Re: [PATCH -mm v9 4/8] proc: add kpagecgroup file

Andrew Morton
2015-07-21T23:34:33Z

On Sun, 19 Jul 2015 15:31:13 +0300 Vladimir Davydov <vdavydov@parallels.com> wrote:

> /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.

> + 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.

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

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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: email@kvack.org