linux-kernel - Re: [PATCH 04/47] LoongArch: Set _PAGE_DIRTY only if _PAGE_WRITE is set in {pmd,pte}

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Date:   Thu, 17 Nov 2022 10:12:07 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Huacai Chen <chenhuacai@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...nel.org>, loongarch@...ts.linux.dev,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        "David S . Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 04/47] LoongArch: Set _PAGE_DIRTY only if _PAGE_WRITE is
 set in {pmd,pte}_mkdirty()

Hi, Huacai,

On Thu, Nov 17, 2022 at 12:25:32PM +0800, Huacai Chen wrote:
> Now {pmd,pte}_mkdirty() set _PAGE_DIRTY bit unconditionally, this causes
> random segmentation fault after commit 0ccf7f168e17bb7e ("mm/thp: carry
> over dirty bit when thp splits on pmd").
> 
> The reason is: when fork(), parent process use pmd_wrprotect() to clear
> huge page's _PAGE_WRITE and _PAGE_DIRTY (for COW);

Is it safe to drop dirty bit when wr-protect?  It means the mm can reclaim
the page directly assuming the page contains rubbish.

Consider after fork() and memory pressure kicks the kswapd, I don't see
anything stops the kswapd from recycling the pages and lose the data in
both processes.

> then pte_mkdirty() set
> _PAGE_DIRTY as well as _PAGE_MODIFIED while splitting dirty huge pages;
> once _PAGE_DIRTY is set, there will be no tlb modify exception so the COW
> machanism fails; and at last memory corruption occurred between parent
> and child processes.
> 
> So, we should set _PAGE_DIRTY only when _PAGE_WRITE is set in {pmd,pte}_
> mkdirty().
> 
> Cc: stable@...r.kernel.org
> Cc: Peter Xu <peterx@...hat.com>
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
> Note: CC sparc maillist because they have similar issues.

I also had a look on sparc64, it seems to not do the same as loongarch
here (not removing dirty in wr-protect):

static inline pmd_t pmd_wrprotect(pmd_t pmd)
{
	pte_t pte = __pte(pmd_val(pmd));

	pte = pte_wrprotect(pte);

	return __pmd(pte_val(pte));
}

static inline pte_t pte_wrprotect(pte_t pte)
{
	unsigned long val = pte_val(pte), tmp;

	__asm__ __volatile__(
	"\n661:	andn		%0, %3, %0\n"
	"	nop\n"
	"\n662:	nop\n"
	"	nop\n"
	"	.section	.sun4v_2insn_patch, \"ax\"\n"
	"	.word		661b\n"
	"	sethi		%%uhi(%4), %1\n"
	"	sllx		%1, 32, %1\n"
	"	.word		662b\n"
	"	or		%1, %%lo(%4), %1\n"
	"	andn		%0, %1, %0\n"
	"	.previous\n"
	: "=r" (val), "=r" (tmp)
	: "0" (val), "i" (_PAGE_WRITE_4U | _PAGE_W_4U),
	  "i" (_PAGE_WRITE_4V | _PAGE_W_4V));

	return __pte(val);
}

>  
>  arch/loongarch/include/asm/pgtable.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
> index 946704bee599..debbe116f105 100644
> --- a/arch/loongarch/include/asm/pgtable.h
> +++ b/arch/loongarch/include/asm/pgtable.h
> @@ -349,7 +349,9 @@ static inline pte_t pte_mkclean(pte_t pte)
>  
>  static inline pte_t pte_mkdirty(pte_t pte)
>  {
> -	pte_val(pte) |= (_PAGE_DIRTY | _PAGE_MODIFIED);
> +	pte_val(pte) |= _PAGE_MODIFIED;
> +	if (pte_val(pte) & _PAGE_WRITE)
> +		pte_val(pte) |= _PAGE_DIRTY;

I'm not sure whether mm has rule to always set write bit then set dirty
bit, need to be careful here because the outcome may differ when use:

  pte_mkdirty(pte_mkwrite(pte))
  (expected)

VS:

  pte_mkwrite(pte_mkdirty(pte))
  (dirty not set)

I had a feeling I miss some arch-specific details here on why loongarch
needs such implementation, but I can't quickly tell.

Thanks,

>  	return pte;
>  }
>  
> @@ -478,7 +480,9 @@ static inline pmd_t pmd_mkclean(pmd_t pmd)
>  
>  static inline pmd_t pmd_mkdirty(pmd_t pmd)
>  {
> -	pmd_val(pmd) |= (_PAGE_DIRTY | _PAGE_MODIFIED);
> +	pmd_val(pmd) |= _PAGE_MODIFIED;
> +	if (pmd_val(pmd) & _PAGE_WRITE)
> +		pmd_val(pmd) |= _PAGE_DIRTY;
>  	return pmd;
>  }
>  
> -- 
> 2.31.1
> 

-- 
Peter Xu