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Message-ID: <20231019142054.GA60597@dev-arch.thelio-3990X>
Date: Thu, 19 Oct 2023 07:20:54 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Mike Kravetz <mike.kravetz@...cle.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
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 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
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