From: Nathan Chancellor <nathan@kernel.org>
To: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Usama Arif <usama.arif@bytedance.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
akpm@linux-foundation.org, muchun.song@linux.dev,
songmuchun@bytedance.com, fam.zheng@bytedance.com,
liangma@liangbit.com, punit.agrawal@bytedance.com,
Konrad Dybcio <konrad.dybcio@linaro.org>,
llvm@lists.linux.dev
Subject: Re: [PATCH] mm: hugetlb: Only prep and add allocated folios for non-gigantic pages
Date: Thu, 19 Oct 2023 07:20:54 -0700 [thread overview]
Message-ID: <20231019142054.GA60597@dev-arch.thelio-3990X> (raw)
In-Reply-To: <20231019043305.GB14965@google.com>
On Thu, Oct 19, 2023 at 01:33:05PM +0900, Sergey Senozhatsky wrote:
> 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".
GCC would warn about this under -Wmaybe-uninitialized but it has been
disabled in a normal build for the past three years, see commit
78a5255ffb6a ("Stop the ad-hoc games with -Wno-maybe-initialized").
In function 'gather_bootmem_prealloc',
inlined from 'hugetlb_init' at mm/hugetlb.c:4299:2:
mm/hugetlb.c:3203:9: warning: 'h' may be used uninitialized [-Wmaybe-uninitialized]
3203 | prep_and_add_allocated_folios(h, &folio_list);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/hugetlb.c: In function 'hugetlb_init':
mm/hugetlb.c:3166:24: note: 'h' was declared here
3166 | struct hstate *h, *prev_h = NULL;
| ^
Clang's -Wconditional-uninitialized would have flagged it too but it
suffers from the same problems as -Wmaybe-uninitialized.
mm/hugetlb.c:3203:32: warning: variable 'h' may be uninitialized when used here [-Wconditional-uninitialized]
3203 | prep_and_add_allocated_folios(h, &folio_list);
| ^
mm/hugetlb.c:3166:18: note: initialize the variable 'h' to silence this warning
3166 | struct hstate *h, *prev_h = NULL;
| ^
| = NULL
I know clang has some handling for loops in -Wsometimes-uninitialized, I
wonder why that does not trigger here...
Cheers,
Nathan
next prev parent reply other threads:[~2023-10-19 14:21 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-09 14:56 [PATCH] mm: hugetlb: Only prep and add allocated folios for non-gigantic pages Usama Arif
2023-10-10 1:23 ` Mike Kravetz
2023-10-10 17:01 ` [External] " Usama Arif
2023-10-10 21:30 ` Mike Kravetz
2023-10-10 21:31 ` Konrad Dybcio
2023-10-12 0:03 ` Nathan Chancellor
2023-10-12 14:53 ` Mike Kravetz
2023-10-13 0:12 ` Mike Kravetz
2023-10-14 0:04 ` Mike Kravetz
2023-10-18 20:54 ` Nick Desaulniers
2023-10-18 22:20 ` Mike Kravetz
2023-10-19 4:33 ` Sergey Senozhatsky
2023-10-19 14:20 ` Nathan Chancellor [this message]
2023-10-19 2:38 ` Mike Kravetz
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