Re: [patch 1/6] procfs: Introduce sequential fdinfo engine

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Thu, Oct 31, 2013 at 02:57:52PM +0400, Cyrill Gorcunov wrote:
> > > At moment the fdinfo operations (ie the output from /proc/$pid/fdinfo/$fd)
> > > are generating output in one pass, which makes useless memory pressue
> > > if the reader/user provides a buffer with a small size.
> > 
> > cat(1) uses 64 KB buffer.
> > The output doesn't exceed one page anyway.
> 
> Yes, good point. I probably need to update changelog (forgot that i'm using
> single_open here). What if we meet a file with that big number of epoll
> or say notifies assigned that the fdinfo won't fit a page? I didn't meet
> such scenario yet, but I think it's possible?

Alexey, would the following changelog sound better? I've been testing the
program which creates 100 pipes, which in turn are watched with epoll
(I don't think someone ever need to do that actully) then tried to read
fdinfo and it failed. Instead if I'm using this series I can see all 100
pipes in the output.
---
From: Cyrill Gorcunov <gorcunov@openvz.org>
Subject: procfs: Introduce sequential fdinfo engine

At moment the fdinfo operations (ie the output from /proc/$pid/fdinfo/$fd)
are generating output in one pass, allocating one page buffer as maximum.
But in case of lots of event polls watchees (100 and more) read procedure
will fail because one page is not enough to provide all information associated
with watch descriptors.

Instead, provide proc_fdinfo structure where each file hook own seq_operations
for fdinfo output and the seq-file engine will be able to traverse over the all
records and provide the reader complete information increasing buffer size if
needed.

Once all callers to fdinfo are converted (addressed in next patches)
the former @show_fdinfo will be deleted.

Note the patch doesn't change current output it's rather preparing
the ground for next patches.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrey Vagin <avagin@openvz.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: James Bottomley <jbottomley@parallels.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Matthew Helsley <matt.helsley@gmail.com>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 fs/proc/fd.c       |  162 ++++++++++++++++++++++++++++++++++++++++++-----------
 include/linux/fs.h |    1 
 2 files changed, 130 insertions(+), 33 deletions(-)

Index: linux-2.6.git/fs/proc/fd.c
===================================================================
--- linux-2.6.git.orig/fs/proc/fd.c
+++ linux-2.6.git/fs/proc/fd.c
@@ -14,60 +14,156 @@
 #include "internal.h"
 #include "fd.h"
 
+struct proc_fdinfo {
+	struct seq_file		seq;		/* Make sure it's first */
+	struct seq_operations	*fdinfo_ops;
+	loff_t			pos;
+	struct file		*f_file;
+	unsigned int		f_flags;
+};
+
+#define seq_info(m)	(container_of(m, struct proc_fdinfo, seq))
+
+static void *seq_start(struct seq_file *m, loff_t *pos)
+{
+	struct proc_fdinfo *info = seq_info(m);
+
+	info->pos = *pos;
+	if (*pos == 0)
+		return info;
+	return info->fdinfo_ops ? info->fdinfo_ops->start(m, pos) : NULL;
+}
+
+static void seq_stop(struct seq_file *m, void *v)
+{
+	struct proc_fdinfo *info = seq_info(m);
+
+	if (info->pos > 0 && info->fdinfo_ops)
+		info->fdinfo_ops->stop(m, v);
+}
+
+static void *seq_next(struct seq_file *m, void *p, loff_t *pos)
+{
+	struct proc_fdinfo *info = seq_info(m);
+	void *v = NULL;
+
+	if (info->fdinfo_ops) {
+		int ret = 0;
+
+		if (*pos == 0) {
+			v = info->fdinfo_ops->start(m, pos);
+			if (v) {
+				ret = info->fdinfo_ops->show(m, v);
+				p = v;
+			} else
+				ret = -1;
+		}
+
+		if (!ret)
+			v = info->fdinfo_ops->next(m, p, pos);
+		else
+			++*pos;
+	} else
+		++*pos;
+
+	info->pos = *pos;
+	return v;
+}
+
 static int seq_show(struct seq_file *m, void *v)
 {
+	struct proc_fdinfo *info = seq_info(m);
+
+	if (info->fdinfo_ops && info->pos > 0)
+		return info->fdinfo_ops->show(m, v);
+
+	seq_printf(m, "pos:\t%lli\nflags:\t0%o\n",
+		   (long long)info->f_file->f_pos, info->f_flags);
+
+	/* Legacy interface */
+	if (info->f_file->f_op->show_fdinfo)
+		return info->f_file->f_op->show_fdinfo(m, info->f_file);
+
+	return 0;
+}
+
+static const struct seq_operations fdinfo_seq_ops = {
+	.start	= seq_start,
+	.stop	= seq_stop,
+	.next	= seq_next,
+	.show	= seq_show,
+};
+
+static int seq_fdinfo_open(struct inode *inode, struct file *file)
+{
 	struct files_struct *files = NULL;
-	int f_flags = 0, ret = -ENOENT;
-	struct file *file = NULL;
+	struct proc_fdinfo *info;
 	struct task_struct *task;
+	int ret;
 
-	task = get_proc_task(m->private);
-	if (!task)
-		return -ENOENT;
-
-	files = get_files_struct(task);
-	put_task_struct(task);
-
-	if (files) {
-		int fd = proc_fd(m->private);
-
-		spin_lock(&files->file_lock);
-		file = fcheck_files(files, fd);
-		if (file) {
-			struct fdtable *fdt = files_fdtable(files);
-
-			f_flags = file->f_flags;
-			if (close_on_exec(fd, fdt))
-				f_flags |= O_CLOEXEC;
+	info = kzalloc(sizeof(*info), GFP_KERNEL);
+	if (!info)
+		return -ENOMEM;
+	file->private_data = &info->seq;
 
-			get_file(file);
-			ret = 0;
+	ret = seq_open(file, &fdinfo_seq_ops);
+	if (!ret) {
+		ret = -ENOENT;
+
+		task = get_proc_task(inode);
+		if (task) {
+			files = get_files_struct(task);
+			put_task_struct(task);
+		}
+
+		if (files) {
+			struct seq_file *m = file->private_data;
+			int fd = proc_fd(inode);
+
+			spin_lock(&files->file_lock);
+			info->f_file = fcheck_files(files, fd);
+			if (info->f_file) {
+				struct fdtable *fdt = files_fdtable(files);
+
+				info->f_flags = info->f_file->f_flags & ~O_CLOEXEC;
+				if (close_on_exec(fd, fdt))
+					info->f_flags |= O_CLOEXEC;
+
+				info->fdinfo_ops = info->f_file->f_op->fdinfo_ops;
+				m->private = info->f_file;
+
+				get_file(info->f_file);
+				ret = 0;
+			}
+			spin_unlock(&files->file_lock);
+			put_files_struct(files);
 		}
-		spin_unlock(&files->file_lock);
-		put_files_struct(files);
 	}
 
-	if (!ret) {
-                seq_printf(m, "pos:\t%lli\nflags:\t0%o\n",
-			   (long long)file->f_pos, f_flags);
-		if (file->f_op->show_fdinfo)
-			ret = file->f_op->show_fdinfo(m, file);
-		fput(file);
+	if (ret) {
+		if (info->f_file)
+			fput(info->f_file);
+		file->private_data = NULL;
+		kfree(info);
 	}
 
 	return ret;
 }
 
-static int seq_fdinfo_open(struct inode *inode, struct file *file)
+static int seq_fdinfo_release(struct inode *inode, struct file *file)
 {
-	return single_open(file, seq_show, inode);
+	struct seq_file *m = file->private_data;
+	struct proc_fdinfo *info = seq_info(m);
+
+	fput(info->f_file);
+	return seq_release(inode, file);
 }
 
 static const struct file_operations proc_fdinfo_file_operations = {
 	.open		= seq_fdinfo_open,
 	.read		= seq_read,
 	.llseek		= seq_lseek,
-	.release	= single_release,
+	.release	= seq_fdinfo_release,
 };
 
 static int tid_fd_revalidate(struct dentry *dentry, unsigned int flags)
Index: linux-2.6.git/include/linux/fs.h
===================================================================
--- linux-2.6.git.orig/include/linux/fs.h
+++ linux-2.6.git/include/linux/fs.h
@@ -1552,6 +1552,7 @@ struct file_operations {
 	long (*fallocate)(struct file *file, int mode, loff_t offset,
 			  loff_t len);
 	int (*show_fdinfo)(struct seq_file *m, struct file *f);
+	struct seq_operations *fdinfo_ops;
 };
 
 struct inode_operations {