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Message-ID: <df2889c0-6027-4f42-a013-b01357fd0005@linux.alibaba.com>
Date: Wed, 7 Jan 2026 14:56:35 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Hongbo Li <lihongbo22@...wei.com>
Cc: djwong@...nel.org, amir73il@...il.com, hch@....de,
linux-fsdevel@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
linux-kernel@...r.kernel.org, Chao Yu <chao@...nel.org>, brauner@...nel.org
Subject: Re: [PATCH v12 07/10] erofs: introduce the page cache share feature
On 2026/1/7 14:48, Hongbo Li wrote:
> Hi, Xiang
>
> On 2026/1/7 14:08, Gao Xiang wrote:
>>
>>
>> On 2025/12/31 17:01, 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 | 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;
>>
>> Why not just:
>>
>> if (erofs_xattr_fill_ishare_fp(&fp, inode, sbi->domain_id))
>> return false;
>>
>
> When erofs_sb_has_ishare_xattrs returns false, erofs_xattr_fill_ishare_fp also considers success.
Then why !test_opt(&sbi->opt, INODE_SHARE) didn't return?
Thanks,
Gao Xiang
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