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Message-ID: <8ed8ef13-e818-42e3-bece-2af1af238b62@linux.alibaba.com>
Date: Fri, 9 Jan 2026 13:50:52 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Hongbo Li <lihongbo22@...wei.com>, chao@...nel.org, brauner@...nel.org
Cc: djwong@...nel.org, amir73il@...il.com, hch@....de,
linux-fsdevel@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 07/10] erofs: introduce the page cache share feature
On 2026/1/9 11:01, Hongbo Li wrote:
> 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 | 161 ++++++++++++++++++++++++++++
> fs/erofs/super.c | 53 ++++++++-
> fs/erofs/xattr.c | 33 ++++++
> fs/erofs/xattr.h | 3 +
> 7 files changed, 285 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..56a955aaeb18
> --- /dev/null
> +++ b/fs/erofs/ishare.c
> @@ -0,0 +1,161 @@
> +// 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 (erofs_xattr_fill_inode_fingerprint(&fp, inode, sbi->domain_id))
> + 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;
just
struct inode *sharedinode = EROFS_I(inode)->sharedinode;
here for simplicity.
`if (file->f_flags & O_DIRECT)` is an error case, so I don't bother
with the check.
> + struct file *realfile;
> +
> + if (file->f_flags & O_DIRECT)
> + return -EINVAL;
> + 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;
> + kiocb_clone(&dedup_iocb, iocb, realfile);
> + nread = filemap_read(&dedup_iocb, to, 0);
> + iocb->ki_pos = dedup_iocb.ki_pos;
> + 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);
> + return PTR_ERR_OR_ZERO(erofs_ishare_mnt);
> +}
> +
> +void erofs_exit_ishare(void)
> +{
> + kern_unmount(erofs_ishare_mnt);
> +}
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index 960da62636ad..a851b47ee579 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 +553,13 @@ static int erofs_fc_parse_param(struct fs_context *fc,
> case Opt_fsoffset:
> sbi->dif0.fsoff = result.uint_64;
> break;
> + case Opt_inode_share:
> +#if defined(CONFIG_EROFS_FS_PAGE_CACHE_SHARE)
> + set_opt(&sbi->opt, INODE_SHARE);
> +#else
> + errorfc(fc, "%s option not supported", erofs_fs_parameters[opt].name);
> +#endif
> + break;
> }
> return 0;
> }
> @@ -649,6 +658,11 @@ 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 (test_opt(&sbi->opt, DAX_ALWAYS) && test_opt(&sbi->opt, INODE_SHARE)) {
> + errorfc(fc, "FSDAX is not allowed when inode_ishare is on");
> + return -EINVAL;
> + }
> +
> sbi->blkszbits = PAGE_SHIFT;
> if (!sb->s_bdev) {
> /*
> @@ -719,6 +733,10 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
> erofs_info(sb, "unsupported blocksize for DAX");
> clear_opt(&sbi->opt, DAX_ALWAYS);
> }
> + if (test_opt(&sbi->opt, INODE_SHARE) && !erofs_sb_has_ishare_xattrs(sbi)) {
> + erofs_info(sb, "inode ishare is unavailable");
erofs_info(sb, "on-disk ishare xattrs not found. Turning off inode_share.");
> + clear_opt(&sbi->opt, INODE_SHARE);
> + }
>
> sb->s_time_gran = 1;
> sb->s_xattr = erofs_xattr_handlers;
> @@ -948,10 +966,31 @@ 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
Drop `#ifdef CONFIG_EROFS_FS_PAGE_CACHE_SHARE` here for simplicity.
> + 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;
ctx = init_pseudo(fc, EROFS_SUPER_MAGIC);
if (!ctx)
return -ENOMEM;
ctx->ops = &erofs_anon_sops;
return 0;
> }
>
> struct file_system_type erofs_anon_fs_type = {
> @@ -986,6 +1025,10 @@ 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 +1036,8 @@ 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 +1055,7 @@ 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 +1108,8 @@ 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 +1120,7 @@ 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..290acbf89fa6 100644
> --- a/fs/erofs/xattr.c
> +++ b/fs/erofs/xattr.c
> @@ -577,3 +577,36 @@ 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_inode_fingerprint(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;
> +
> + if (!test_opt(&sbi->opt, INODE_SHARE))
> + return -EOPNOTSUPP;
> + 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;
fp->size = valuelen + (domain_id ? strlen(domain_id) : 0);
fp->opaque = kmalloc(fp->size, GFP_KERNEL);
...
> + memcpy(fp->opaque + valuelen, domain_id, domainlen);
> + fp->size = valuelen + domainlen;
Then kill this line.
Thanks,
Gao Xiang
> + return 0;
> +}
> +#endif
> diff --git a/fs/erofs/xattr.h b/fs/erofs/xattr.h
> index 6317caa8413e..bf75a580b8f1 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_inode_fingerprint(struct erofs_inode_fingerprint *fp,
> + struct inode *inode, const char *domain_id);
> +
> #endif
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