Re: [RFC PATCH 2/3] mm: Create snapshot_page()

Re: [RFC PATCH 2/3] mm: Create snapshot_page()

[kernel test robot]

Hi Matthew,

[This is a private test report for your RFC patch.]
kernel test robot noticed the following build warnings:

[auto build test WARNING on akpm-mm/mm-everything]
[also build test WARNING on brauner-vfs/vfs.all linus/master v6.14-rc2 next-20250210]
[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/Matthew-Wilcox-Oracle/mm-Constify-folio_mapping-and-swapcache_mapping/20250211-052512
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20250210212142.4002210-3-willy%40infradead.org
patch subject: [RFC PATCH 2/3] mm: Create snapshot_page()
config: i386-buildonly-randconfig-006-20250211 (https://download.01.org/0day-ci/archive/20250211/202502112000.w8too7ue-lkp@intel.com/config)
compiler: clang version 19.1.3 (https://github.com/llvm/llvm-project ab51eccf88f5321e7c60591c5546b254b6afab99)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250211/202502112000.w8too7ue-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202502112000.w8too7ue-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> mm/util.c:1275:6: warning: variable 'idx' is uninitialized when used here [-Wuninitialized]
    1275 |         if (idx < MAX_FOLIO_NR_PAGES || folio_test_hugetlb(folio)) {
         |             ^~~
   mm/util.c:1256:19: note: initialize the variable 'idx' to silence this warning
    1256 |         unsigned long idx, nr_pages = 1;
         |                          ^
         |                           = 0
   1 warning generated.


vim +/idx +1275 mm/util.c

  1242	
  1243	/*
  1244	 * If you have an unstable reference to a page, use this to get a
  1245	 * somewhat-consistent (potentially outdated) snapshot.  The consistency
  1246	 * is limited to the page being contained in the folio.  You need to pass in
  1247	 * a scratch folio and scratch page, probably allocated on the stack.
  1248	 * You get back a pointer to the scratch folio you passed in, marked
  1249	 * as const to remind you not to modify this.
  1250	 */
  1251	const struct folio *snapshot_page(struct folio *foliop, struct page *precise,
  1252			unsigned long *idxp, const struct page *unstable)
  1253	{
  1254		struct folio *folio;
  1255		unsigned long head;
  1256		unsigned long idx, nr_pages = 1;
  1257		int loops = 5;
  1258	
  1259	again:
  1260		memcpy(precise, unstable, sizeof(struct page));
  1261		head = precise->compound_head;
  1262		/* Open-coded !PageTail because page_is_fake_head() doesn't work here */
  1263		if ((head & 1) == 0) {
  1264			folio = (struct folio *)precise;
  1265			*idxp = 0;
  1266			/* Not a tail, not a head, we have a single page */
  1267			if (!folio_test_large(folio))
  1268				goto out;
  1269			folio = (struct folio *)unstable;
  1270		} else {
  1271			folio = (struct folio *)(head - 1);
  1272			*idxp = folio_page_idx(folio, unstable);
  1273		}
  1274	
> 1275		if (idx < MAX_FOLIO_NR_PAGES || folio_test_hugetlb(folio)) {

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