[PATCH RFC 2/4] erofs: introduce page cache share feature
Hongzhen Luo
hongzhen at linux.alibaba.com
Sat Jul 5 11:09:26 AEST 2025
On 2025/7/3 20:23, Christian Brauner wrote:
> From: Hongzhen Luo <hongzhen at linux.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. During the mkfs
> phase, the file content is hashed and the hash value is stored in the
> `trusted.erofs.fingerprint` extended attribute. Inodes of files with the
> same `trusted.erofs.fingerprint` are mapped to the same anonymous inode
> (indicated by the `ano_inode` field). When a read request occurs, the
> anonymous inode serves as a "container" whose page cache is shared. The
> actual operations involving the iomap are carried out by the original
> inode which is mapped to the anonymous inode.
>
> Below is the memory usage for reading all files in two different minor
> versions of container images:
>
> +-------------------+------------------+-------------+---------------+
> | Image | Page Cache Share | Memory (MB) | Memory |
> | | | | Reduction (%) |
> +-------------------+------------------+-------------+---------------+
> | | No | 241 | - |
> | redis +------------------+-------------+---------------+
> | 7.2.4 & 7.2.5 | Yes | 163 | 33% |
> +-------------------+------------------+-------------+---------------+
> | | No | 872 | - |
> | postgres +------------------+-------------+---------------+
> | 16.1 & 16.2 | Yes | 630 | 28% |
> +-------------------+------------------+-------------+---------------+
> | | No | 2771 | - |
> | tensorflow +------------------+-------------+---------------+
> | 1.11.0 & 2.11.1 | Yes | 2340 | 16% |
> +-------------------+------------------+-------------+---------------+
> | | No | 926 | - |
> | mysql +------------------+-------------+---------------+
> | 8.0.11 & 8.0.12 | Yes | 735 | 21% |
> +-------------------+------------------+-------------+---------------+
> | | No | 390 | - |
> | nginx +------------------+-------------+---------------+
> | 7.2.4 & 7.2.5 | Yes | 219 | 44% |
> +-------------------+------------------+-------------+---------------+
> | tomcat | No | 924 | - |
> | 10.1.25 & 10.1.26 +------------------+-------------+---------------+
> | | Yes | 474 | 49% |
> +-------------------+------------------+-------------+---------------+
>
> Additionally, the table below shows the runtime memory usage of the
> container:
>
> +-------------------+------------------+-------------+---------------+
> | Image | Page Cache Share | Memory (MB) | Memory |
> | | | | Reduction (%) |
> +-------------------+------------------+-------------+---------------+
> | | No | 35 | - |
> | redis +------------------+-------------+---------------+
> | 7.2.4 & 7.2.5 | Yes | 28 | 20% |
> +-------------------+------------------+-------------+---------------+
> | | No | 149 | - |
> | postgres +------------------+-------------+---------------+
> | 16.1 & 16.2 | Yes | 95 | 37% |
> +-------------------+------------------+-------------+---------------+
> | | No | 1028 | - |
> | tensorflow +------------------+-------------+---------------+
> | 1.11.0 & 2.11.1 | Yes | 930 | 10% |
> +-------------------+------------------+-------------+---------------+
> | | No | 155 | - |
> | mysql +------------------+-------------+---------------+
> | 8.0.11 & 8.0.12 | Yes | 132 | 15% |
> +-------------------+------------------+-------------+---------------+
> | | No | 25 | - |
> | nginx +------------------+-------------+---------------+
> | 7.2.4 & 7.2.5 | Yes | 20 | 20% |
> +-------------------+------------------+-------------+---------------+
> | tomcat | No | 186 | - |
> | 10.1.25 & 10.1.26 +------------------+-------------+---------------+
> | | Yes | 98 | 48% |
> +-------------------+------------------+-------------+---------------+
>
> Signed-off-by: Hongzhen Luo <hongzhen at linux.alibaba.com>
> Link: https://lore.kernel.org/20240902110620.2202586-3-hongzhen@linux.alibaba.com
> Signed-off-by: Christian Brauner <brauner at kernel.org>
> ---
> fs/erofs/Kconfig | 10 +++
> fs/erofs/Makefile | 1 +
> fs/erofs/internal.h | 4 +
> fs/erofs/pagecache_share.c | 204 +++++++++++++++++++++++++++++++++++++++++++++
> fs/erofs/pagecache_share.h | 20 +++++
> 5 files changed, 239 insertions(+)
>
> diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
> index 6beeb7063871..553770068fee 100644
> --- a/fs/erofs/Kconfig
> +++ b/fs/erofs/Kconfig
> @@ -192,3 +192,13 @@ config EROFS_FS_PCPU_KTHREAD_HIPRI
> at higher priority.
>
> If unsure, say N.
> +
> +config EROFS_FS_PAGE_CACHE_SHARE
> + bool "EROFS page cache share support"
> + depends on EROFS_FS
> + default n
> + help
> + This permits EROFS to share page cache for files with same
> + fingerprints.
> +
> + If unsure, say N.
> diff --git a/fs/erofs/Makefile b/fs/erofs/Makefile
> index 549abc424763..f4141fdfcb0b 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) += pagecache_share.o
> diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
> index 30380f7baf5e..47136894d17d 100644
> --- a/fs/erofs/internal.h
> +++ b/fs/erofs/internal.h
> @@ -273,6 +273,9 @@ struct erofs_inode {
> };
> #endif /* CONFIG_EROFS_FS_ZIP */
> };
> +#ifdef CONFIG_EROFS_FS_PAGE_CACHE_SHARE
> + struct inode *ano_inode;
> +#endif
> /* the corresponding vfs inode */
> struct inode vfs_inode;
> };
> @@ -369,6 +372,7 @@ 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_pcs_file_fops;
>
> extern const struct iomap_ops z_erofs_iomap_report_ops;
>
> diff --git a/fs/erofs/pagecache_share.c b/fs/erofs/pagecache_share.c
> new file mode 100644
> index 000000000000..309b33cc6c30
> --- /dev/null
> +++ b/fs/erofs/pagecache_share.c
> @@ -0,0 +1,204 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Copyright (C) 2024, Alibaba Cloud
> + */
> +#include <linux/xxhash.h>
> +#include <linux/refcount.h>
> +#include "pagecache_share.h"
> +#include "internal.h"
> +#include "xattr.h"
> +
> +#define PCS_FPRT_IDX 4
> +#define PCS_FPRT_NAME "erofs.fingerprint"
> +#define PCS_FPRT_MAXLEN (sizeof(size_t) + 1024)
> +
> +static DEFINE_MUTEX(pseudo_mnt_lock);
> +static refcount_t pseudo_mnt_count;
> +static struct vfsmount *erofs_pcs_mnt;
> +
> +int erofs_pcs_init_mnt(void)
> +{
> + struct vfsmount *mnt;
> +
> + if (refcount_inc_not_zero(&pseudo_mnt_count))
> + return 0;
> +
> + guard(mutex)(&pseudo_mnt_lock);
> + if (erofs_pcs_mnt) {
> + refcount_inc(&pseudo_mnt_count);
> + return 0;
> + }
> +
> + mnt = kern_mount(&erofs_anon_fs_type);
> + if (IS_ERR(mnt))
> + return PTR_ERR(mnt);
> +
> + rcu_read_lock();
> + rcu_assign_pointer(erofs_pcs_mnt, mnt);
> + rcu_read_unlock();
> + refcount_set_release(&pseudo_mnt_count, 1);
> + return 0;
> +}
> +
> +void erofs_pcs_free_mnt(void)
> +{
> + struct vfsmount *mnt = NULL;
> +
> + if (refcount_dec_not_one(&pseudo_mnt_count))
> + return;
> +
> + scoped_guard(mutex, &pseudo_mnt_lock) {
> + rcu_read_lock();
> + if (refcount_dec_and_test(&pseudo_mnt_count))
> + mnt = rcu_replace_pointer(erofs_pcs_mnt, NULL, true);
> + rcu_read_unlock();
> + }
> + if (mnt)
> + kern_unmount(mnt);
> +}
> +
> +static int erofs_pcs_eq(struct inode *inode, void *data)
> +{
> + return inode->i_private && memcmp(inode->i_private, data,
> + sizeof(size_t) + *(size_t *)data) == 0 ? 1 : 0;
> +}
> +
> +static int erofs_pcs_set_fprt(struct inode *inode, void *data)
> +{
> + /* fprt length and content */
> + inode->i_private = kmalloc(*(size_t *)data + sizeof(size_t),
> + GFP_KERNEL);
> + memcpy(inode->i_private, data, sizeof(size_t) + *(size_t *)data);
> + return 0;
> +}
> +
> +void erofs_pcs_fill_inode(struct inode *inode)
> +{
> + struct erofs_inode *vi = EROFS_I(inode);
> + char fprt[PCS_FPRT_MAXLEN];
> + struct inode *ano_inode;
> + unsigned long fprt_hash;
> + size_t fprt_len;
> +
> + vi->ano_inode = NULL;
> + fprt_len = erofs_getxattr(inode, PCS_FPRT_IDX, PCS_FPRT_NAME,
> + fprt + sizeof(size_t), PCS_FPRT_MAXLEN);
> + if (fprt_len > 0 && fprt_len <= PCS_FPRT_MAXLEN) {
> + *(size_t *)fprt = fprt_len;
> + fprt_hash = xxh32(fprt + sizeof(size_t), fprt_len, 0);
> + ano_inode = iget5_locked(erofs_pcs_mnt->mnt_sb, fprt_hash,
> + erofs_pcs_eq, erofs_pcs_set_fprt,
> + fprt);
> + vi->ano_inode = ano_inode;
> + if (ano_inode->i_state & I_NEW) {
> + if (erofs_inode_is_data_compressed(vi->datalayout))
> + ano_inode->i_mapping->a_ops = &z_erofs_aops;
> + else
> + ano_inode->i_mapping->a_ops = &erofs_aops;
> + ano_inode->i_size = inode->i_size;
> + unlock_new_inode(ano_inode);
> + }
> + }
> +}
> +
> +/*
> + * TODO: Hm, could we leverage our fancy new backing file infrastructure
> + * as for overlayfs and fuse?
> + */
> +static struct file *erofs_pcs_alloc_file(struct file *file,
> + struct inode *ano_inode)
> +{
> + struct file *ano_file;
> +
> + ano_file = alloc_file_pseudo(ano_inode, erofs_pcs_mnt, "[erofs_pcs_f]",
> + O_RDONLY, &erofs_file_fops);
> + file_ra_state_init(&ano_file->f_ra, file->f_mapping);
> + ano_file->private_data = EROFS_I(file_inode(file));
> + return ano_file;
> +}
> +
> +static int erofs_pcs_file_open(struct inode *inode, struct file *file)
> +{
> + struct file *ano_file;
> + struct inode *ano_inode;
> + struct erofs_inode *vi = EROFS_I(inode);
> +
> + ano_inode = vi->ano_inode;
> + if (!ano_inode)
> + return -EINVAL;
> +
> + ano_file = erofs_pcs_alloc_file(file, ano_inode);
> + if (IS_ERR(ano_file))
> + return PTR_ERR(ano_file);
> +
> + file->private_data = ano_file;
> + return 0;
> +}
> +
> +static int erofs_pcs_file_release(struct inode *inode, struct file *file)
> +{
> + struct file *ano_file __free(fput) = NULL;
> +
> + if (WARN_ON_ONCE(!file->private_data))
> + return -EINVAL;
> +
> + swap(file->private_data, ano_file);
> + return 0;
> +}
> +
> +static ssize_t erofs_pcs_file_read_iter(struct kiocb *iocb,
> + struct iov_iter *to)
> +{
> + struct file *file, *ano_file;
> + struct kiocb ano_iocb;
> + ssize_t res;
> +
> + if (!iov_iter_count(to))
> + return 0;
> +
> +#ifdef CONFIG_FS_DAX
> + if (IS_DAX(inode))
> + return iocb->ki_filp->f_op->read_iter(iocb, to);
> +#endif
> + if (iocb->ki_flags & IOCB_DIRECT)
> + return iocb->ki_filp->f_op->read_iter(iocb, to);
> +
> + memcpy(&ano_iocb, iocb, sizeof(struct kiocb));
> + file = iocb->ki_filp;
> + ano_file = file->private_data;
> + if (WARN_ON_ONCE(!ano_file))
> + return -EINVAL;
> + ano_iocb.ki_filp = ano_file;
> + res = filemap_read(&ano_iocb, to, 0);
> + memcpy(iocb, &ano_iocb, sizeof(struct kiocb));
> + iocb->ki_filp = file;
> + file_accessed(file);
> + return res;
> +}
> +
> +/*
> + * TODO: Amir, you've got some experience in this area due to overlayfs
> + * and fuse. Does that work?
> + */
> +static int erofs_pcs_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> + struct file *ano_file = file->private_data;
> +
> + vma_set_file(vma, ano_file);
> + vma->vm_ops = &generic_file_vm_ops;
> + return 0;
> +}
> +
> +const struct file_operations erofs_pcs_file_fops = {
> + .open = erofs_pcs_file_open,
> + /*
> + * TODO: Why doesn't .llseek require similar treatment as
> + * .read_iter?
> + */
.llseek only needs to calculate the offset and requires no excessive
handling for regular files, while .read_iter
involves actual data retrieval (EROFS-specific logic such as
decompressing compressed data and cross-block reads),
thus necessitating page cache sharing logic.
Thanks,
Hongzhen
> + .llseek = generic_file_llseek,
> + .read_iter = erofs_pcs_file_read_iter,
> + .mmap = erofs_pcs_mmap,
> + .release = erofs_pcs_file_release,
> + .get_unmapped_area = thp_get_unmapped_area,
> + .splice_read = filemap_splice_read,
> +};
> diff --git a/fs/erofs/pagecache_share.h b/fs/erofs/pagecache_share.h
> new file mode 100644
> index 000000000000..b8111291cf79
> --- /dev/null
> +++ b/fs/erofs/pagecache_share.h
> @@ -0,0 +1,20 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + * Copyright (C) 2024, Alibaba Cloud
> + */
> +#ifndef __EROFS_PAGECACHE_SHARE_H
> +#define __EROFS_PAGECACHE_SHARE_H
> +
> +#include <linux/fs.h>
> +#include <linux/mount.h>
> +#include <linux/rwlock.h>
> +#include <linux/mutex.h>
> +#include "internal.h"
> +
> +int erofs_pcs_init_mnt(void);
> +void erofs_pcs_free_mnt(void);
> +void erofs_pcs_fill_inode(struct inode *inode);
> +
> +extern const struct vm_operations_struct generic_file_vm_ops;
> +
> +#endif
>
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