Re: [PATCH v5 10/19] mm: pagewalk: Add p4d_entry() and pgd_entry()

[Steven Price]

On 21/03/2019 21:15, Mike Rapoport wrote:
> On Thu, Mar 21, 2019 at 02:19:44PM +0000, Steven Price wrote:
>> pgd_entry() and pud_entry() were removed by commit 0b1fbfe50006c410
>> ("mm/pagewalk: remove pgd_entry() and pud_entry()") because there were
>> no users. We're about to add users so reintroduce them, along with
>> p4d_entry() as we now have 5 levels of tables.
>>
>> Note that commit a00cc7d9dd93d66a ("mm, x86: add support for
>> PUD-sized transparent hugepages") already re-added pud_entry() but with
>> different semantics to the other callbacks. Since there have never
>> been upstream users of this, revert the semantics back to match the
>> other callbacks. This means pud_entry() is called for all entries, not
>> just transparent huge pages.
>>
>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>> ---
>>  include/linux/mm.h |  9 ++++++---
>>  mm/pagewalk.c      | 27 ++++++++++++++++-----------
>>  2 files changed, 22 insertions(+), 14 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 76769749b5a5..2983f2396a72 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1367,10 +1367,9 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>>
>>  /**
>>   * mm_walk - callbacks for walk_page_range
>> + * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
>> + * @p4d_entry: if set, called for each non-empty P4D (1st-level) entry
> 
> IMHO, p4d implies the 4th level :)

You have a good point there... I was simply working back from the
existing definitions (below) of PTE:4th, PMD:3rd, PUD:2nd. But it's
already somewhat broken by PGD:0th and my cop-out was calling it "top".

> I think it would make more sense to start counting from PTE rather than
> from PGD. Then it would be consistent across architectures with fewer
> levels.

It would also be the opposite way round to architectures such as Arm
which number their levels, for example [1] refers to levels 0-3 (with 3
being PTE in Linux terms).

[1]
https://developer.arm.com/docs/100940/latest/translation-tables-in-armv8-a

Probably the least confusing thing is to drop the level numbers in
brackets since I don't believe they directly match any architecture, and
hopefully any user of the page walking code is already familiar with the
P?D terms used by the kernel.

Steve

>>   * @pud_entry: if set, called for each non-empty PUD (2nd-level) entry
>> - *	       this handler should only handle pud_trans_huge() puds.
>> - *	       the pmd_entry or pte_entry callbacks will be used for
>> - *	       regular PUDs.
>>   * @pmd_entry: if set, called for each non-empty PMD (3rd-level) entry
>>   *	       this handler is required to be able to handle
>>   *	       pmd_trans_huge() pmds.  They may simply choose to
>> @@ -1390,6 +1389,10 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>>   * (see the comment on walk_page_range() for more details)
>>   */
>>  struct mm_walk {
>> +	int (*pgd_entry)(pgd_t *pgd, unsigned long addr,
>> +			 unsigned long next, struct mm_walk *walk);
>> +	int (*p4d_entry)(p4d_t *p4d, unsigned long addr,
>> +			 unsigned long next, struct mm_walk *walk);
>>  	int (*pud_entry)(pud_t *pud, unsigned long addr,
>>  			 unsigned long next, struct mm_walk *walk);
>>  	int (*pmd_entry)(pmd_t *pmd, unsigned long addr,
>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>> index c3084ff2569d..98373a9f88b8 100644
>> --- a/mm/pagewalk.c
>> +++ b/mm/pagewalk.c
>> @@ -90,15 +90,9 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>>  		}
>>
>>  		if (walk->pud_entry) {
>> -			spinlock_t *ptl = pud_trans_huge_lock(pud, walk->vma);
>> -
>> -			if (ptl) {
>> -				err = walk->pud_entry(pud, addr, next, walk);
>> -				spin_unlock(ptl);
>> -				if (err)
>> -					break;
>> -				continue;
>> -			}
>> +			err = walk->pud_entry(pud, addr, next, walk);
>> +			if (err)
>> +				break;
>>  		}
>>
>>  		split_huge_pud(walk->vma, pud, addr);
>> @@ -131,7 +125,12 @@ static int walk_p4d_range(pgd_t *pgd, unsigned long addr, unsigned long end,
>>  				break;
>>  			continue;
>>  		}
>> -		if (walk->pmd_entry || walk->pte_entry)
>> +		if (walk->p4d_entry) {
>> +			err = walk->p4d_entry(p4d, addr, next, walk);
>> +			if (err)
>> +				break;
>> +		}
>> +		if (walk->pud_entry || walk->pmd_entry || walk->pte_entry)
>>  			err = walk_pud_range(p4d, addr, next, walk);
>>  		if (err)
>>  			break;
>> @@ -157,7 +156,13 @@ static int walk_pgd_range(unsigned long addr, unsigned long end,
>>  				break;
>>  			continue;
>>  		}
>> -		if (walk->pmd_entry || walk->pte_entry)
>> +		if (walk->pgd_entry) {
>> +			err = walk->pgd_entry(pgd, addr, next, walk);
>> +			if (err)
>> +				break;
>> +		}
>> +		if (walk->p4d_entry || walk->pud_entry || walk->pmd_entry ||
>> +				walk->pte_entry)
>>  			err = walk_p4d_range(pgd, addr, next, walk);
>>  		if (err)
>>  			break;
>> -- 
>> 2.20.1
>>
>