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Message-ID: <20241119094555.660666-2-mjguzik@gmail.com>
Date: Tue, 19 Nov 2024 10:45:53 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
jack@...e.cz,
linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
hughd@...gle.com,
linux-ext4@...r.kernel.org,
tytso@....edu,
linux-mm@...ck.org,
Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH v2 1/3] vfs: support caching symlink lengths in inodes
When utilized it dodges strlen() in vfs_readlink(), giving about 1.5%
speed up when issuing readlink on /initrd.img on ext4.
Filesystems opt in by calling inode_set_cached_link() when creating an
inode.
The size is stored in what used to be a 4-byte hole. If necessary the
field can be made smaller and converted into a union with something not
used with symlinks.
Churn-wise the current readlink_copy() helper is patched to accept the
size instead of calculating it.
Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---
fs/namei.c | 34 +++++++++++++++++++---------------
fs/proc/namespaces.c | 2 +-
include/linux/fs.h | 12 ++++++++++--
security/apparmor/apparmorfs.c | 2 +-
4 files changed, 31 insertions(+), 19 deletions(-)
diff --git a/fs/namei.c b/fs/namei.c
index 9d30c7aa9aa6..e56c29a22d26 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -5272,19 +5272,16 @@ SYSCALL_DEFINE2(rename, const char __user *, oldname, const char __user *, newna
getname(newname), 0);
}
-int readlink_copy(char __user *buffer, int buflen, const char *link)
+int readlink_copy(char __user *buffer, int buflen, const char *link, int linklen)
{
- int len = PTR_ERR(link);
- if (IS_ERR(link))
- goto out;
+ int copylen;
- len = strlen(link);
- if (len > (unsigned) buflen)
- len = buflen;
- if (copy_to_user(buffer, link, len))
- len = -EFAULT;
-out:
- return len;
+ copylen = linklen;
+ if (unlikely(copylen > (unsigned) buflen))
+ copylen = buflen;
+ if (copy_to_user(buffer, link, copylen))
+ copylen = -EFAULT;
+ return copylen;
}
/**
@@ -5304,6 +5301,9 @@ int vfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
const char *link;
int res;
+ if (inode->i_opflags & IOP_CACHED_LINK)
+ return readlink_copy(buffer, buflen, inode->i_link, inode->i_linklen);
+
if (unlikely(!(inode->i_opflags & IOP_DEFAULT_READLINK))) {
if (unlikely(inode->i_op->readlink))
return inode->i_op->readlink(dentry, buffer, buflen);
@@ -5322,7 +5322,7 @@ int vfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
if (IS_ERR(link))
return PTR_ERR(link);
}
- res = readlink_copy(buffer, buflen, link);
+ res = readlink_copy(buffer, buflen, link, strlen(link));
do_delayed_call(&done);
return res;
}
@@ -5391,10 +5391,14 @@ EXPORT_SYMBOL(page_put_link);
int page_readlink(struct dentry *dentry, char __user *buffer, int buflen)
{
+ const char *link;
+ int res;
+
DEFINE_DELAYED_CALL(done);
- int res = readlink_copy(buffer, buflen,
- page_get_link(dentry, d_inode(dentry),
- &done));
+ link = page_get_link(dentry, d_inode(dentry), &done);
+ res = PTR_ERR(link);
+ if (!IS_ERR(link))
+ res = readlink_copy(buffer, buflen, link, strlen(link));
do_delayed_call(&done);
return res;
}
diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
index 8e159fc78c0a..c610224faf10 100644
--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c
@@ -83,7 +83,7 @@ static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int bufl
if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
res = ns_get_name(name, sizeof(name), task, ns_ops);
if (res >= 0)
- res = readlink_copy(buffer, buflen, name);
+ res = readlink_copy(buffer, buflen, name, strlen(name));
}
put_task_struct(task);
return res;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 972147da71f9..30e332fb399d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -626,6 +626,7 @@ is_uncached_acl(struct posix_acl *acl)
#define IOP_XATTR 0x0008
#define IOP_DEFAULT_READLINK 0x0010
#define IOP_MGTIME 0x0020
+#define IOP_CACHED_LINK 0x0040
/*
* Keep mostly read-only and often accessed (especially for
@@ -686,7 +687,7 @@ struct inode {
/* Misc */
u32 i_state;
- /* 32-bit hole */
+ int i_linklen; /* for symlinks */
struct rw_semaphore i_rwsem;
unsigned long dirtied_when; /* jiffies of first dirtying */
@@ -749,6 +750,13 @@ struct inode {
void *i_private; /* fs or device private pointer */
} __randomize_layout;
+static inline void inode_set_cached_link(struct inode *inode, char *link, int linklen)
+{
+ inode->i_link = link;
+ inode->i_linklen = linklen;
+ inode->i_opflags |= IOP_CACHED_LINK;
+}
+
/*
* Get bit address from inode->i_state to use with wait_var_event()
* infrastructre.
@@ -3351,7 +3359,7 @@ extern const struct file_operations generic_ro_fops;
#define special_file(m) (S_ISCHR(m)||S_ISBLK(m)||S_ISFIFO(m)||S_ISSOCK(m))
-extern int readlink_copy(char __user *, int, const char *);
+extern int readlink_copy(char __user *, int, const char *, int);
extern int page_readlink(struct dentry *, char __user *, int);
extern const char *page_get_link(struct dentry *, struct inode *,
struct delayed_call *);
diff --git a/security/apparmor/apparmorfs.c b/security/apparmor/apparmorfs.c
index 01b923d97a44..60959cfba672 100644
--- a/security/apparmor/apparmorfs.c
+++ b/security/apparmor/apparmorfs.c
@@ -2611,7 +2611,7 @@ static int policy_readlink(struct dentry *dentry, char __user *buffer,
res = snprintf(name, sizeof(name), "%s:[%lu]", AAFS_NAME,
d_inode(dentry)->i_ino);
if (res > 0 && res < sizeof(name))
- res = readlink_copy(buffer, buflen, name);
+ res = readlink_copy(buffer, buflen, name, strlen(name));
else
res = -ENOENT;
--
2.43.0
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