Re: [PATCH] [PATCH v2] mm: initialize 'seq' in gup_fast to remove -Wmaybe-uninitialized warning

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Mon,  2 Mar 2026 22:34:05 +0300 Alexey Suchkov <aleks.koyf@gmail.com> wrote:

> The local variable 'seq' in gup_fast (mm/gup.c) was declared
> without initialization, which can trigger:
> 
> mm/gup.c:3165:20: warning: ‘seq’ may be used uninitialized [-Wmaybe-uninitialized]
> 
> Initialize 'seq' to 0. This does not change behavior, since
> read_seqcount_retry() always writes to it before use.

Hi Alexey, I hope you're doing well!

Unfortunately I think that this patch is still unneeded. The variable seq is
used in two places, raw_seqcount_try_begin and read_seqcount_retry, and nowhere
else in the function. Both are guarded by if (gup_flags & FOLL_PIN).

raw_seqcount_try_begin is a macro that sets the value of seq.
read_seqcount_retry is a macro that indeed reads the value of seq, but at
that point seq has already been initialized by raw_seqcount_try_begin.

So I am having a hard time seeing where it would ever be used in an
uninitialized context. The compiler should be smart enough to expand the
macro, see that the writer and consumer are under the same if conditions,
and realize that it is always initialized before being used.

I'm curious to know if the compilation warning above is theoretical, or
if you've actually it it during compilation.

Let me know what you think. Have a great day!
Joshua

> Changes since v1:
> - Removes the accidental formatting from v1.
> 
> Signed-off-by: Alexey Suchkov <dywoq.contact@gmail.com>
> ---
>  mm/gup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 8e7dc2c6e..879798de5 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3131,7 +3131,7 @@ static unsigned long gup_fast(unsigned long start, unsigned long end,
>  {
>  	unsigned long flags;
>  	int nr_pinned = 0;
> -	unsigned seq;
> +	unsigned int seq = 0;
>  
>  	if (!IS_ENABLED(CONFIG_HAVE_GUP_FAST) ||
>  	    !gup_fast_permitted(start, end))
> -- 
> 2.53.0