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Subject[PATCH] fs/seq_file: remove kmalloc(ops) for single_open seqfiles
Date
single_open() currently allocates seq_operations with kmalloc(). This is
suboptimal, because that's four pointers, of which three are constant, and
only the 'show' op differs. We also have to be careful to use single_release()
to avoid leaking the ops structure.

Instead of this we can have a fixed single_show() function and constant ops
structure for these seq_files. We can store the pointer to the 'show' op as
a new field of struct seq_file. That's also not terribly elegant, because the
field is there also for non-single_open() seq files, but it's a single pointer
in an already existing (and already relatively large) structure instead of
an extra kmalloc of four pointers, so the tradeoff is OK.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
Documentation/filesystems/seq_file.txt | 5 +++-
fs/seq_file.c | 40 ++++++++++++--------------
include/linux/seq_file.h | 5 ++--
3 files changed, 25 insertions(+), 25 deletions(-)

diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt
index 9de4303201e1..ed61495abee8 100644
--- a/Documentation/filesystems/seq_file.txt
+++ b/Documentation/filesystems/seq_file.txt
@@ -335,4 +335,7 @@ When output time comes, the show() function will be called once. The data
value given to single_open() can be found in the private field of the
seq_file structure. When using single_open(), the programmer should use
single_release() instead of seq_release() in the file_operations structure
-to avoid a memory leak.
+to avoid a memory leak. Note that the implementation has changed and current
+kernels will not leak anymore, but it's better to keep using single_release()
+in case the implementation details change again.
+
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 4cc090b50cc5..3fd2ded04d93 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -563,22 +563,27 @@ static void single_stop(struct seq_file *p, void *v)
{
}

+static int single_show(struct seq_file *p, void *v)
+{
+ return p->single_show_op(p, v);
+}
+
+static const struct seq_operations single_seq_op = {
+ .start = single_start,
+ .next = single_next,
+ .stop = single_stop,
+ .show = single_show
+};
+
int single_open(struct file *file, int (*show)(struct seq_file *, void *),
void *data)
{
- struct seq_operations *op = kmalloc(sizeof(*op), GFP_KERNEL_ACCOUNT);
- int res = -ENOMEM;
-
- if (op) {
- op->start = single_start;
- op->next = single_next;
- op->stop = single_stop;
- op->show = show;
- res = seq_open(file, op);
- if (!res)
- ((struct seq_file *)file->private_data)->private = data;
- else
- kfree(op);
+ int res;
+
+ res = seq_open(file, &single_seq_op);
+ if (!res) {
+ ((struct seq_file *)file->private_data)->private = data;
+ ((struct seq_file *)file->private_data)->single_show_op = show;
}
return res;
}
@@ -602,15 +607,6 @@ int single_open_size(struct file *file, int (*show)(struct seq_file *, void *),
}
EXPORT_SYMBOL(single_open_size);

-int single_release(struct inode *inode, struct file *file)
-{
- const struct seq_operations *op = ((struct seq_file *)file->private_data)->op;
- int res = seq_release(inode, file);
- kfree(op);
- return res;
-}
-EXPORT_SYMBOL(single_release);
-
int seq_release_private(struct inode *inode, struct file *file)
{
struct seq_file *seq = file->private_data;
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index a121982af0f5..c9a70c584a7d 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -24,9 +24,10 @@ struct seq_file {
u64 version;
struct mutex lock;
const struct seq_operations *op;
- int poll_event;
+ int (*single_show_op)(struct seq_file *, void *);
const struct file *file;
void *private;
+ int poll_event;
};

struct seq_operations {
@@ -140,7 +141,7 @@ int seq_path_root(struct seq_file *m, const struct path *path,

int single_open(struct file *, int (*)(struct seq_file *, void *), void *);
int single_open_size(struct file *, int (*)(struct seq_file *, void *), void *, size_t);
-int single_release(struct inode *, struct file *);
+#define single_release seq_release
void *__seq_open_private(struct file *, const struct seq_operations *, int);
int seq_open_private(struct file *, const struct seq_operations *, int);
int seq_release_private(struct inode *, struct file *);
--
2.18.0
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