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[PATCH 19/37] x86, mm: simplify get_user_pages() PTE bit handling

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  [PATCH 19/37] x86, mm: simplify get_user_pages() PTE bit handling Dave Hansen <dave@sr71.net> - 2015-11-17 04:40 +0100
    Re: [PATCH 19/37] x86, mm: simplify get_user_pages() PTE bit  handling Thomas Gleixner <tglx@linutronix.de> - 2015-11-27 11:20 +0100
      Re: [PATCH 19/37] x86, mm: simplify get_user_pages() PTE bit handling Dave Hansen <dave@sr71.net> - 2015-11-30 17:30 +0100

#1270796 — [PATCH 19/37] x86, mm: simplify get_user_pages() PTE bit handling

From: Dave Hansen <dave.hansen@linux.intel.com>

The current get_user_pages() code is a wee bit more complicated
than it needs to be for pte bit checking.  Currently, it establishes
a mask of required pte _PAGE_* bits and ensures that the pte it
goes after has all those bits.

This consolidates the three identical copies of this code.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---

 b/arch/x86/mm/gup.c |   46 +++++++++++++++++++++++++++++-----------------
 1 file changed, 29 insertions(+), 17 deletions(-)

diff -puN arch/x86/mm/gup.c~pkeys-16-gup-swizzle arch/x86/mm/gup.c
--- a/arch/x86/mm/gup.c~pkeys-16-gup-swizzle	2015-11-16 12:35:43.690550937 -0800
+++ b/arch/x86/mm/gup.c	2015-11-16 12:35:43.693551073 -0800
@@ -63,6 +63,31 @@ retry:
 #endif
 }
 
+static inline int pte_allows_gup(pte_t pte, int write)
+{
+	/*
+	 * 'entry_flags' can come from a pte, pmd or pud.  Make
+	 * sure to only check flags here that are valid *and* the
+	 * same value on all 3 types.  (PAT is currently the only
+	 * one where that is true and is not checked here).
+	 */
+	if (!(pte_flags(pte) & (_PAGE_PRESENT|_PAGE_USER)))
+		return 0;
+	if (write && !(pte_write(pte)))
+		return 0;
+	return 1;
+}
+
+static inline int pmd_allows_gup(pmd_t pmd, int write)
+{
+	return pte_allows_gup(*(pte_t *)&pmd, write);
+}
+
+static inline int pud_allows_gup(pud_t pud, int write)
+{
+	return pte_allows_gup(*(pte_t *)&pud, write);
+}
+
 /*
  * The performance critical leaf functions are made noinline otherwise gcc
  * inlines everything into a single function which results in too much
@@ -71,13 +96,8 @@ retry:
 static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
 		unsigned long end, int write, struct page **pages, int *nr)
 {
-	unsigned long mask;
 	pte_t *ptep;
 
-	mask = _PAGE_PRESENT|_PAGE_USER;
-	if (write)
-		mask |= _PAGE_RW;
-
 	ptep = pte_offset_map(&pmd, addr);
 	do {
 		pte_t pte = gup_get_pte(ptep);
@@ -88,8 +108,8 @@ static noinline int gup_pte_range(pmd_t
 			pte_unmap(ptep);
 			return 0;
 		}
-
-		if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
+		if (!pte_allows_gup(pte, write) ||
+		    pte_special(pte)) {
 			pte_unmap(ptep);
 			return 0;
 		}
@@ -117,14 +137,10 @@ static inline void get_head_page_multipl
 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
 		unsigned long end, int write, struct page **pages, int *nr)
 {
-	unsigned long mask;
 	struct page *head, *page;
 	int refs;
 
-	mask = _PAGE_PRESENT|_PAGE_USER;
-	if (write)
-		mask |= _PAGE_RW;
-	if ((pmd_flags(pmd) & mask) != mask)
+	if (!pmd_allows_gup(pmd, write))
 		return 0;
 	/* hugepages are never "special" */
 	VM_BUG_ON(pmd_flags(pmd) & _PAGE_SPECIAL);
@@ -193,14 +209,10 @@ static int gup_pmd_range(pud_t pud, unsi
 static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
 		unsigned long end, int write, struct page **pages, int *nr)
 {
-	unsigned long mask;
 	struct page *head, *page;
 	int refs;
 
-	mask = _PAGE_PRESENT|_PAGE_USER;
-	if (write)
-		mask |= _PAGE_RW;
-	if ((pud_flags(pud) & mask) != mask)
+	if (!pud_allows_gup(pud, write))
 		return 0;
 	/* hugepages are never "special" */
 	VM_BUG_ON(pud_flags(pud) & _PAGE_SPECIAL);
_
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#1278703 — Re: [PATCH 19/37] x86, mm: simplify get_user_pages() PTE bit handling

On Mon, 16 Nov 2015, Dave Hansen wrote:
> +static inline int pte_allows_gup(pte_t pte, int write)
> +{
> +	/*
> +	 * 'entry_flags' can come from a pte, pmd or pud.  Make
> +	 * sure to only check flags here that are valid *and* the
> +	 * same value on all 3 types.  (PAT is currently the only
> +	 * one where that is true and is not checked here).

I have a hard time to understand that comment.

       /*
        * 'entry_flags' can come from a pte, pmd or pud. We only check
	* _PAGE_PRESENT, _PAGE_USER and _PAGE_RW here, which are the
	* same for all types.
        */

Is that what you wanted to say?

> +	 */
> +	if (!(pte_flags(pte) & (_PAGE_PRESENT|_PAGE_USER)))
> +		return 0;
> +	if (write && !(pte_write(pte)))
> +		return 0;
> +	return 1;
> +}
> +
> +static inline int pmd_allows_gup(pmd_t pmd, int write)
> +{
> +	return pte_allows_gup(*(pte_t *)&pmd, write);
> +}
> +
> +static inline int pud_allows_gup(pud_t pud, int write)
> +{
> +	return pte_allows_gup(*(pte_t *)&pud, write);
> +}
> +

>  static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
>  		unsigned long end, int write, struct page **pages, int *nr)
>  {
> -	unsigned long mask;
>  	struct page *head, *page;
>  	int refs;
>  
> -	mask = _PAGE_PRESENT|_PAGE_USER;
> -	if (write)
> -		mask |= _PAGE_RW;
> -	if ((pmd_flags(pmd) & mask) != mask)
> +	if (!pmd_allows_gup(pmd, write))

Why do you need that extra indirection here of rereading the pmd?
You have the pmd already.

Thanks,

	tglx
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#1280070

On 11/27/2015 02:12 AM, Thomas Gleixner wrote:
> On Mon, 16 Nov 2015, Dave Hansen wrote:
>> +static inline int pte_allows_gup(pte_t pte, int write)
>> +{
>> +	/*
>> +	 * 'entry_flags' can come from a pte, pmd or pud.  Make
>> +	 * sure to only check flags here that are valid *and* the
>> +	 * same value on all 3 types.  (PAT is currently the only
>> +	 * one where that is true and is not checked here).
> 
> I have a hard time to understand that comment.
> 
>        /*
>         * 'entry_flags' can come from a pte, pmd or pud. We only check
> 	* _PAGE_PRESENT, _PAGE_USER and _PAGE_RW here, which are the
> 	* same for all types.
>         */
> 
> Is that what you wanted to say?

Yeah, that's a much better way to say it.  I'll fix it up.

>> +	 */
>> +	if (!(pte_flags(pte) & (_PAGE_PRESENT|_PAGE_USER)))
>> +		return 0;
>> +	if (write && !(pte_write(pte)))
>> +		return 0;
>> +	return 1;
>> +}
>> +
>> +static inline int pmd_allows_gup(pmd_t pmd, int write)
>> +{
>> +	return pte_allows_gup(*(pte_t *)&pmd, write);
>> +}
>> +
>> +static inline int pud_allows_gup(pud_t pud, int write)
>> +{
>> +	return pte_allows_gup(*(pte_t *)&pud, write);
>> +}
>> +
> 
>>  static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
>>  		unsigned long end, int write, struct page **pages, int *nr)
>>  {
>> -	unsigned long mask;
>>  	struct page *head, *page;
>>  	int refs;
>>  
>> -	mask = _PAGE_PRESENT|_PAGE_USER;
>> -	if (write)
>> -		mask |= _PAGE_RW;
>> -	if ((pmd_flags(pmd) & mask) != mask)
>> +	if (!pmd_allows_gup(pmd, write))
> 
> Why do you need that extra indirection here of rereading the pmd?
> You have the pmd already.

The intention there was not to re-read the PMD, but only to cast the
structure over to a pte_t.  I expected the compiler to be able to figure
out what was going on and not actually re-read anything.

Is that a bad assumption?

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