Message-ID: <20251231090118.541061-8-lihongbo22@huawei.com>
Date: Wed, 31 Dec 2025 09:01:15 +0000
From: Hongbo Li <lihongbo22@...wei.com>
To: <hsiangkao@...ux.alibaba.com>, <chao@...nel.org>, <brauner@...nel.org>
CC: <djwong@...nel.org>, <amir73il@...il.com>, <hch@....de>,
	<lihongbo22@...wei.com>, <linux-fsdevel@...r.kernel.org>,
	<linux-erofs@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH v12 07/10] erofs: introduce the page cache share feature

From: Hongzhen Luo <hongzhen@...ux.alibaba.com>

Currently, reading files with different paths (or names) but the same
content will consume multiple copies of the page cache, even if the
content of these page caches is the same. For example, reading
identical files (e.g., *.so files) from two different minor versions of
container images will cost multiple copies of the same page cache,
since different containers have different mount points. Therefore,
sharing the page cache for files with the same content can save memory.

This introduces the page cache share feature in erofs. It allocate a
deduplicated inode and use its page cache as shared. Reads for files
with identical content will ultimately be routed to the page cache of
the deduplicated inode. In this way, a single page cache satisfies
multiple read requests for different files with the same contents.

We introduce inode_share mount option to enable the page sharing mode
during mounting.

Signed-off-by: Hongzhen Luo <hongzhen@...ux.alibaba.com>
Signed-off-by: Hongbo Li <lihongbo22@...wei.com>
---
 Documentation/filesystems/erofs.rst |   5 +
 fs/erofs/Makefile                   |   1 +
 fs/erofs/internal.h                 |  31 +++++
 fs/erofs/ishare.c                   | 170 ++++++++++++++++++++++++++++
 fs/erofs/super.c                    |  55 ++++++++-
 fs/erofs/xattr.c                    |  34 ++++++
 fs/erofs/xattr.h                    |   3 +
 7 files changed, 297 insertions(+), 2 deletions(-)
 create mode 100644 fs/erofs/ishare.c

diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
index 08194f194b94..27d3caa3c73c 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst
@@ -128,7 +128,12 @@ device=%s              Specify a path to an extra device to be used together.
 fsid=%s                Specify a filesystem image ID for Fscache back-end.
 domain_id=%s           Specify a domain ID in fscache mode so that different images
                        with the same blobs under a given domain ID can share storage.
+                       Also used for inode page sharing mode which defines a sharing
+                       domain.
 fsoffset=%llu          Specify block-aligned filesystem offset for the primary device.
+inode_share            Enable inode page sharing for this filesystem.  Inodes with
+                       identical content within the same domain ID can share the
+                       page cache.
 ===================    =========================================================
 
 Sysfs Entries
diff --git a/fs/erofs/Makefile b/fs/erofs/Makefile
index 549abc424763..a80e1762b607 100644
--- a/fs/erofs/Makefile
+++ b/fs/erofs/Makefile
@@ -10,3 +10,4 @@ erofs-$(CONFIG_EROFS_FS_ZIP_ZSTD) += decompressor_zstd.o
 erofs-$(CONFIG_EROFS_FS_ZIP_ACCEL) += decompressor_crypto.o
 erofs-$(CONFIG_EROFS_FS_BACKED_BY_FILE) += fileio.o
 erofs-$(CONFIG_EROFS_FS_ONDEMAND) += fscache.o
+erofs-$(CONFIG_EROFS_FS_PAGE_CACHE_SHARE) += ishare.o
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index ec79e8b44d3b..6ef1cdd9d651 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -179,6 +179,7 @@ struct erofs_sb_info {
 #define EROFS_MOUNT_DAX_ALWAYS		0x00000040
 #define EROFS_MOUNT_DAX_NEVER		0x00000080
 #define EROFS_MOUNT_DIRECT_IO		0x00000100
+#define EROFS_MOUNT_INODE_SHARE		0x00000200
 
 #define clear_opt(opt, option)	((opt)->mount_opt &= ~EROFS_MOUNT_##option)
 #define set_opt(opt, option)	((opt)->mount_opt |= EROFS_MOUNT_##option)
@@ -269,6 +270,11 @@ static inline u64 erofs_nid_to_ino64(struct erofs_sb_info *sbi, erofs_nid_t nid)
 /* default readahead size of directories */
 #define EROFS_DIR_RA_BYTES	16384
 
+struct erofs_inode_fingerprint {
+	u8 *opaque;
+	int size;
+};
+
 struct erofs_inode {
 	erofs_nid_t nid;
 
@@ -304,6 +310,18 @@ struct erofs_inode {
 		};
 #endif	/* CONFIG_EROFS_FS_ZIP */
 	};
+#ifdef CONFIG_EROFS_FS_PAGE_CACHE_SHARE
+	struct list_head ishare_list;
+	union {
+		/* for each anon shared inode */
+		struct {
+			struct erofs_inode_fingerprint fingerprint;
+			spinlock_t ishare_lock;
+		};
+		/* for each real inode */
+		struct inode *sharedinode;
+	};
+#endif
 	/* the corresponding vfs inode */
 	struct inode vfs_inode;
 };
@@ -410,6 +428,7 @@ extern const struct inode_operations erofs_dir_iops;
 
 extern const struct file_operations erofs_file_fops;
 extern const struct file_operations erofs_dir_fops;
+extern const struct file_operations erofs_ishare_fops;
 
 extern const struct iomap_ops z_erofs_iomap_report_ops;
 
@@ -541,6 +560,18 @@ static inline struct bio *erofs_fscache_bio_alloc(struct erofs_map_dev *mdev) {
 static inline void erofs_fscache_submit_bio(struct bio *bio) {}
 #endif
 
+#ifdef CONFIG_EROFS_FS_PAGE_CACHE_SHARE
+int __init erofs_init_ishare(void);
+void erofs_exit_ishare(void);
+bool erofs_ishare_fill_inode(struct inode *inode);
+void erofs_ishare_free_inode(struct inode *inode);
+#else
+static inline int erofs_init_ishare(void) { return 0; }
+static inline void erofs_exit_ishare(void) {}
+static inline bool erofs_ishare_fill_inode(struct inode *inode) { return false; }
+static inline void erofs_ishare_free_inode(struct inode *inode) {}
+#endif // CONFIG_EROFS_FS_PAGE_CACHE_SHARE
+
 long erofs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg);
 long erofs_compat_ioctl(struct file *filp, unsigned int cmd,
 			unsigned long arg);
diff --git a/fs/erofs/ishare.c b/fs/erofs/ishare.c
new file mode 100644
index 000000000000..e93d379d4a3a
--- /dev/null
+++ b/fs/erofs/ishare.c
@@ -0,0 +1,170 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2024, Alibaba Cloud
+ */
+#include <linux/xxhash.h>
+#include <linux/mount.h>
+#include "internal.h"
+#include "xattr.h"
+
+#include "../internal.h"
+
+static struct vfsmount *erofs_ishare_mnt;
+
+static int erofs_ishare_iget5_eq(struct inode *inode, void *data)
+{
+	struct erofs_inode_fingerprint *fp1 = &EROFS_I(inode)->fingerprint;
+	struct erofs_inode_fingerprint *fp2 = data;
+
+	return fp1->size == fp2->size &&
+		!memcmp(fp1->opaque, fp2->opaque, fp2->size);
+}
+
+static int erofs_ishare_iget5_set(struct inode *inode, void *data)
+{
+	struct erofs_inode *vi = EROFS_I(inode);
+
+	vi->fingerprint = *(struct erofs_inode_fingerprint *)data;
+	INIT_LIST_HEAD(&vi->ishare_list);
+	spin_lock_init(&vi->ishare_lock);
+	return 0;
+}
+
+bool erofs_ishare_fill_inode(struct inode *inode)
+{
+	struct erofs_sb_info *sbi = EROFS_SB(inode->i_sb);
+	struct erofs_inode *vi = EROFS_I(inode);
+	struct erofs_inode_fingerprint fp;
+	struct inode *sharedinode;
+	unsigned long hash;
+
+	if (!test_opt(&sbi->opt, INODE_SHARE))
+		return false;
+	(void)erofs_xattr_fill_ishare_fp(&fp, inode, sbi->domain_id);
+	if (!fp.size)
+		return false;
+	hash = xxh32(fp.opaque, fp.size, 0);
+	sharedinode = iget5_locked(erofs_ishare_mnt->mnt_sb, hash,
+				   erofs_ishare_iget5_eq, erofs_ishare_iget5_set,
+				   &fp);
+	if (!sharedinode) {
+		kfree(fp.opaque);
+		return false;
+	}
+
+	vi->sharedinode = sharedinode;
+	if (inode_state_read_once(sharedinode) & I_NEW) {
+		if (erofs_inode_is_data_compressed(vi->datalayout))
+			sharedinode->i_mapping->a_ops = &z_erofs_aops;
+		else
+			sharedinode->i_mapping->a_ops = &erofs_aops;
+		sharedinode->i_mode = vi->vfs_inode.i_mode;
+		sharedinode->i_size = vi->vfs_inode.i_size;
+		unlock_new_inode(sharedinode);
+	} else {
+		kfree(fp.opaque);
+	}
+	INIT_LIST_HEAD(&vi->ishare_list);
+	spin_lock(&EROFS_I(sharedinode)->ishare_lock);
+	list_add(&vi->ishare_list, &EROFS_I(sharedinode)->ishare_list);
+	spin_unlock(&EROFS_I(sharedinode)->ishare_lock);
+	return true;
+}
+
+void erofs_ishare_free_inode(struct inode *inode)
+{
+	struct erofs_inode *vi = EROFS_I(inode);
+	struct inode *sharedinode = vi->sharedinode;
+
+	if (!sharedinode)
+		return;
+	spin_lock(&EROFS_I(sharedinode)->ishare_lock);
+	list_del(&vi->ishare_list);
+	spin_unlock(&EROFS_I(sharedinode)->ishare_lock);
+	iput(sharedinode);
+	vi->sharedinode = NULL;
+}
+
+static int erofs_ishare_file_open(struct inode *inode, struct file *file)
+{
+	struct inode *sharedinode;
+	struct file *realfile;
+
+	sharedinode = EROFS_I(inode)->sharedinode;
+	realfile = alloc_empty_backing_file(O_RDONLY|O_NOATIME, current_cred());
+	if (IS_ERR(realfile))
+		return PTR_ERR(realfile);
+	ihold(sharedinode);
+	realfile->f_op = &erofs_file_fops;
+	realfile->f_inode = sharedinode;
+	realfile->f_mapping = sharedinode->i_mapping;
+	path_get(&file->f_path);
+	backing_file_set_user_path(realfile, &file->f_path);
+
+	file_ra_state_init(&realfile->f_ra, file->f_mapping);
+	realfile->private_data = EROFS_I(inode);
+	file->private_data = realfile;
+	return 0;
+}
+
+static int erofs_ishare_file_release(struct inode *inode, struct file *file)
+{
+	struct file *realfile = file->private_data;
+
+	iput(realfile->f_inode);
+	fput(realfile);
+	file->private_data = NULL;
+	return 0;
+}
+
+static ssize_t erofs_ishare_file_read_iter(struct kiocb *iocb,
+					   struct iov_iter *to)
+{
+	struct file *realfile = iocb->ki_filp->private_data;
+	struct kiocb dedup_iocb;
+	ssize_t nread;
+
+	if (!iov_iter_count(to))
+		return 0;
+
+	/* fallback to the original file in DIRECT mode */
+	if (iocb->ki_flags & IOCB_DIRECT)
+		realfile = iocb->ki_filp;
+
+	kiocb_clone(&dedup_iocb, iocb, realfile);
+	nread = filemap_read(&dedup_iocb, to, 0);
+	iocb->ki_pos = dedup_iocb.ki_pos;
+	file_accessed(iocb->ki_filp);
+	return nread;
+}
+
+static int erofs_ishare_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct file *realfile = file->private_data;
+
+	vma_set_file(vma, realfile);
+	return generic_file_readonly_mmap(file, vma);
+}
+
+const struct file_operations erofs_ishare_fops = {
+	.open		= erofs_ishare_file_open,
+	.llseek		= generic_file_llseek,
+	.read_iter	= erofs_ishare_file_read_iter,
+	.mmap		= erofs_ishare_mmap,
+	.release	= erofs_ishare_file_release,
+	.get_unmapped_area = thp_get_unmapped_area,
+	.splice_read	= filemap_splice_read,
+};
+
+int __init erofs_init_ishare(void)
+{
+	erofs_ishare_mnt = kern_mount(&erofs_anon_fs_type);
+	if (IS_ERR(erofs_ishare_mnt))
+		return PTR_ERR(erofs_ishare_mnt);
+	return 0;
+}
+
+void erofs_exit_ishare(void)
+{
+	kern_unmount(erofs_ishare_mnt);
+}
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 960da62636ad..6489241c5e42 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -396,6 +396,7 @@ static void erofs_default_options(struct erofs_sb_info *sbi)
 enum {
 	Opt_user_xattr, Opt_acl, Opt_cache_strategy, Opt_dax, Opt_dax_enum,
 	Opt_device, Opt_fsid, Opt_domain_id, Opt_directio, Opt_fsoffset,
+	Opt_inode_share,
 };
 
 static const struct constant_table erofs_param_cache_strategy[] = {
@@ -423,6 +424,7 @@ static const struct fs_parameter_spec erofs_fs_parameters[] = {
 	fsparam_string("domain_id",	Opt_domain_id),
 	fsparam_flag_no("directio",	Opt_directio),
 	fsparam_u64("fsoffset",		Opt_fsoffset),
+	fsparam_flag("inode_share",	Opt_inode_share),
 	{}
 };
 
@@ -551,6 +553,14 @@ static int erofs_fc_parse_param(struct fs_context *fc,
 	case Opt_fsoffset:
 		sbi->dif0.fsoff = result.uint_64;
 		break;
+#if defined(CONFIG_EROFS_FS_PAGE_CACHE_SHARE)
+	case Opt_inode_share:
+		set_opt(&sbi->opt, INODE_SHARE);
+#else
+	case Opt_inode_share:
+		errorfc(fc, "%s option not supported", erofs_fs_parameters[opt].name);
+#endif
+		break;
 	}
 	return 0;
 }
@@ -649,6 +659,16 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_maxbytes = MAX_LFS_FILESIZE;
 	sb->s_op = &erofs_sops;
 
+	if (sbi->domain_id &&
+		(!sbi->fsid && !test_opt(&sbi->opt, INODE_SHARE))) {
+		errorfc(fc, "domain_id should be with fsid or inode_share option");
+		return -EINVAL;
+	}
+	if (test_opt(&sbi->opt, DAX_ALWAYS) && test_opt(&sbi->opt, INODE_SHARE)) {
+		errorfc(fc, "dax is not allowed when inode_share is on");
+		return -EINVAL;
+	}
+
 	sbi->blkszbits = PAGE_SHIFT;
 	if (!sb->s_bdev) {
 		/*
@@ -948,10 +968,31 @@ static struct file_system_type erofs_fs_type = {
 };
 MODULE_ALIAS_FS("erofs");
 
-#if defined(CONFIG_EROFS_FS_ONDEMAND)
+#if defined(CONFIG_EROFS_FS_ONDEMAND) || defined(CONFIG_EROFS_FS_PAGE_CACHE_SHARE)
+static void erofs_free_anon_inode(struct inode *inode)
+{
+	struct erofs_inode *vi = EROFS_I(inode);
+
+#ifdef CONFIG_EROFS_FS_PAGE_CACHE_SHARE
+	kfree(vi->fingerprint.opaque);
+#endif
+	kmem_cache_free(erofs_inode_cachep, vi);
+}
+
+static const struct super_operations erofs_anon_sops = {
+	.alloc_inode = erofs_alloc_inode,
+	.free_inode = erofs_free_anon_inode,
+};
+
 static int erofs_anon_init_fs_context(struct fs_context *fc)
 {
-	return init_pseudo(fc, EROFS_SUPER_MAGIC) ? 0 : -ENOMEM;
+	struct pseudo_fs_context *ctx;
+
+	ctx = init_pseudo(fc, EROFS_SUPER_MAGIC);
+	if (ctx)
+		ctx->ops = &erofs_anon_sops;
+
+	return ctx ? 0 : -ENOMEM;
 }
 
 struct file_system_type erofs_anon_fs_type = {
@@ -986,6 +1027,10 @@ static int __init erofs_module_init(void)
 	if (err)
 		goto sysfs_err;
 
+	err = erofs_init_ishare();
+	if (err)
+		goto ishare_err;
+
 	err = register_filesystem(&erofs_fs_type);
 	if (err)
 		goto fs_err;
@@ -993,6 +1038,8 @@ static int __init erofs_module_init(void)
 	return 0;
 
 fs_err:
+	erofs_exit_ishare();
+ishare_err:
 	erofs_exit_sysfs();
 sysfs_err:
 	z_erofs_exit_subsystem();
@@ -1010,6 +1057,7 @@ static void __exit erofs_module_exit(void)
 	/* Ensure all RCU free inodes / pclusters are safe to be destroyed. */
 	rcu_barrier();
 
+	erofs_exit_ishare();
 	erofs_exit_sysfs();
 	z_erofs_exit_subsystem();
 	erofs_exit_shrinker();
@@ -1062,6 +1110,8 @@ static int erofs_show_options(struct seq_file *seq, struct dentry *root)
 		seq_printf(seq, ",domain_id=%s", sbi->domain_id);
 	if (sbi->dif0.fsoff)
 		seq_printf(seq, ",fsoffset=%llu", sbi->dif0.fsoff);
+	if (test_opt(opt, INODE_SHARE))
+		seq_puts(seq, ",inode_share");
 	return 0;
 }
 
@@ -1072,6 +1122,7 @@ static void erofs_evict_inode(struct inode *inode)
 		dax_break_layout_final(inode);
 #endif
 
+	erofs_ishare_free_inode(inode);
 	truncate_inode_pages_final(&inode->i_data);
 	clear_inode(inode);
 }
diff --git a/fs/erofs/xattr.c b/fs/erofs/xattr.c
index ae61f20cb861..ec7a0f5dc172 100644
--- a/fs/erofs/xattr.c
+++ b/fs/erofs/xattr.c
@@ -577,3 +577,37 @@ struct posix_acl *erofs_get_acl(struct inode *inode, int type, bool rcu)
 	return acl;
 }
 #endif
+
+#ifdef CONFIG_EROFS_FS_PAGE_CACHE_SHARE
+int erofs_xattr_fill_ishare_fp(struct erofs_inode_fingerprint *fp,
+			       struct inode *inode, const char *domain_id)
+{
+	struct erofs_sb_info *sbi = EROFS_SB(inode->i_sb);
+	struct erofs_xattr_prefix_item *prefix;
+	const char *infix;
+	int valuelen, base_index, domainlen;
+
+	fp->size = 0;
+	if (!erofs_sb_has_ishare_xattrs(sbi))
+		return 0;
+	prefix = sbi->xattr_prefixes + sbi->ishare_xattr_prefix_id;
+	infix = prefix->prefix->infix;
+	base_index = prefix->prefix->base_index;
+	valuelen = erofs_getxattr(inode, base_index, infix, NULL, 0);
+	if (valuelen <= 0 || valuelen > (1 << sbi->blkszbits))
+		return -EFSCORRUPTED;
+	domainlen = domain_id ? strlen(domain_id) : 0;
+	fp->opaque = kmalloc(valuelen + domainlen, GFP_KERNEL);
+	if (!fp->opaque)
+		return -ENOMEM;
+	if (valuelen != erofs_getxattr(inode, base_index, infix,
+				       fp->opaque, valuelen)) {
+		kfree(fp->opaque);
+		fp->opaque = NULL;
+		return -EFSCORRUPTED;
+	}
+	memcpy(fp->opaque + valuelen, domain_id, domainlen);
+	fp->size = valuelen + domainlen;
+	return 0;
+}
+#endif
diff --git a/fs/erofs/xattr.h b/fs/erofs/xattr.h
index 6317caa8413e..8656ec8d931e 100644
--- a/fs/erofs/xattr.h
+++ b/fs/erofs/xattr.h
@@ -67,4 +67,7 @@ struct posix_acl *erofs_get_acl(struct inode *inode, int type, bool rcu);
 #define erofs_get_acl	(NULL)
 #endif
 
+int erofs_xattr_fill_ishare_fp(struct erofs_inode_fingerprint *fp,
+			       struct inode *inode, const char *domain_id);
+
 #endif
-- 
2.22.0

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