linux-kernel - Re: Simplify load_unaligned_zeropad() (was Re: [GIT PULL] Ceph updates for 5.20-rc1)

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Date:   Mon, 15 Aug 2022 09:17:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        clang-built-linux <llvm@...ts.linux.dev>
Subject: Re: Simplify load_unaligned_zeropad() (was Re: [GIT PULL] Ceph
 updates for 5.20-rc1)

On Sun, Aug 14, 2022 at 02:14:08PM -0700, Linus Torvalds wrote:

> PeterZ - you've touched both the load_unaligned_zeropad() and the
> exception code last, so let's run this past you, but this really does
> seem to not only fix the code generation issue in fs/dcache.s, it just
> looks simpler too. Comments?

Ha, freshly back from vacation and I barely know what a computer is :-)

>  arch/x86/include/asm/extable_fixup_types.h |  2 ++
>  arch/x86/include/asm/word-at-a-time.h      | 50 +++---------------------------
>  arch/x86/mm/extable.c                      | 30 ++++++++++++++++++
>  3 files changed, 37 insertions(+), 45 deletions(-)
> 
> diff --git a/arch/x86/include/asm/extable_fixup_types.h b/arch/x86/include/asm/extable_fixup_types.h
> index 503622627400..b53f1919710b 100644
> --- a/arch/x86/include/asm/extable_fixup_types.h
> +++ b/arch/x86/include/asm/extable_fixup_types.h
> @@ -64,4 +64,6 @@
>  #define	EX_TYPE_UCOPY_LEN4		(EX_TYPE_UCOPY_LEN | EX_DATA_IMM(4))
>  #define	EX_TYPE_UCOPY_LEN8		(EX_TYPE_UCOPY_LEN | EX_DATA_IMM(8))

(weird tab stuff there, but that's for another day I suppose)

> +#define EX_TYPE_ZEROPAD			20 /* load ax from dx zero-padded */
> +
>  #endif
> diff --git a/arch/x86/include/asm/word-at-a-time.h b/arch/x86/include/asm/word-at-a-time.h
> index 8338b0432b50..4893f1b30dd6 100644
> --- a/arch/x86/include/asm/word-at-a-time.h
> +++ b/arch/x86/include/asm/word-at-a-time.h
> @@ -77,58 +77,18 @@ static inline unsigned long find_zero(unsigned long mask)
>   * and the next page not being mapped, take the exception and
>   * return zeroes in the non-existing part.
>   */
>  static inline unsigned long load_unaligned_zeropad(const void *addr)
>  {
>  	unsigned long ret;
>  
> +	asm volatile(
> +		"1:	mov (%[addr]), %[ret]\n"
>  		"2:\n"
> +		_ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_ZEROPAD)
> +		: [ret] "=a" (ret)
> +		: [addr] "d" (addr));
>  
>  	return ret;
>  }
>  
>  #endif /* _ASM_WORD_AT_A_TIME_H */
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 331310c29349..58c79077a496 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -41,6 +41,34 @@ static bool ex_handler_default(const struct exception_table_entry *e,
>  	return true;
>  }
>  
> +/*
> + * This is the *very* rare case where we do a "load_unaligned_zeropad()"
> + * and it's a page crosser into a non-existent page.
> + *
> + * This happens when we optimistically load a pathname a word-at-a-time
> + * and the name is less than the full word and the  next page is not
> + * mapped. Typically that only happens for CONFIG_DEBUG_PAGEALLOC.
> + *
> + * NOTE! The load is always of the form "mov (%edx),%eax" to make the
> + * fixup simple.

So obviously we could use _ASM_EXTABLE_TYPE_REG together with something
like: "mov (%[reg]), %[reg]" to not depend on these fixed registers, but
yeah, that doesn't seem needed. Code-gen is fine as is.

> + */
> +static bool ex_handler_zeropad(const struct exception_table_entry *e,
> +			       struct pt_regs *regs,
> +			       unsigned long fault_addr)
> +{
> +	const unsigned long mask = sizeof(long) - 1;
> +	unsigned long offset, addr;
> +
> +	offset = regs->dx & mask;
> +	addr = regs->dx & ~mask;
> +	if (fault_addr != addr + sizeof(long))
> +		return false;
> +
> +	regs->ax = *(unsigned long *)addr >> (offset * 8);


> +	regs->ip = ex_fixup_addr(e);
> +	return true;

I think the convention here is to do:

	return ex_handler_default(e, regs);

instead, that ensures there a bit of common post code.

> +}
> +
>  static bool ex_handler_fault(const struct exception_table_entry *fixup,
>  			     struct pt_regs *regs, int trapnr)
>  {

But yeah, looks good to me.