Re: [PATCH] mm/filemap: Implement fast short reads

[kernel test robot <lkp@intel.com>]

Hi Kiryl,

kernel test robot noticed the following build warnings:

[auto build test WARNING on akpm-mm/mm-everything]

url:    https://github.com/intel-lab-lkp/linux/commits/Kiryl-Shutsemau/mm-filemap-Implement-fast-short-reads/20251017-221655
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20251017141536.577466-1-kirill%40shutemov.name
patch subject: [PATCH] mm/filemap: Implement fast short reads
config: i386-defconfig (https://download.01.org/0day-ci/archive/20251018/202510181215.jcL2gJMQ-lkp@intel.com/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/20251018/202510181215.jcL2gJMQ-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/202510181215.jcL2gJMQ-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> mm/filemap.c:2753:22: warning: stack frame size (1096) exceeds limit (1024) in 'filemap_read_fast' [-Wframe-larger-than]
    2753 | static noinline bool filemap_read_fast(struct kiocb *iocb, struct iov_iter *iter,
         |                      ^
   1 warning generated.


vim +/filemap_read_fast +2753 mm/filemap.c

  2752	
> 2753	static noinline bool filemap_read_fast(struct kiocb *iocb, struct iov_iter *iter,
  2754					       ssize_t *already_read)
  2755	{
  2756		struct address_space *mapping = iocb->ki_filp->f_mapping;
  2757		struct file_ra_state *ra = &iocb->ki_filp->f_ra;
  2758		char buffer[FAST_READ_BUF_SIZE];
  2759		size_t count;
  2760	
  2761		if (ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE)
  2762			return false;
  2763	
  2764		if (iov_iter_count(iter) > sizeof(buffer))
  2765			return false;
  2766	
  2767		count = iov_iter_count(iter);
  2768	
  2769		/* Let's see if we can just do the read under RCU */
  2770		rcu_read_lock();
  2771		count = filemap_read_fast_rcu(mapping, iocb->ki_pos, buffer, count);
  2772		rcu_read_unlock();
  2773	
  2774		if (!count)
  2775			return false;
  2776	
  2777		count = copy_to_iter(buffer, count, iter);
  2778		if (unlikely(!count))
  2779			return false;
  2780	
  2781		iocb->ki_pos += count;
  2782		ra->prev_pos = iocb->ki_pos;
  2783		file_accessed(iocb->ki_filp);
  2784		*already_read += count;
  2785	
  2786		return !iov_iter_count(iter);
  2787	}
  2788	

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