Re: [PATCH v9 11/21] mm: pagewalk: Add p4d_entry() and pgd_entry()

[Steven Price <steven.price@arm.com>]

On 23/07/2019 11:14, Mark Rutland wrote:
> On Mon, Jul 22, 2019 at 04:42:00PM +0100, 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@arm.com>
>> ---
>>  include/linux/mm.h | 15 +++++++++------
>>  mm/pagewalk.c      | 27 ++++++++++++++++-----------
>>  2 files changed, 25 insertions(+), 17 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 0334ca97c584..b22799129128 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1432,15 +1432,14 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>>  
>>  /**
>>   * mm_walk - callbacks for walk_page_range
>> - * @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
>> + * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
>> + * @p4d_entry: if set, called for each non-empty P4D entry
>> + * @pud_entry: if set, called for each non-empty PUD entry
>> + * @pmd_entry: if set, called for each non-empty PMD entry
> 
> How are these expected to work with folding?
> 
> For example, on arm64 with 64K pages and 42-bit VA, you can have 2-level
> tables where the PGD is P4D, PUD, and PMD. IIUC we'd invoke the
> callbacks for each of those levels where we found an entry in the pgd.
> 
> Either the callee handle that, or we should inhibit the callbacks when
> levels are folded, and I think that needs to be explcitly stated either
> way.
> 
> IIRC on x86 the p4d folding is dynamic depending on whether the HW
> supports 5-level page tables. Maybe that implies the callee has to
> handle that.

Yes, my assumption is that it has to be up to the callee to handle that
because folding can be dynamic. I believe this also was how these
callbacks work before they were removed. However I'll add a comment
explaining that here as it's probably non-obvious.

Steve