linux-kernel - Re: [PATCH 30/44] fs: use min() or umin() instead of min

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Message-ID: <aWVs2gVB418WiMVa@redhat.com>
Date: Mon, 12 Jan 2026 16:51:22 -0500
From: Brian Masney <bmasney@...hat.com>
To: david.laight.linux@...il.com
Cc: linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Christian Brauner <brauner@...nel.org>, Kees Cook <kees@...nel.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH 30/44] fs: use min() or umin() instead of min_t()

Hi David,

On Wed, Nov 19, 2025 at 10:41:26PM +0000, david.laight.linux@...il.com wrote:
> From: David Laight <david.laight.linux@...il.com>
> 
> min_t(unsigned int, a, b) casts an 'unsigned long' to 'unsigned int'.
> Use min(a, b) instead as it promotes any 'unsigned int' to 'unsigned long'
> and so cannot discard significant bits.
> 
> A couple of places need umin() because of loops like:
> 	nfolios = DIV_ROUND_UP(ret + start, PAGE_SIZE);
> 
> 	for (i = 0; i < nfolios; i++) {
> 		struct folio *folio = page_folio(pages[i]);
> 		...
> 		unsigned int len = umin(ret, PAGE_SIZE - start);
> 		...
> 		ret -= len;
> 		...
> 	}
> where the compiler doesn't track things well enough to know that
> 'ret' is never negative.
> 
> The alternate loop:
>         for (i = 0; ret > 0; i++) {
>                 struct folio *folio = page_folio(pages[i]);
>                 ...
>                 unsigned int len = min(ret, PAGE_SIZE - start);
>                 ...
>                 ret -= len;
>                 ...
>         }
> would be equivalent and doesn't need 'nfolios'.
> 
> Most of the 'unsigned long' actually come from PAGE_SIZE.
> 
> Detected by an extra check added to min_t().
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>

When doing a mips cross compile from an arm64 host
(via ARCH=mips CROSS_COMPILE=mips64-linux-gnu- make), the following
build error occurs in linux-next and goes away when I revert this
commit.

In file included from <command-line>:                                                                                               
In function ‘fuse_wr_pages’,                                                                                                        
    inlined from ‘fuse_perform_write’ at fs/fuse/file.c:1347:27:                                                                    
././include/linux/compiler_types.h:667:45: error: call to ‘__compiletime_assert_405’ declared with attribute error: min(((pos + len 
- 1) >> 12) - (pos >> 12) + 1, max_pages) signedness error                                                                          
  667 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)                                             
      |                                             ^                                                                               
././include/linux/compiler_types.h:648:25: note: in definition of macro ‘__compiletime_assert’                                      
  648 |                         prefix ## suffix();                             \                                                   
      |                         ^~~~~~                                                                                              
././include/linux/compiler_types.h:667:9: note: in expansion of macro ‘_compiletime_assert’                                         
  667 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)                                             
      |         ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro ‘compiletime_assert’
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
./include/linux/minmax.h:93:9: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
   93 |         BUILD_BUG_ON_MSG(!__types_ok(ux, uy),           \
      |         ^~~~~~~~~~~~~~~~
./include/linux/minmax.h:98:9: note: in expansion of macro ‘__careful_cmp_once’
   98 |         __careful_cmp_once(op, x, y, __UNIQUE_ID(x_), __UNIQUE_ID(y_))
      |         ^~~~~~~~~~~~~~~~~~
./include/linux/minmax.h:105:25: note: in expansion of macro ‘__careful_cmp’
  105 | #define min(x, y)       __careful_cmp(min, x, y)
      |                         ^~~~~~~~~~~~~
fs/fuse/file.c:1326:16: note: in expansion of macro ‘min’
 1326 |         return min(((pos + len - 1) >> PAGE_SHIFT) - (pos >> PAGE_SHIFT) + 1,
      |                ^~~

This is on a cento-stream-10 host running
gcc version 14.3.1 20250617 (Red Hat 14.3.1-2) (GCC). I didn't look into
this in detail, and I'm not entirely sure what the correct fix here
should be.

Brian