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;
+}
+