memory. Each HDD contains approximately 2 million files,
resulting in a total of ~20 million cached dentries after initialization.

To minimize dentry reclamation, we set vfs_cache_pressure to 1. Despite
this configuration, memory pressure conditions can still trigger
reclamation of up to 50% of cached dentries, reducing the cache from 20
million to approximately 10 million entries. During the subsequent cache
rebuild period, any HDFS datanode restart operation incurs substantial
latency penalties until full cache recovery completes.

To maintain service stability, we need to preserve more dentries during
memory reclamation. The current minimum reclaim ratio (1/100 of total
dentries) remains too aggressive for our workload. This patch introduces
vfs_cache_pressure_denom for more granular cache pressure control. The
configuration [vfs_cache_pressure=1, vfs_cache_pressure_denom=10000]
effectively maintains the full 20 million dentry cache under memory
pressure, preventing datanode restart performance degradation.

Link: https://hadoop.apache.org/docs/r1.2.1/hdfs_design.html#NameNode+and+DataNodes [0]

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/20250511083624.9305-1-laoar.shao@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Documentation/admin-guide/sysctl/vm.rst

@@ -75,6 +75,7 @@ Currently, these files are in /proc/sys/vm:
 - unprivileged_userfaultfd
 - user_reserve_kbytes
 - vfs_cache_pressure
+- vfs_cache_pressure_denom
 - watermark_boost_factor
 - watermark_scale_factor
 - zone_reclaim_mode
@@ -1017,19 +1018,28 @@ vfs_cache_pressure
 This percentage value controls the tendency of the kernel to reclaim
 the memory which is used for caching of directory and inode objects.
 
-At the default value of vfs_cache_pressure=100 the kernel will attempt to
-reclaim dentries and inodes at a "fair" rate with respect to pagecache and
-swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
-to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
-never reclaim dentries and inodes due to memory pressure and this can easily
-lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
-causes the kernel to prefer to reclaim dentries and inodes.
+At the default value of vfs_cache_pressure=vfs_cache_pressure_denom the kernel
+will attempt to reclaim dentries and inodes at a "fair" rate with respect to
+pagecache and swapcache reclaim.  Decreasing vfs_cache_pressure causes the
+kernel to prefer to retain dentry and inode caches. When vfs_cache_pressure=0,
+the kernel will never reclaim dentries and inodes due to memory pressure and
+this can easily lead to out-of-memory conditions. Increasing vfs_cache_pressure
+beyond vfs_cache_pressure_denom causes the kernel to prefer to reclaim dentries
+and inodes.
 
-Increasing vfs_cache_pressure significantly beyond 100 may have negative
-performance impact. Reclaim code needs to take various locks to find freeable
-directory and inode objects. With vfs_cache_pressure=1000, it will look for
-ten times more freeable objects than there are.
+Increasing vfs_cache_pressure significantly beyond vfs_cache_pressure_denom may
+have negative performance impact. Reclaim code needs to take various locks to
+find freeable directory and inode objects. When vfs_cache_pressure equals
+(10 * vfs_cache_pressure_denom), it will look for ten times more freeable
+objects than there are.
 
+Note: This setting should always be used together with vfs_cache_pressure_denom.
+
+vfs_cache_pressure_denom
+========================
+
+Defaults to 100 (minimum allowed value). Requires corresponding
+vfs_cache_pressure setting to take effect.
 
 watermark_boost_factor
 ======================

fs/dcache.c

@@ -74,10 +74,11 @@
  * arbitrary, since it's serialized on rename_lock
  */
 static int sysctl_vfs_cache_pressure __read_mostly = 100;
+static int sysctl_vfs_cache_pressure_denom __read_mostly = 100;
 
 unsigned long vfs_pressure_ratio(unsigned long val)
 {
-	return mult_frac(val, sysctl_vfs_cache_pressure, 100);
+	return mult_frac(val, sysctl_vfs_cache_pressure, sysctl_vfs_cache_pressure_denom);
 }
 EXPORT_SYMBOL_GPL(vfs_pressure_ratio);
 
@@ -225,6 +226,14 @@ static const struct ctl_table vm_dcache_sysctls[] = {
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= SYSCTL_ZERO,
 	},
+	{
+		.procname	= "vfs_cache_pressure_denom",
+		.data		= &sysctl_vfs_cache_pressure_denom,
+		.maxlen		= sizeof(sysctl_vfs_cache_pressure_denom),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ONE_HUNDRED,
+	},
 };
 
 static int __init init_fs_dcache_sysctls(void)