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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Sun, 14 Aug 2022 14:20:04 -0700
Message-ID: <CAKwvOdnL=icZ7j=-i-OY2CaFxgF9eJsNQo=7ARDXUeAraNzhaA@mail.gmail.com>
Subject: Re: [GIT PULL] Ceph updates for 5.20-rc1
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Nathan Chancellor <nathan@kernel.org>,
        Jeff Layton <jlayton@kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>, ceph-devel@vger.kernel.org,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
        clang-built-linux <llvm@lists.linux.dev>
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On Sun, Aug 14, 2022 at 1:29 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I dug into it some more, and it is really "load_unaligned_zeropad()"
> that makes clang really uncomfortable.
>
> The problem ends up being that clang sees that it's inside that inner
> loop, and tries very hard to optimize the shift-and-mask that happens
> if the exception happens.
>
> The fact that the exception *never* happens unless DEBUG_PAGEALLOC is
> enabled - and very very seldom even then - is not something we can
> really explain to clang.

Probably if we could express that the do_exception label in
load_unaligned_zeropad was cold then that might help.
https://github.com/llvm/llvm-project/issues/46831

Otherwise, could we put the exceptional case statements in a noinline
or cold attributed function?
-- 
Thanks,
~Nick Desaulniers