Hi Nhat,

I love your patch! Perhaps something to improve:

[auto build test WARNING on shuah-kselftest/next]
[also build test WARNING on shuah-kselftest/fixes tip/x86/asm akpm-mm/mm-everything linus/master v6.1-rc8 next-20221206]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Nhat-Pham/cachestat-a-new-syscall-for-page-cache-state-of-files/20221206-015308
base:   https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git next
patch link:    https://lore.kernel.org/r/20221205175140.1543229-4-nphamcs%40gmail.com
patch subject: [PATCH v2 3/4] cachestat: implement cachestat syscall
config: i386-randconfig-m021-20221205
compiler: gcc-11 (Debian 11.3.0-8) 11.3.0

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>

smatch warnings:
mm/cachestat.c:53 __do_sys_cachestat() warn: unsigned 'len' is never less than zero.
mm/cachestat.c:55 __do_sys_cachestat() warn: unsigned 'cstat_size' is never less than zero.
mm/cachestat.c:53 __do_sys_cachestat() warn: unsigned 'len' is never less than zero.
mm/cachestat.c:55 __do_sys_cachestat() warn: unsigned 'cstat_size' is never less than zero.

vim +/len +53 mm/cachestat.c

    15	
    16	/*
    17	 * The cachestat(4) system call.
    18	 *
    19	 * cachestat() returns the page cache status of a file in the
    20	 * bytes specified by `off` and `len`: number of cached pages,
    21	 * number of dirty pages, number of pages marked for writeback,
    22	 * number of (recently) evicted pages.
    23	 *
    24	 * `off` and `len` must be non-negative integers. If `len` > 0,
    25	 * the queried range is [`off`, `off` + `len`]. If `len` == 0,
    26	 * we will query in the range from `off` to the end of the file.
    27	 *
    28	 * `cstat_size` allows users to obtain partial results. The syscall
    29	 * will copy the first `csstat_size` bytes to the specified userspace
    30	 * memory. It also makes the cachestat struct extensible - new fields
    31	 * can be added in the future without breaking existing usage.
    32	 * `cstat_size` must be a non-negative value that is no larger than
    33	 * the current size of the cachestat struct.
    34	 *
    35	 * Because the status of a page can change after cachestat() checks it
    36	 * but before it returns to the application, the returned values may
    37	 * contain stale information.
    38	 *
    39	 * return values:
    40	 *  zero    - success
    41	 *  -EFAULT - cstat points to an illegal address
    42	 *  -EINVAL - invalid arguments
    43	 *  -EBADF	- invalid file descriptor
    44	 */
    45	SYSCALL_DEFINE5(cachestat, unsigned int, fd, off_t, off, size_t, len,
    46		size_t, cstat_size, struct cachestat __user *, cstat)
    47	{
    48		struct fd f;
    49		struct cachestat cs;
    50	
    51		memset(&cs, 0, sizeof(struct cachestat));
    52	
  > 53		if (off < 0 || len < 0 ||
    54			cstat_size > sizeof(struct cachestat) ||
  > 55			cstat_size < 0)
    56			return -EINVAL;
    57	
    58		if (!access_ok(cstat, sizeof(struct cachestat)))
    59			return -EFAULT;
    60	
    61		f = fdget(fd);
    62		if (f.file) {
    63			struct address_space *mapping = f.file->f_mapping;
    64			pgoff_t first_index = off >> PAGE_SHIFT;
    65			pgoff_t last_index =
    66				len == 0 ? ULONG_MAX : (off + len - 1) >> PAGE_SHIFT;
    67			XA_STATE(xas, &mapping->i_pages, first_index);
    68			struct folio *folio;
    69	
    70			rcu_read_lock();
    71			xas_for_each(&xas, folio, last_index) {
    72				if (xas_retry(&xas, folio) || !folio)
    73					continue;
    74	
    75				if (xa_is_value(folio)) {
    76					/* page is evicted */
    77					void *shadow = (void *)folio;
    78					bool workingset; /* not used */
    79	
    80					cs.nr_evicted += 1;
    81	

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