On Tue, Jul 21, 2015 at 04:34:33PM -0700, Andrew Morton wrote:

On Sun, 19 Jul 2015 15:31:13 +0300 Vladimir Davydov [off-list ref] wrote:

quoted

/proc/kpagecgroup contains a 64-bit inode number of the memory cgroup
each page is charged to, indexed by PFN. Having this information is
useful for estimating a cgroup working set size.

The file is present if CONFIG_PROC_PAGE_MONITOR && CONFIG_MEMCG.

...

@@ -225,10 +226,62 @@ static const struct file_operations proc_kpageflags_operations = {
 	.read = kpageflags_read,
 };
 
+#ifdef CONFIG_MEMCG
+static ssize_t kpagecgroup_read(struct file *file, char __user *buf,
+				size_t count, loff_t *ppos)
+{
+	u64 __user *out = (u64 __user *)buf;
+	struct page *ppage;
+	unsigned long src = *ppos;
+	unsigned long pfn;
+	ssize_t ret = 0;
+	u64 ino;
+
+	pfn = src / KPMSIZE;
+	count = min_t(unsigned long, count, (max_pfn * KPMSIZE) - src);
+	if (src & KPMMASK || count & KPMMASK)
+		return -EINVAL;

The user-facing documentation should explain that reads must be
performed in multiple-of-8 sizes.

It does. It's in the end of Documentation/vm/pagemap.c:

: Other notes:
: 
: Reading from any of the files will return -EINVAL if you are not starting
: the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
: into the file), or if the size of the read is not a multiple of 8 bytes.

quoted

+	while (count > 0) {
+		if (pfn_valid(pfn))
+			ppage = pfn_to_page(pfn);
+		else
+			ppage = NULL;
+
+		if (ppage)
+			ino = page_cgroup_ino(ppage);
+		else
+			ino = 0;
+
+		if (put_user(ino, out)) {
+			ret = -EFAULT;

Here we do the usual procfs violation of read() behaviour.  read()
normally only returns an error if it read nothing.  This code will
transfer a megabyte then return -EFAULT so userspace doesn't know that
it got that megabyte.

Yeah, that's how it works. I did it preliminary for /proc/kpagecgroup to
work exactly like /proc/kpageflags and /proc/kpagecount.

FWIW, the man page I have on my system already warns about this
peculiarity of read(2):

: On error, -1 is returned, and errno is set appropriately. In this
: case, it is left unspecified whether the file position (if any)
: changes.

That's easy to fix, but procfs files do this all over the place anyway :(

quoted

+			break;
+		}
+
+		pfn++;
+		out++;
+		count -= KPMSIZE;
+	}
+
+	*ppos += (char __user *)out - buf;
+	if (!ret)
+		ret = (char __user *)out - buf;
+	return ret;
+}
+