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vfs: support caching symlink lengths in inodes

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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 a new union utilizing the same space as i_devices,
thus avoiding growing the struct or taking up any more space.

Churn-wise the current readlink_copy() helper is patched to accept the
size instead of calculating it.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20241120112037.822078-2-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Mateusz Guzik 2024-11-20 12:20:34 +01:00 committed by Christian Brauner
parent 6740021e90
commit 8cdcbe085a
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
4 changed files with 34 additions and 19 deletions
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34
fs/namei.c
View File

@ -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;
}

2
fs/proc/namespaces.c
View File

@ -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;

15
include/linux/fs.h
View File

@ -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
@ -723,7 +724,10 @@ struct inode {
};
struct file_lock_context *i_flctx;
struct address_space i_data;
struct list_head i_devices;
union {
struct list_head i_devices;
int i_linklen;
};
union {
struct pipe_inode_info *i_pipe;
struct cdev *i_cdev;
@ -749,6 +753,13 @@ 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 +3362,7 @@ 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 *);

2
security/apparmor/apparmorfs.c
View File

@ -2612,7 +2612,7 @@ 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;