[RFC] proc interface to show file page cache usage details

[Vladimir Shebordaev]

Hi,

I would like to suggest an interface to list inodes that currently
occupy page cache in human readable form.

A piece of code below creates a dedicated proc entry, namely,
/proc/kpagecache. Upon read request it traverses all the inodes of
each superblock and shows their page cache usage summary. It is done
in a stateful way, so it needs to access super_blocks list and has to
get and put superblocks on its own.

I am not quite sure who will give a fuck. Actually, it was a task for
my recent job interview. I still don't know what they exactly meant. I
just think it would be anyway nice to have such an interface.

In the hope it helps.

--
Regards,
Vladimir

diff -u a/fs/proc/Makefile b/fs/proc/Makefile
--- a/fs/proc/Makefile
+++ b/fs/proc/Makefile
@@ -29,4 +29,4 @@ proc-$(CONFIG_NET) += proc_net.o
 proc-$(CONFIG_PROC_KCORE) += kcore.o
 proc-$(CONFIG_PROC_VMCORE) += vmcore.o
 proc-$(CONFIG_PRINTK) += kmsg.o
-proc-$(CONFIG_PROC_PAGE_MONITOR) += page.o
+proc-$(CONFIG_PROC_PAGE_MONITOR) += page.o pagecache.o
diff -u a/fs/internal.h b/fs/internal.h
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -88,6 +88,9 @@ extern struct dentry *mount_fs(struct file_system_type *,
        int, const char *, void *);
 extern struct super_block *user_get_super(dev_t);

+extern void __put_super(struct super_block *sb);
+extern void put_super(struct super_block *sb);
+
 /*
  * open.c
  */
diff -u a/fs/super.c b/fs/super.c
--- a/fs/super.c
+++ b/fs/super.c
@@ -242,7 +242,7 @@ fail:
 /*
  * Drop a superblock's refcount.  The caller must hold sb_lock.
  */
-static void __put_super(struct super_block *sb)
+void __put_super(struct super_block *sb)
 {
  if (!--sb->s_count) {
  list_del_init(&sb->s_list);
@@ -257,7 +257,7 @@ static void __put_super(struct super_block *sb)
  * Drops a temporary reference, frees superblock if there's no
  * references left.
  */
-static void put_super(struct super_block *sb)
+void put_super(struct super_block *sb)
 {
  spin_lock(&sb_lock);
  __put_super(sb);
diff -u a/fs/proc/pagecache.c b/fs/proc/pagecache.c
--- /dev/null
+++ b/fs/proc/pagecache.c
@@ -0,0 +1,412 @@
+/*
+ *  fs/proc/pagecache.c
+ *
+ *  Copyright (C) 2014
+ *
+ *  Author: Vladimir Shebordaev <vshebordaev@xxxxxxx>
+ *
+ *  /proc/kpagecache interface to show file page cache usage
+ */
+
+#include <linux/version.h>
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+#include <linux/nsproxy.h>
+#include <linux/backing-dev.h>
+#include <linux/page-flags.h>
+#include <linux/slab.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/path.h>
+#include <linux/fs_struct.h>
+#include <linux/mount.h>
+#include <linux/ctype.h>
+#include <linux/unistd.h>
+
+#include <linux/seq_file.h>
+#include <linux/proc_fs.h>
+#include <linux/errno.h>
+
+#include "../internal.h"
+
+#define NR_PAGES (PAGE_ALIGN(PATH_MAX) >> PAGE_SHIFT)
+#define BUFSIZE (NR_PAGES << PAGE_SHIFT)
+
+struct iter {
+ struct inode *inode;
+ char *buf;
+};
+
+struct iter *iter_next(struct iter *iter)
+{
+ struct super_block *sb, *p;
+ struct inode *inode, *prev;
+
+ inode = iter->inode;
+ prev = inode;
+ sb = inode->i_sb;
+
+ spin_lock(&inode_sb_list_lock);
+next:
+ inode = list_next_entry(inode, i_sb_list);
+check:
+ if (&inode->i_sb_list == &sb->s_inodes)
+ inode = NULL;
+ if (inode) {
+ spin_lock(&inode->i_lock);
+ if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
+   !(mapping_cap_writeback_dirty(inode->i_mapping)) ||
+    (inode->i_mapping->nrpages == 0) ||
+    hlist_empty(&inode->i_dentry)) {
+ spin_unlock(&inode->i_lock);
+ goto next;
+ }
+ __iget(inode);
+ spin_unlock(&inode->i_lock);
+ }
+ spin_unlock(&inode_sb_list_lock);
+
+ iput(prev);
+ prev = NULL;
+
+ if (inode)
+ goto out;
+
+ up_read(&sb->s_umount);
+ p = sb;
+ spin_lock(&sb_lock);
+retry:
+ sb = list_next_entry(sb, s_list);
+ if (&sb->s_list == &super_blocks)
+ sb = NULL;
+ if (sb) {
+ if (hlist_unhashed(&sb->s_instances))
+ goto retry;
+ sb->s_count++;
+ }
+ if (p) {
+ __put_super(p);
+ p = NULL;
+ }
+ spin_unlock(&sb_lock);
+
+ if (sb) {
+ down_read(&sb->s_umount);
+ if (!sb->s_root || !(sb->s_flags & MS_BORN) || !sb->s_bdi ||
+    !bdi_cap_writeback_dirty(sb->s_bdi)) {
+ up_read(&sb->s_umount);
+ p = sb;
+ spin_lock(&sb_lock);
+ goto retry;
+ }
+ spin_lock(&inode_sb_list_lock);
+ if (list_empty(&sb->s_inodes)) {
+ spin_unlock(&inode_sb_list_lock);
+ up_read(&sb->s_umount);
+ p = sb;
+ spin_lock(&sb_lock);
+ goto retry;
+ }
+ inode = list_first_entry(&sb->s_inodes, struct inode, i_sb_list);
+ goto check;
+ }
+out:
+ iter->inode = inode;
+ return inode ? iter : NULL;
+}
+
+struct iter *iter_first(struct iter *iter)
+{
+ struct super_block *sb, *p;
+ struct inode *inode;
+
+ inode = NULL;
+ p = NULL;
+
+ spin_lock(&sb_lock);
+ sb = list_first_entry(&super_blocks, struct super_block, s_list);
+check:
+ if (&sb->s_list == &super_blocks)
+ sb = NULL;
+ if (sb) {
+ if (hlist_unhashed(&sb->s_instances)) {
+retry:
+ sb = list_next_entry(sb, s_list);
+ goto check;
+ }
+ sb->s_count++;
+ }
+ if (p) {
+ __put_super(p);
+ p = NULL;
+ }
+ spin_unlock(&sb_lock);
+
+ if (!sb)
+ goto out;
+
+ down_read(&sb->s_umount);
+ if (!sb->s_root || !(sb->s_flags & MS_BORN) || !sb->s_bdi ||
+    !bdi_cap_writeback_dirty(sb->s_bdi)) {
+ up_read(&sb->s_umount);
+ p = sb;
+ spin_lock(&sb_lock);
+ goto retry;
+ }
+
+ spin_lock(&inode_sb_list_lock);
+ if (list_empty(&sb->s_inodes)) {
+ spin_unlock(&inode_sb_list_lock);
+ up_read(&sb->s_umount);
+ p = sb;
+ spin_lock(&sb_lock);
+ goto retry;
+ }
+
+ inode = list_first_entry(&sb->s_inodes, struct inode, i_sb_list);
+next:
+ if (&inode->i_sb_list == &sb->s_inodes) {
+ spin_unlock(&inode_sb_list_lock);
+ up_read(&sb->s_umount);
+ inode = NULL;
+ p = sb;
+ spin_lock(&sb_lock);
+ goto retry;
+ }
+
+ if (inode) {
+ spin_lock(&inode->i_lock);
+ if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
+   !(mapping_cap_writeback_dirty(inode->i_mapping)) ||
+    (inode->i_mapping->nrpages == 0) ||
+    hlist_empty(&inode->i_dentry)) {
+ spin_unlock(&inode->i_lock);
+ inode = list_next_entry(inode, i_sb_list);
+ goto next;
+ }
+ __iget(inode);
+ spin_unlock(&inode->i_lock);
+ }
+ spin_unlock(&inode_sb_list_lock);
+out:
+ iter->inode = inode;
+ return inode ? iter : NULL;
+}
+
+static int iter_init(struct iter *iter)
+{
+ memset(iter, 0, sizeof(*iter));
+ iter->buf = (char *)__get_free_pages(GFP_TEMPORARY, order_base_2(NR_PAGES));
+
+ return iter->buf ? 0 : -ENOMEM;
+}
+
+static void iter_destroy(struct iter *iter)
+{
+ free_pages((unsigned long)iter->buf, order_base_2(NR_PAGES));
+}
+
+struct inode_stat {
+ unsigned long nr_pages;
+ unsigned long nr_shadow;
+ unsigned long nr_dirty;
+ unsigned long nr_active;
+ unsigned long nr_mlocked;
+ unsigned long nr_locked;
+ unsigned long nr_reclaim;
+};
+
+static int get_inode_stat(struct inode *inode, struct inode_stat *stat)
+{
+ int ret;
+ void **slot;
+ struct radix_tree_iter iter;
+
+ ret = 0;
+ memset(stat, 0, sizeof(*stat));
+
+ rcu_read_lock();
+retry:
+ radix_tree_for_each_slot(slot, &inode->i_mapping->page_tree, &iter, 0) {
+ struct page *page;
+
+ page = radix_tree_deref_slot(slot);
+ if (unlikely(!page))
+ continue;
+ if (radix_tree_exception(page)) {
+ if (radix_tree_deref_retry(page))
+ goto retry;
+ /* we are to avoid swap backed mappings */
+ BUG();
+ }
+ if (PageDirty(page))
+ ++stat->nr_dirty;
+ if (PageLocked(page))
+ ++stat->nr_locked;
+ if (PageActive(page))
+ ++stat->nr_active;
+ if (PageMlocked(page))
+ ++stat->nr_mlocked;
+ if (PageReclaim(page))
+ ++stat->nr_reclaim;
+ ++ret;
+ }
+ rcu_read_unlock();
+
+ stat->nr_pages = ret;
+
+ return ret;
+}
+
+static int seq_show(struct seq_file *m, void *priv)
+{
+ int ret;
+ struct iter *iter;
+ struct inode *inode;
+ struct inode_stat stat;
+ struct path path;
+
+ if (unlikely(priv == SEQ_START_TOKEN)) {
+ seq_printf(m, "              pages               "
+      "\t              device/path\n"
+      "    lo     ml     di     ac     re  total\n");
+ return 0;
+ }
+
+ iter = priv;
+ inode = iter->inode;
+
+ ret = get_inode_stat(inode, &stat);
+ if (ret < 0)
+ goto out;
+
+ get_fs_root(current->fs, &path);
+ dput(path.dentry);
+
+ /* only the name of the last instantiated link is displayed */
+ path.dentry = hlist_entry(inode->i_dentry.first, struct dentry, d_alias);
+
+ seq_printf(m, "% 6ld % 6ld % 6ld % 6ld % 6ld % 6ld\t(%u:%u)%s\n",
+ stat.nr_locked, stat.nr_mlocked, stat.nr_dirty,
+ stat.nr_active, stat.nr_reclaim, stat.nr_pages,
+ MAJOR(inode->i_sb->s_dev),
+ MINOR(inode->i_sb->s_dev),
+ d_path(&path, iter->buf, BUFSIZE));
+
+ mntput(path.mnt);
+out:
+ return 0;
+}
+
+static void *seq_next(struct seq_file *m, void *priv, loff_t *pos)
+{
+ ++(*pos);
+ return (priv == SEQ_START_TOKEN) ?
+ iter_first(m->private) :
+ iter_next(priv);
+}
+
+static void *seq_start(struct seq_file *m, loff_t *pos)
+{
+ struct iter *iter;
+ loff_t off;
+
+ if (!*pos)
+ return SEQ_START_TOKEN;
+
+ iter = iter_first(m->private);
+
+ for (off = 1; iter && off < *pos; ++off)
+ iter = iter_next(iter);
+
+ return iter;
+}
+
+static void seq_stop(struct seq_file *m, void *priv)
+{
+ struct iter *iter;
+ struct inode *inode;
+ struct super_block *sb;
+
+ if (priv == SEQ_START_TOKEN)
+ return;
+
+ iter = priv;
+ if (!iter)
+ return;
+
+ inode = iter->inode;
+ if (inode) {
+ sb = inode->i_sb;
+ iput(inode);
+ up_read(&sb->s_umount);
+ put_super(sb);
+ }
+}
+
+static const struct seq_operations seq_ops = {
+ .start = seq_start,
+ .next = seq_next,
+ .stop = seq_stop,
+ .show = seq_show
+};
+
+static int page_cache_open(struct inode *inode, struct file *file)
+{
+ int ret;
+ struct iter *iter;
+
+ ret = -ENOMEM;
+ iter = __seq_open_private(file, &seq_ops, sizeof(*iter));
+ if (!iter)
+ goto out;
+
+ ret = iter_init(iter);
+out:
+ return ret;
+}
+
+static int page_cache_release(struct inode *inode, struct file *file)
+{
+ struct seq_file *seq;
+
+ seq = file->private_data;
+ iter_destroy(seq->private);
+ kfree(seq->private);
+ return seq_release(inode, file);
+}
+
+static const struct file_operations page_cache_fops = {
+ .open = page_cache_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = page_cache_release
+};
+
+#ifndef PROCENTRY
+#define PROCENTRY "kpagecache"
+#endif
+
+static int __init page_cache_init(void)
+{
+ int ret;
+
+ ret = -ENOENT;
+
+ if (!proc_create(PROCENTRY, S_IFREG|0400, NULL, &page_cache_fops))
+ goto out;
+
+ ret = 0;
+out:
+ return ret;
+}
+module_init(page_cache_init);
+
+static void __exit page_cache_exit(void)
+{
+ remove_proc_entry(PROCENTRY, NULL);
+}
+module_exit(page_cache_exit);
+
+MODULE_LICENSE("GPL");
+
--
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