Re: [GIT PULL] hotfixes for 6.2

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Feb 13, 2023 at 2:08 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> Kuan-Ying Lee (1):
>       mm/gup: add folio to list when folio_isolate_lru() succeed

Ugh. I really hate fixes like this.

The problem came from mis-understanding the return value of
folio_isolate_lru(), and thinking that it was a boolean
success/failure thing.

It wasn't, it was an integer "success/errno" thing, and the sense of
the test was wrong. So the patch is

-       if (!folio_isolate_lru(folio))
+       if (folio_isolate_lru(folio))
                continue;

but at no point was the code *clarified*.

Wouldn't it have been much better to write the new code to be

        if (folio_isolate_lru(folio) < 0)
                continue;

to actually make it clear that this is a "negative error return check".

I've pulled this, but I really think that when somebody notices that
we had a silly bug because of a misunderstanding like this, it's not
just that the bug should be fixed, the code should also be *clarified*
at the same time.

                 Linus