linux-kernel - Re: [PATCH] mm: hugetlb: Only prep and add allocated folios for non-gigantic pages

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Date:   Thu, 19 Oct 2023 13:33:05 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Usama Arif <usama.arif@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        muchun.song@...ux.dev, songmuchun@...edance.com,
        fam.zheng@...edance.com, liangma@...ngbit.com,
        punit.agrawal@...edance.com,
        Konrad Dybcio <konrad.dybcio@...aro.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH] mm: hugetlb: Only prep and add allocated folios for
 non-gigantic pages

On (23/10/18 15:20), Mike Kravetz wrote:
> > I think you need to initialize h, otherwise what value is passed to
> > prep_and_add_bootmem_folios if the loop is not run because the list is
> > empty.  The compiler sees `h` is only given a value in the loop, so
> > the loop must be run.  That's obviously hazardous, but the compiler
> > assumes there's no UB. At least that's my limited understanding
> > looking at the IR diff Nathan got me in
> > https://github.com/ClangBuiltLinux/linux/issues/1946.
> 
> Thanks for looking closer at this Nick and Nathan!
> 
> I think you are saying the compiler is running the loop because it wants
> to initialize h before passing the value to another function.  It does
> this even if the explicit loop entry condition is false.  Is that correct?

The loop is getting promoted to "infinite" loop, there is no
&pos->member != (head) condition check in the generated code
at all (at least on my machine).

I wish we could at least get the "possibly uninitialized variable"
warning from the compiler in this case, which we'd translate to
"hold my beer, I'm going to try one thing".