linux-kernel - Re: [GIT PULL] Ceph updates for 5.20-rc1

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Date:   Sun, 14 Aug 2022 14:20:04 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Nathan Chancellor <nathan@...nel.org>,
        Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        clang-built-linux <llvm@...ts.linux.dev>
Subject: Re: [GIT PULL] Ceph updates for 5.20-rc1

On Sun, Aug 14, 2022 at 1:29 PM Linus Torvalds
<torvalds@...ux-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