[RFC PATCH 0/6] erofs: support page cache sharing between EROFS images in fscache mode

[RFC PATCH 0/6] erofs: support page cache sharing between EROFS images in fscache mode

[Jingbo Xu]

[Background]
=============
Erofs already supports chunk deduplication across different images to
minimize disk usage since v6.1.  Furthermore, we can make inodes among
different images share page cache for these deduplicated chunks to
reduce the memory usage.  This shall be much usable in container
scenarios as deduplication is requisite for container images.


[Implementation]
================
This is achieved by managing page cache of deduplicated chunks in
blob's address space.  In this way, all inodes sharing the deduplicated
chunk will refer to and share the page cache in the blob's address
space.


[Restriction]
==============
The page cache sharing feature also supports .mmap().  The reverse
mapping requires that one vma can not be shared among inodes and can
be linked to only one inode.  As the vma will be finally linked to the
blob's address space when page cache sharing enabled, the restriction of
the reverse mapping actually requires that the mapped file area can not
be mapped to multiple blobs.  Thus page cache sharing can only be
enabled for those files mapped to one blob.

The chunk based data layout guarantees that a chunk will not cross the
device (blob) boundary.  Thus in chunk based data layout, those files
smaller than the chunk size shall be guaranteed to be mapped to one
blob.  As chunk size is tunable at a per-file basis, this restriction
can be relaxed at image building phase.  As long as we ensure that the
file can not be deduplicated, the file's chunk size can be set to a
reasonable value larger than the file size, so that the file contains
only one chunk, in which case page cache sharing feature can be enabled
on this file later.


[Effect]
========
The final optimization result of this feature depends on the following
factors:

1. The number of deduplicated (shared) chunks.  Images sharing most of
the layers (e.g. a base image and v1 image based on the base image) will
achieve better optimization.

2. As the restriction mentioned above, the number of files for which
page cache sharing can ben enabled among the files accessed.


I test the workload of starting up Tensorflow, which will access quite
many (~5K) files among the startup phase.  I get the base image of
Tensorflow from [1] and build a new image (e.g. v1 image) on top of this
base image.

Since the image got from [1] is in OCI format, I have to convert it to
erofs format with buildkit[2], with default chunk size of 1MB.

I run containers from these two images with containerd (base image first,
v2 image secondly).  The (page cache) memory usage of the rootfs
(container image) is shown as below:

			| page cache sharing	| page cache sharing
			| disabled		| enabled
------------------------|-----------------------|-------------------
First container       	|      			|
page cache usage (MB) 	| 150      		| 150
------------------------+-----------------------|-------------------
Second container      	|      			|
page cache usage (MB) 	| 150			| 7

It can be seen that most (~95%, 143MB/150MB) memory usage reduced under
this workload (when starting following containers sharing container image
layers).

The remained 7MB memory usage is consumed by directories, since page
cache sharing is enabled only for regular files in this RFC
implementation.


[1] docker.io/tensorflow/tensorflow:2.10.0
[2] https://github.com/moby/buildkit


Jingbo Xu (6):
  erofs: remove unused device mapping in the meta routine
  erofs: unify anonymous inodes for blob
  erofs: alloc anonymous file for blob in share domain mode
  erofs: implement .read_iter for page cache sharing
  erofs: implement .mmap for page cache sharing
  erofs: enable page cache sharing in fscache mode

 fs/erofs/fscache.c  | 231 ++++++++++++++++++++++++++++++++------------
 fs/erofs/inode.c    |  27 ++++++
 fs/erofs/internal.h |   8 +-
 fs/erofs/super.c    |   2 +
 4 files changed, 202 insertions(+), 66 deletions(-)

-- 
2.19.1.6.gb485710b

[RFC PATCH 1/6] erofs: remove unused device mapping in the meta routine

[Jingbo Xu]

Currently there're two anonymous inodes (inode and anon_inode in struct
erofs_fscache) may be used for each blob.  The former is only for
bootstrap and used as the address_space of page cache, while the latter
is used for both bootstrap and data blobs when share domain mode enabled
and behaves as a sentinel in the shared domain.

In prep for the following support for page cache sharing, following
patch will unify these two anonymous inodes.  That is, the unified
anonymous inode not only acts as the address_space of page cache, but
also a sentinel in share domain mode.

However the current meta routine can't work if above change applied.
Current meta routine will make a device mapping, and superblock of the
filesystem is required to do the device mapping.  Currently the
superblock is derived from the input meta folio, which is reasonable
since the anonymous inode (used for the address_space of page cache) is
always allocated from the filesystem's sb.  However after anonymous
inodes are unified, that is no longer always true.  For example, in
share domain mode, the unified anonymous inode will be allocated from
pseudo mnt, and the superblock derived from the folio is actually a
pseudo sb, which can't be used for the device mapping at all.

As for the meta routine itself, currently metadata is always on
bootstrap, which means device mapping is not needed so far.  After
removing the redundant device mapping logic, we can derive the required
fscache_ctx from anonymous inode's i_private.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 fs/erofs/fscache.c | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index 014e20962376..03de4dc99302 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -164,18 +164,8 @@ static int erofs_fscache_read_folios_async(struct fscache_cookie *cookie,
 static int erofs_fscache_meta_read_folio(struct file *data, struct folio *folio)
 {
 	int ret;
-	struct super_block *sb = folio_mapping(folio)->host->i_sb;
+	struct erofs_fscache *ctx = folio_mapping(folio)->host->i_private;
 	struct erofs_fscache_request *req;
-	struct erofs_map_dev mdev = {
-		.m_deviceid = 0,
-		.m_pa = folio_pos(folio),
-	};
-
-	ret = erofs_map_dev(sb, &mdev);
-	if (ret) {
-		folio_unlock(folio);
-		return ret;
-	}
 
 	req = erofs_fscache_req_alloc(folio_mapping(folio),
 				folio_pos(folio), folio_size(folio));
@@ -184,8 +174,8 @@ static int erofs_fscache_meta_read_folio(struct file *data, struct folio *folio)
 		return PTR_ERR(req);
 	}
 
-	ret = erofs_fscache_read_folios_async(mdev.m_fscache->cookie,
-				req, mdev.m_pa, folio_size(folio));
+	ret = erofs_fscache_read_folios_async(ctx->cookie, req,
+				folio_pos(folio), folio_size(folio));
 	if (ret)
 		req->error = ret;
 
@@ -469,6 +459,7 @@ struct erofs_fscache *erofs_fscache_acquire_cookie(struct super_block *sb,
 		inode->i_size = OFFSET_MAX;
 		inode->i_mapping->a_ops = &erofs_fscache_meta_aops;
 		mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
+		inode->i_private = ctx;
 
 		ctx->inode = inode;
 	}
-- 
2.19.1.6.gb485710b

[RFC PATCH 2/6] erofs: unify anonymous inodes for blob

[Jingbo Xu]

Currently only bootstrap will allocate anonymous inode for the
address_space of page cache.  In prep for the following support for page
cache sharing, as the first step, always allocate anonymous inode for
this use for both bootstrap and data blobs.

Since now anonymous inode is also allocated for data blobs, release
these anonymous inodes when .put_super() is called, or we'll get
"VFS: Busy inodes after unmount." warning.

Similarly, the fscache contexts for data blobs are initialized prior to
the root inode, thus .kill_sb() shall also contain the cleanup routine,
so that these fscache contexts can be cleaned up when mount fails while
root inode has not been initialized yet.

Also remove the redundant set_nlink() when initializing anonymous inode,
since i_nlink has already been initialized to 1 when the inode gets
allocated.

Until then there're two anonymous inodes (inode and anon_inode in struct
erofs_fscache) may be used for each blob.  The former is used as the
address_space of page cache, while the latter is used as a sentinel in
the shared domain.

In prep for the following support for page cache sharing, unify these
two anonymous inodes.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 fs/erofs/fscache.c  | 97 +++++++++++++++++++++------------------------
 fs/erofs/internal.h |  6 +--
 fs/erofs/super.c    |  2 +
 3 files changed, 49 insertions(+), 56 deletions(-)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index 03de4dc99302..4d7785a70926 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -318,8 +318,6 @@ const struct address_space_operations erofs_fscache_access_aops = {
 
 static void erofs_fscache_domain_put(struct erofs_domain *domain)
 {
-	if (!domain)
-		return;
 	mutex_lock(&erofs_domain_list_lock);
 	if (refcount_dec_and_test(&domain->ref)) {
 		list_del(&domain->list);
@@ -423,12 +421,13 @@ static int erofs_fscache_register_domain(struct super_block *sb)
 
 static
 struct erofs_fscache *erofs_fscache_acquire_cookie(struct super_block *sb,
-						   char *name,
-						   unsigned int flags)
+						   char *name)
 {
 	struct fscache_volume *volume = EROFS_SB(sb)->volume;
 	struct erofs_fscache *ctx;
 	struct fscache_cookie *cookie;
+	struct super_block *psb = sb;
+	struct inode *inode;
 	int ret;
 
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
@@ -442,33 +441,29 @@ struct erofs_fscache *erofs_fscache_acquire_cookie(struct super_block *sb,
 		ret = -EINVAL;
 		goto err;
 	}
-
 	fscache_use_cookie(cookie, false);
-	ctx->cookie = cookie;
-
-	if (flags & EROFS_REG_COOKIE_NEED_INODE) {
-		struct inode *const inode = new_inode(sb);
 
-		if (!inode) {
-			erofs_err(sb, "failed to get anon inode for %s", name);
-			ret = -ENOMEM;
-			goto err_cookie;
-		}
-
-		set_nlink(inode, 1);
-		inode->i_size = OFFSET_MAX;
-		inode->i_mapping->a_ops = &erofs_fscache_meta_aops;
-		mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
-		inode->i_private = ctx;
-
-		ctx->inode = inode;
+	if (EROFS_SB(sb)->domain_id)
+		psb = erofs_pseudo_mnt->mnt_sb;
+	inode = new_inode(psb);
+	if (!inode) {
+		erofs_err(sb, "failed to get anon inode for %s", name);
+		ret = -ENOMEM;
+		goto err_cookie;
 	}
 
+	inode->i_size = OFFSET_MAX;
+	inode->i_mapping->a_ops = &erofs_fscache_meta_aops;
+	mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
+	inode->i_private = ctx;
+
+	ctx->cookie = cookie;
+	ctx->inode = inode;
 	return ctx;
 
 err_cookie:
-	fscache_unuse_cookie(ctx->cookie, NULL, NULL);
-	fscache_relinquish_cookie(ctx->cookie, false);
+	fscache_unuse_cookie(cookie, NULL, NULL);
+	fscache_relinquish_cookie(cookie, false);
 err:
 	kfree(ctx);
 	return ERR_PTR(ret);
@@ -485,38 +480,34 @@ static void erofs_fscache_relinquish_cookie(struct erofs_fscache *ctx)
 
 static
 struct erofs_fscache *erofs_fscache_domain_init_cookie(struct super_block *sb,
-						       char *name,
-						       unsigned int flags)
+						       char *name)
 {
-	int err;
-	struct inode *inode;
 	struct erofs_fscache *ctx;
 	struct erofs_domain *domain = EROFS_SB(sb)->domain;
 
-	ctx = erofs_fscache_acquire_cookie(sb, name, flags);
+	ctx = erofs_fscache_acquire_cookie(sb, name);
 	if (IS_ERR(ctx))
 		return ctx;
 
 	ctx->name = kstrdup(name, GFP_KERNEL);
 	if (!ctx->name) {
-		err = -ENOMEM;
-		goto out;
+		erofs_fscache_relinquish_cookie(ctx);
+		return ERR_PTR(-ENOMEM);
 	}
 
-	inode = new_inode(erofs_pseudo_mnt->mnt_sb);
-	if (!inode) {
-		err = -ENOMEM;
-		goto out;
-	}
+	/*
+	 * In share domain scenarios, the unified anonymous inode is not only
+	 * used as the address_space of shared page cache, but also a sentinel
+	 * in pseudo mount.  The initial refcount is used for the former and
+	 * will be killed when the cookie finally gets relinquished.  For the
+	 * latter, the refcount is increased every time the cookie in the domain
+	 * gets referred to.
+	 */
+	igrab(ctx->inode);
 
 	ctx->domain = domain;
-	ctx->anon_inode = inode;
-	inode->i_private = ctx;
 	refcount_inc(&domain->ref);
 	return ctx;
-out:
-	erofs_fscache_relinquish_cookie(ctx);
-	return ERR_PTR(err);
 }
 
 static
@@ -547,7 +538,7 @@ struct erofs_fscache *erofs_domain_register_cookie(struct super_block *sb,
 		return ctx;
 	}
 	spin_unlock(&psb->s_inode_list_lock);
-	ctx = erofs_fscache_domain_init_cookie(sb, name, flags);
+	ctx = erofs_fscache_domain_init_cookie(sb, name);
 	mutex_unlock(&erofs_domain_cookies_lock);
 	return ctx;
 }
@@ -558,7 +549,7 @@ struct erofs_fscache *erofs_fscache_register_cookie(struct super_block *sb,
 {
 	if (EROFS_SB(sb)->domain_id)
 		return erofs_domain_register_cookie(sb, name, flags);
-	return erofs_fscache_acquire_cookie(sb, name, flags);
+	return erofs_fscache_acquire_cookie(sb, name);
 }
 
 void erofs_fscache_unregister_cookie(struct erofs_fscache *ctx)
@@ -568,18 +559,21 @@ void erofs_fscache_unregister_cookie(struct erofs_fscache *ctx)
 
 	if (!ctx)
 		return;
+
 	domain = ctx->domain;
 	if (domain) {
 		mutex_lock(&erofs_domain_cookies_lock);
-		drop = atomic_read(&ctx->anon_inode->i_count) == 1;
-		iput(ctx->anon_inode);
+		/* drop the ref for the sentinel in pseudo mount */
+		iput(ctx->inode);
+		drop = atomic_read(&ctx->inode->i_count) == 1;
+		if (drop)
+			erofs_fscache_relinquish_cookie(ctx);
 		mutex_unlock(&erofs_domain_cookies_lock);
-		if (!drop)
-			return;
+		if (drop)
+			erofs_fscache_domain_put(domain);
+	} else {
+		erofs_fscache_relinquish_cookie(ctx);
 	}
-
-	erofs_fscache_relinquish_cookie(ctx);
-	erofs_fscache_domain_put(domain);
 }
 
 int erofs_fscache_register_fs(struct super_block *sb)
@@ -587,7 +581,7 @@ int erofs_fscache_register_fs(struct super_block *sb)
 	int ret;
 	struct erofs_sb_info *sbi = EROFS_SB(sb);
 	struct erofs_fscache *fscache;
-	unsigned int flags;
+	unsigned int flags = 0;
 
 	if (sbi->domain_id)
 		ret = erofs_fscache_register_domain(sb);
@@ -606,7 +600,6 @@ int erofs_fscache_register_fs(struct super_block *sb)
 	 *
 	 * Acquired domain/volume will be relinquished in kill_sb() on error.
 	 */
-	flags = EROFS_REG_COOKIE_NEED_INODE;
 	if (sbi->domain_id)
 		flags |= EROFS_REG_COOKIE_NEED_NOEXIST;
 	fscache = erofs_fscache_register_cookie(sb, sbi->fsid, flags);
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index bb8501c0ff5b..b3d04bc2d279 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -108,8 +108,7 @@ struct erofs_domain {
 
 struct erofs_fscache {
 	struct fscache_cookie *cookie;
-	struct inode *inode;
-	struct inode *anon_inode;
+	struct inode *inode;	/* anonymous indoe for the blob */
 	struct erofs_domain *domain;
 	char *name;
 };
@@ -604,8 +603,7 @@ static inline int z_erofs_load_lzma_config(struct super_block *sb,
 #endif	/* !CONFIG_EROFS_FS_ZIP */
 
 /* flags for erofs_fscache_register_cookie() */
-#define EROFS_REG_COOKIE_NEED_INODE	1
-#define EROFS_REG_COOKIE_NEED_NOEXIST	2
+#define EROFS_REG_COOKIE_NEED_NOEXIST	1
 
 /* fscache.c */
 #ifdef CONFIG_EROFS_FS_ONDEMAND
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 481788c24a68..4ce00285a2e4 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -970,6 +970,8 @@ static void erofs_put_super(struct super_block *sb)
 	iput(sbi->packed_inode);
 	sbi->packed_inode = NULL;
 #endif
+	erofs_free_dev_context(sbi->devs);
+	sbi->devs = NULL;
 	erofs_fscache_unregister_fs(sb);
 }
 
-- 
2.19.1.6.gb485710b

[RFC PATCH 3/6] erofs: alloc anonymous file for blob in share domain mode

[Jingbo Xu]

In prep for the following support for page cache sharing based mmap,
allocate an anonymous file for each blob, so that we can link
associated vma to blobs later.

Since page cache sharing will be enabled only for share domain mode,
prepare anonymous file only in share domain mode.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 fs/erofs/fscache.c  | 24 +++++++++++++++++++++++-
 fs/erofs/internal.h |  1 +
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index 4d7785a70926..ea276884f043 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -4,6 +4,8 @@
  * Copyright (C) 2022, Bytedance Inc. All rights reserved.
  */
 #include <linux/fscache.h>
+#include <linux/file.h>
+#include <linux/anon_inodes.h>
 #include "internal.h"
 
 static DEFINE_MUTEX(erofs_domain_list_lock);
@@ -316,6 +318,8 @@ const struct address_space_operations erofs_fscache_access_aops = {
 	.readahead = erofs_fscache_readahead,
 };
 
+static const struct file_operations erofs_fscache_meta_fops = {};
+
 static void erofs_fscache_domain_put(struct erofs_domain *domain)
 {
 	mutex_lock(&erofs_domain_list_lock);
@@ -428,6 +432,7 @@ struct erofs_fscache *erofs_fscache_acquire_cookie(struct super_block *sb,
 	struct fscache_cookie *cookie;
 	struct super_block *psb = sb;
 	struct inode *inode;
+	struct file *file;
 	int ret;
 
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
@@ -457,10 +462,24 @@ struct erofs_fscache *erofs_fscache_acquire_cookie(struct super_block *sb,
 	mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
 	inode->i_private = ctx;
 
+	if (EROFS_SB(sb)->domain_id) {
+		ihold(inode);
+		file = alloc_file_pseudo(inode, erofs_pseudo_mnt, "[erofs]",
+				O_RDONLY, &erofs_fscache_meta_fops);
+		if (IS_ERR(file)) {
+			ret = PTR_ERR(file);
+			iput(inode);
+			goto err_inode;
+		}
+		ctx->file = file;
+	}
+
 	ctx->cookie = cookie;
 	ctx->inode = inode;
 	return ctx;
 
+err_inode:
+	iput(inode);
 err_cookie:
 	fscache_unuse_cookie(cookie, NULL, NULL);
 	fscache_relinquish_cookie(cookie, false);
@@ -473,6 +492,8 @@ static void erofs_fscache_relinquish_cookie(struct erofs_fscache *ctx)
 {
 	fscache_unuse_cookie(ctx->cookie, NULL, NULL);
 	fscache_relinquish_cookie(ctx->cookie, false);
+	if (ctx->file)
+		fput(ctx->file);
 	iput(ctx->inode);
 	kfree(ctx->name);
 	kfree(ctx);
@@ -565,7 +586,8 @@ void erofs_fscache_unregister_cookie(struct erofs_fscache *ctx)
 		mutex_lock(&erofs_domain_cookies_lock);
 		/* drop the ref for the sentinel in pseudo mount */
 		iput(ctx->inode);
-		drop = atomic_read(&ctx->inode->i_count) == 1;
+		/* one initial ref, and one ref for anonymous file */
+		drop = atomic_read(&ctx->inode->i_count) == 2;
 		if (drop)
 			erofs_fscache_relinquish_cookie(ctx);
 		mutex_unlock(&erofs_domain_cookies_lock);
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index b3d04bc2d279..24d471fe2fa4 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -109,6 +109,7 @@ struct erofs_domain {
 struct erofs_fscache {
 	struct fscache_cookie *cookie;
 	struct inode *inode;	/* anonymous indoe for the blob */
+	struct file *file;	/* anonymous file */
 	struct erofs_domain *domain;
 	char *name;
 };
-- 
2.19.1.6.gb485710b

[RFC PATCH 4/6] erofs: implement .read_iter for page cache sharing

[Jingbo Xu]

When page cache sharing enabled, page caches are managed in the address
space of blobs rather than erofs inodes.  All erofs inodes sharing one
chunk will refer to and share the page cache in the blob's address
space.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 fs/erofs/fscache.c  | 64 +++++++++++++++++++++++++++++++++++++++++++++
 fs/erofs/internal.h |  1 +
 2 files changed, 65 insertions(+)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index ea276884f043..1f2a42dd1590 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -320,6 +320,70 @@ const struct address_space_operations erofs_fscache_access_aops = {
 
 static const struct file_operations erofs_fscache_meta_fops = {};
 
+static ssize_t erofs_fscache_share_file_read_iter(struct kiocb *iocb,
+						  struct iov_iter *to)
+{
+	struct file *filp = iocb->ki_filp;
+	struct inode *inode = file_inode(filp);
+	/* since page cache sharing is enabled only when i_size <= chunk_size */
+	struct erofs_map_blocks map = {}; /* .m_la = 0 */
+	struct erofs_map_dev mdev;
+	struct folio *folio;
+	ssize_t already_read = 0;
+	int ret = 0;
+
+	/* no need taking (shared) inode lock since it's a ro filesystem */
+	if (!iov_iter_count(to))
+		return 0;
+
+	if (IS_DAX(inode) || iocb->ki_flags & IOCB_DIRECT)
+		return -EOPNOTSUPP;
+
+	ret = erofs_map_blocks(inode, &map, EROFS_GET_BLOCKS_RAW);
+	if (ret)
+		return ret;
+
+	mdev = (struct erofs_map_dev) {
+		.m_deviceid = map.m_deviceid,
+		.m_pa = map.m_pa,
+	};
+	ret = erofs_map_dev(inode->i_sb, &mdev);
+	if (ret)
+		return ret;
+
+	do {
+		size_t bytes, copied, offset, fsize;
+		pgoff_t index = (mdev.m_pa >> PAGE_SHIFT) + (iocb->ki_pos >> PAGE_SHIFT);
+
+		folio = read_cache_folio(mdev.m_fscache->inode->i_mapping, index, NULL, NULL);
+		if (IS_ERR(folio)) {
+			ret = PTR_ERR(folio);
+			break;
+		}
+
+		fsize = folio_size(folio);
+		offset = iocb->ki_pos & (fsize - 1);
+		bytes = min_t(size_t, inode->i_size - iocb->ki_pos, iov_iter_count(to));
+		bytes = min_t(size_t, bytes, fsize - offset);
+		copied = copy_folio_to_iter(folio, offset, bytes, to);
+		folio_put(folio);
+		iocb->ki_pos += copied;
+		already_read += copied;
+		if (copied < bytes) {
+			ret = -EFAULT;
+			break;
+		}
+	} while (iov_iter_count(to) && iocb->ki_pos < inode->i_size);
+
+	file_accessed(filp);
+	return already_read ? already_read : ret;
+}
+
+const struct file_operations erofs_fscache_share_file_fops = {
+	.llseek		= generic_file_llseek,
+	.read_iter	= erofs_fscache_share_file_read_iter,
+};
+
 static void erofs_fscache_domain_put(struct erofs_domain *domain)
 {
 	mutex_lock(&erofs_domain_list_lock);
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 24d471fe2fa4..386e2fd4c025 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -617,6 +617,7 @@ struct erofs_fscache *erofs_fscache_register_cookie(struct super_block *sb,
 void erofs_fscache_unregister_cookie(struct erofs_fscache *fscache);
 
 extern const struct address_space_operations erofs_fscache_access_aops;
+extern const struct file_operations erofs_fscache_share_file_fops;
 #else
 static inline int erofs_fscache_register_fs(struct super_block *sb)
 {
-- 
2.19.1.6.gb485710b

[RFC PATCH 5/6] erofs: implement .mmap for page cache sharing

[Jingbo Xu]

In mmap(2), replace vma->vm_file with the anonymous file associated with
the blob, so that the vma will be linked to the address_space of the
blob.

One thing worth noting is that, we return error early in mmap(2) if
users attempt to map beyond the file size.  Normally filesystems won't
restrict this in mmap(2).  The checking is done in the fault handler,
and SIGBUS will be signaled to users if they actually attempt to access
the area beyond the end of the file.  However since vma->vm_file has
been changed to the anonymous file in mmap(2), we can no way derive the
file size of the original file.  As file size is immutable in ro
filesystem, let's fail early in mmap(2) in this case.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 fs/erofs/fscache.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index 1f2a42dd1590..98341d4d9c0d 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -379,9 +379,46 @@ static ssize_t erofs_fscache_share_file_read_iter(struct kiocb *iocb,
 	return already_read ? already_read : ret;
 }
 
+static const struct vm_operations_struct erofs_fscache_share_file_vm_ops = {
+	.fault = filemap_fault,
+};
+
+static int erofs_fscache_share_file_mmap(struct file *file,
+					 struct vm_area_struct *vma)
+{
+	struct inode *inode = file_inode(file);
+	/* since page cache sharing is enabled only when i_size <= chunk_size */
+	struct erofs_map_blocks map = {}; /* .m_la = 0 */
+	struct erofs_map_dev mdev;
+	int ret;
+
+	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
+		return -EINVAL;
+
+	ret = erofs_map_blocks(inode, &map, EROFS_GET_BLOCKS_RAW);
+	if (ret)
+		return ret;
+
+	mdev = (struct erofs_map_dev) {
+		.m_deviceid = map.m_deviceid,
+		.m_pa = map.m_pa,
+	};
+	ret = erofs_map_dev(inode->i_sb, &mdev);
+	if (ret)
+		return ret;
+
+	vma_set_file(vma, mdev.m_fscache->file);
+	vma->vm_pgoff = (mdev.m_pa >> PAGE_SHIFT) + vma->vm_pgoff;
+	vma->vm_ops = &erofs_fscache_share_file_vm_ops;
+
+	file_accessed(file);
+	return 0;
+}
+
 const struct file_operations erofs_fscache_share_file_fops = {
 	.llseek		= generic_file_llseek,
 	.read_iter	= erofs_fscache_share_file_read_iter,
+	.mmap		= erofs_fscache_share_file_mmap,
 };
 
 static void erofs_fscache_domain_put(struct erofs_domain *domain)
-- 
2.19.1.6.gb485710b

[RFC PATCH 6/6] erofs: enable page cache sharing in fscache mode

[Jingbo Xu]

Erofs supports chunk deduplication to reduce disk usage.  Furthermore we
can make inodes share page cache of these deduplicated chunks to reduce
the memory usage.  This shall be much usable in container scenarios as
deduplication is requisite for container image.

This can be achieved by managing page cache of deduplicated chunks in
blob's address space.  In this way, all inodes sharing the deduplicated
chunk will refer to and share the page cache in the blob's address
space.

So far there are some restrictions for enabling this feature.

The page cache sharing feature also supports .mmap().  The reverse
mapping requires that one vma can not be shared among inodes and can
be linked to only one inode.  As the vma will be finally linked to the
blob's address space when page cache sharing enabled, the restriction of
the reverse mapping actually requires that the mapped file area can not
be mapped to multiple blobs.  Thus page cache sharing can only be
enabled for those files mapped to one blob.

The chunk based data layout guarantees that a chunk will not cross the
device (blob) boundary.  Thus in chunk based data layout, those files
smaller than the chunk size shall be guaranteed to be mapped to one
blob.  As chunk size is tunable at a per-file basis, this restriction
can be relaxed at image building phase.  As long as we ensure that the
file can not be deduplicated, the file's chunk size can be set to a
reasonable value larger than the file size, so that the page cache
sharing feature can be enabled on this file later.

The second restriction is that EROFS_BLKSIZ mus be multiples of
PAGE_SIZE to avoid data leakage.  Otherwise unrelated data may be
exposed at the end of the last page, since file's data is arranged in
unit of EROFS_BLKSIZ in the image.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 fs/erofs/inode.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/fs/erofs/inode.c b/fs/erofs/inode.c
index d3b8736fa124..8fe9b29422b5 100644
--- a/fs/erofs/inode.c
+++ b/fs/erofs/inode.c
@@ -241,6 +241,29 @@ static int erofs_fill_symlink(struct inode *inode, void *kaddr,
 	return 0;
 }
 
+static bool erofs_can_share_page_cache(struct inode *inode)
+{
+	struct erofs_inode *vi = EROFS_I(inode);
+
+	/* enable page cache sharing only in share domain mode */
+	if (!erofs_is_fscache_mode(inode->i_sb) ||
+	    !EROFS_SB(inode->i_sb)->domain_id)
+		return false;
+
+	if (vi->datalayout != EROFS_INODE_CHUNK_BASED)
+		return false;
+
+	/* avoid crossing multi devicces/blobs */
+	if (inode->i_size > 1UL << vi->chunkbits)
+		return false;
+
+	/* avoid data leakage in mmap routine */
+	if (EROFS_BLKSIZ % PAGE_SIZE)
+		return false;
+
+	return true;
+}
+
 static int erofs_fill_inode(struct inode *inode)
 {
 	struct erofs_inode *vi = EROFS_I(inode);
@@ -262,6 +285,10 @@ static int erofs_fill_inode(struct inode *inode)
 		inode->i_op = &erofs_generic_iops;
 		if (erofs_inode_is_data_compressed(vi->datalayout))
 			inode->i_fop = &generic_ro_fops;
+#ifdef CONFIG_EROFS_FS_ONDEMAND
+		else if (erofs_can_share_page_cache(inode))
+			inode->i_fop = &erofs_fscache_share_file_fops;
+#endif
 		else
 			inode->i_fop = &erofs_file_fops;
 		break;
-- 
2.19.1.6.gb485710b