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 <[email protected]> Signed-off-by: Vlastimil Babka <[email protected]> --- 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
