Date:   Mon, 10 Sep 2018 02:03:26 +0000
From:   "Yang, Bin" <bin.yang@...el.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "Gross, Mark" <mark.gross@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [patch 01/10] x86/mm/cpa: Split, rename and clean up
 try_preserve_large_page()

On Fri, 2018-09-07 at 17:01 +0200, Thomas Gleixner wrote:
> Avoid the extra variable and gotos by splitting the function into the
> actual algorithm and a callable function which contains the lock
> protection.
> 
> Rename it to should_split_large_page() while at it so the return values make
> actually sense.
> 
> Clean up the code flow, comments and general whitespace damage while at it. No
> functional change.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  arch/x86/mm/pageattr.c |  121 ++++++++++++++++++++++++-------------------------
>  1 file changed, 60 insertions(+), 61 deletions(-)
> 
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -421,18 +421,18 @@ pte_t *lookup_address_in_pgd(pgd_t *pgd,
>   */
>  pte_t *lookup_address(unsigned long address, unsigned int *level)
>  {
> -        return lookup_address_in_pgd(pgd_offset_k(address), address, level);
> +	return lookup_address_in_pgd(pgd_offset_k(address), address, level);
>  }
>  EXPORT_SYMBOL_GPL(lookup_address);
>  
>  static pte_t *_lookup_address_cpa(struct cpa_data *cpa, unsigned long address,
>  				  unsigned int *level)
>  {
> -        if (cpa->pgd)
> +	if (cpa->pgd)
>  		return lookup_address_in_pgd(cpa->pgd + pgd_index(address),
>  					       address, level);
>  
> -        return lookup_address(address, level);
> +	return lookup_address(address, level);
>  }
>  
>  /*
> @@ -549,27 +549,22 @@ static pgprot_t pgprot_clear_protnone_bi
>  	return prot;
>  }
>  
> -static int
> -try_preserve_large_page(pte_t *kpte, unsigned long address,
> -			struct cpa_data *cpa)
> +static int __should_split_large_page(pte_t *kpte, unsigned long address,
> +				     struct cpa_data *cpa)
>  {
> -	unsigned long nextpage_addr, numpages, pmask, psize, addr, pfn, old_pfn;
> -	pte_t new_pte, old_pte, *tmp;
> +	unsigned long numpages, pmask, psize, lpaddr, addr, pfn, old_pfn;
>  	pgprot_t old_prot, new_prot, req_prot;
> -	int i, do_split = 1;
> +	pte_t new_pte, old_pte, *tmp;
>  	enum pg_level level;
> +	int i;
>  
> -	if (cpa->force_split)
> -		return 1;
> -
> -	spin_lock(&pgd_lock);
>  	/*
>  	 * Check for races, another CPU might have split this page
>  	 * up already:
>  	 */
>  	tmp = _lookup_address_cpa(cpa, address, &level);
>  	if (tmp != kpte)
> -		goto out_unlock;
> +		return 1;
>  
>  	switch (level) {
>  	case PG_LEVEL_2M:
> @@ -581,8 +576,7 @@ try_preserve_large_page(pte_t *kpte, uns
>  		old_pfn = pud_pfn(*(pud_t *)kpte);
>  		break;
>  	default:
> -		do_split = -EINVAL;
> -		goto out_unlock;
> +		return -EINVAL;
>  	}
>  
>  	psize = page_level_size(level);
> @@ -592,8 +586,8 @@ try_preserve_large_page(pte_t *kpte, uns
>  	 * Calculate the number of pages, which fit into this large
>  	 * page starting at address:
>  	 */
> -	nextpage_addr = (address + psize) & pmask;
> -	numpages = (nextpage_addr - address) >> PAGE_SHIFT;
> +	lpaddr = (address + psize) & pmask;
> +	numpages = (lpaddr - address) >> PAGE_SHIFT;
>  	if (numpages < cpa->numpages)
>  		cpa->numpages = numpages;
>  
> @@ -620,57 +614,62 @@ try_preserve_large_page(pte_t *kpte, uns
>  		pgprot_val(req_prot) |= _PAGE_PSE;
>  
>  	/*
> -	 * old_pfn points to the large page base pfn. So we need
> -	 * to add the offset of the virtual address:
> +	 * old_pfn points to the large page base pfn. So we need to add the
> +	 * offset of the virtual address:
>  	 */
>  	pfn = old_pfn + ((address & (psize - 1)) >> PAGE_SHIFT);
>  	cpa->pfn = pfn;
>  
> -	new_prot = static_protections(req_prot, address, pfn);
> +	/*
> +	 * Calculate the large page base address and the number of 4K pages
> +	 * in the large page
> +	 */
> +	lpaddr = address & pmask;
> +	numpages = psize >> PAGE_SHIFT;
>  
>  	/*
> -	 * We need to check the full range, whether
> -	 * static_protection() requires a different pgprot for one of
> -	 * the pages in the range we try to preserve:
> +	 * Make sure that the requested pgprot does not violate the static
> +	 * protections. Check the full large page whether one of the pages
> +	 * in it results in a different pgprot than the first one of the
> +	 * requested range. If yes, then the page needs to be split.
>  	 */
> -	addr = address & pmask;
> +	new_prot = static_protections(req_prot, address, pfn, 1);

"npg" is introduced by patch #3. It might be better to keep old API in
this patch.

>  	pfn = old_pfn;
> -	for (i = 0; i < (psize >> PAGE_SHIFT); i++, addr += PAGE_SIZE, pfn++) {
> +	for (i = 0, addr = lpaddr; i < numpages; i++, addr += PAGE_SIZE, pfn++) {
>  		pgprot_t chk_prot = static_protections(req_prot, addr, pfn);
>  
>  		if (pgprot_val(chk_prot) != pgprot_val(new_prot))
> -			goto out_unlock;
> +			return 1;
>  	}
>  
> -	/*
> -	 * If there are no changes, return. maxpages has been updated
> -	 * above:
> -	 */
> -	if (pgprot_val(new_prot) == pgprot_val(old_prot)) {
> -		do_split = 0;
> -		goto out_unlock;
> -	}
> +	/* If there are no changes, return. */
> +	if (pgprot_val(new_prot) == pgprot_val(old_prot))
> +		return 0;
>  
>  	/*
> -	 * We need to change the attributes. Check, whether we can
> -	 * change the large page in one go. We request a split, when
> -	 * the address is not aligned and the number of pages is
> -	 * smaller than the number of pages in the large page. Note
> -	 * that we limited the number of possible pages already to
> -	 * the number of pages in the large page.
> +	 * Verify that the address is aligned and the number of pages
> +	 * covers the full page.
>  	 */
> -	if (address == (address & pmask) && cpa->numpages == (psize >> PAGE_SHIFT)) {
> -		/*
> -		 * The address is aligned and the number of pages
> -		 * covers the full page.
> -		 */
> -		new_pte = pfn_pte(old_pfn, new_prot);
> -		__set_pmd_pte(kpte, address, new_pte);
> -		cpa->flags |= CPA_FLUSHTLB;
> -		do_split = 0;
> -	}
> +	if (address != lpaddr || cpa->numpages != numpages)
> +		return 1;
> +
> +	/* All checks passed. Update the large page mapping. */
> +	new_pte = pfn_pte(old_pfn, new_prot);
> +	__set_pmd_pte(kpte, address, new_pte);
> +	cpa->flags |= CPA_FLUSHTLB;
> +	return 0;
> +}
>  
> -out_unlock:
> +static int should_split_large_page(pte_t *kpte, unsigned long address,
> +				   struct cpa_data *cpa)
> +{
> +	int do_split;
> +
> +	if (cpa->force_split)
> +		return 1;
> +
> +	spin_lock(&pgd_lock);
> +	do_split = __should_split_large_page(kpte, address, cpa);
>  	spin_unlock(&pgd_lock);
>  
>  	return do_split;
> @@ -1273,7 +1272,7 @@ static int __change_page_attr(struct cpa
>  	 * Check, whether we can keep the large page intact
>  	 * and just change the pte:
>  	 */
> -	do_split = try_preserve_large_page(kpte, address, cpa);
> +	do_split = should_split_large_page(kpte, address, cpa);
>  	/*
>  	 * When the range fits into the existing large page,
>  	 * return. cp->numpages and cpa->tlbflush have been updated in
> @@ -1288,23 +1287,23 @@ static int __change_page_attr(struct cpa
>  	err = split_large_page(cpa, kpte, address);
>  	if (!err) {
>  		/*
> -	 	 * Do a global flush tlb after splitting the large page
> -	 	 * and before we do the actual change page attribute in the PTE.
> -	 	 *
> -	 	 * With out this, we violate the TLB application note, that says
> -	 	 * "The TLBs may contain both ordinary and large-page
> +		 * Do a global flush tlb after splitting the large page
> +		 * and before we do the actual change page attribute in the PTE.
> +		 *
> +		 * With out this, we violate the TLB application note, that says
> +		 * "The TLBs may contain both ordinary and large-page
>  		 *  translations for a 4-KByte range of linear addresses. This
>  		 *  may occur if software modifies the paging structures so that
>  		 *  the page size used for the address range changes. If the two
>  		 *  translations differ with respect to page frame or attributes
>  		 *  (e.g., permissions), processor behavior is undefined and may
>  		 *  be implementation-specific."
> -	 	 *
> -	 	 * We do this global tlb flush inside the cpa_lock, so that we
> +		 *
> +		 * We do this global tlb flush inside the cpa_lock, so that we
>  		 * don't allow any other cpu, with stale tlb entries change the
>  		 * page attribute in parallel, that also falls into the
>  		 * just split large page entry.
> -	 	 */
> +		 */
>  		flush_tlb_all();
>  		goto repeat;
>  	}
> 
> 

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