Re: [PATCH 1/2] iov_iter: iterate_folioq: fix handling of offset >= folio size

[kernel test robot]

Hi Dominique,

kernel test robot noticed the following build warnings:

[auto build test WARNING on 8f5ae30d69d7543eee0d70083daf4de8fe15d585]

url:    https://github.com/intel-lab-lkp/linux/commits/Dominique-Martinet-via-B4-Relay/iov_iter-iterate_folioq-fix-handling-of-offset-folio-size/20250811-154319
base:   8f5ae30d69d7543eee0d70083daf4de8fe15d585
patch link:    https://lore.kernel.org/r/20250811-iot_iter_folio-v1-1-d9c223adf93c%40codewreck.org
patch subject: [PATCH 1/2] iov_iter: iterate_folioq: fix handling of offset >= folio size
config: i386-buildonly-randconfig-002-20250811 (https://download.01.org/0day-ci/archive/20250812/202508120250.Eooq2ydr-lkp@xxxxxxxxx/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250812/202508120250.Eooq2ydr-lkp@xxxxxxxxx/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@xxxxxxxxx>
| Closes: https://lore.kernel.org/oe-kbuild-all/202508120250.Eooq2ydr-lkp@xxxxxxxxx/

All warnings (new ones prefixed by >>):

   In file included from lib/iov_iter.c:14:
>> include/linux/iov_iter.h:171:7: warning: variable 'remain' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
     171 |                 if (skip >= fsize)
         |                     ^~~~~~~~~~~~~
   include/linux/iov_iter.h:190:7: note: uninitialized use occurs here
     190 |                 if (remain)
         |                     ^~~~~~
   include/linux/iov_iter.h:171:3: note: remove the 'if' if its condition is always false
     171 |                 if (skip >= fsize)
         |                 ^~~~~~~~~~~~~~~~~~
     172 |                         goto next;
         |                         ~~~~~~~~~
   include/linux/iov_iter.h:163:22: note: initialize the variable 'remain' to silence this warning
     163 |                 size_t part, remain, consumed;
         |                                    ^
         |                                     = 0
   1 warning generated.


vim +171 include/linux/iov_iter.h

   143	
   144	/*
   145	 * Handle ITER_FOLIOQ.
   146	 */
   147	static __always_inline
   148	size_t iterate_folioq(struct iov_iter *iter, size_t len, void *priv, void *priv2,
   149			      iov_step_f step)
   150	{
   151		const struct folio_queue *folioq = iter->folioq;
   152		unsigned int slot = iter->folioq_slot;
   153		size_t progress = 0, skip = iter->iov_offset;
   154	
   155		if (slot == folioq_nr_slots(folioq)) {
   156			/* The iterator may have been extended. */
   157			folioq = folioq->next;
   158			slot = 0;
   159		}
   160	
   161		do {
   162			struct folio *folio = folioq_folio(folioq, slot);
   163			size_t part, remain, consumed;
   164			size_t fsize;
   165			void *base;
   166	
   167			if (!folio)
   168				break;
   169	
   170			fsize = folioq_folio_size(folioq, slot);
 > 171			if (skip >= fsize)
   172				goto next;
   173			base = kmap_local_folio(folio, skip);
   174			part = umin(len, PAGE_SIZE - skip % PAGE_SIZE);
   175			remain = step(base, progress, part, priv, priv2);
   176			kunmap_local(base);
   177			consumed = part - remain;
   178			len -= consumed;
   179			progress += consumed;
   180			skip += consumed;
   181			if (skip >= fsize) {
   182	next:
   183				skip = 0;
   184				slot++;
   185				if (slot == folioq_nr_slots(folioq) && folioq->next) {
   186					folioq = folioq->next;
   187					slot = 0;
   188				}
   189			}
   190			if (remain)
   191				break;
   192		} while (len);
   193	
   194		iter->folioq_slot = slot;
   195		iter->folioq = folioq;
   196		iter->iov_offset = skip;
   197		iter->count -= progress;
   198		return progress;
   199	}
   200	

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