Re: [RFC V2 1/2] arm64/mm: Change THP helpers per generic memory semantics

[Anshuman Khandual]

On 07/02/2020 05:41 PM, Catalin Marinas wrote:
> Hi Anshuman,

Hi Catalin,

> 
> On Mon, Jun 15, 2020 at 06:45:17PM +0530, Anshuman Khandual wrote:
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -353,15 +353,92 @@ static inline int pmd_protnone(pmd_t pmd)
>>  }
>>  #endif
>>  
>> +#define pmd_table(pmd)	((pmd_val(pmd) & PMD_TYPE_MASK) ==  PMD_TYPE_TABLE)
>> +#define pmd_sect(pmd)	((pmd_val(pmd) & PMD_TYPE_MASK) ==  PMD_TYPE_SECT)
>> +
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>  /*
>> - * THP definitions.
>> + * PMD Level Encoding (THP Enabled)
>> + *
>> + * 0b00 - Not valid	Not present	NA
>> + * 0b10 - Not valid	Present		Huge  (Splitting)
>> + * 0b01 - Valid		Present		Huge  (Mapped)
>> + * 0b11 - Valid		Present		Table (Mapped)
>>   */
> 
> I wonder whether it would be easier to read if we add a dedicated
> PMD_SPLITTING bit, only when bit 0 is cleared. This bit can be high (say
> 59), it doesn't really matter as the entry is not valid.

Could make (PMD[0b00] = 0b10) be represented as PMD_SPLITTING just for
better reading purpose. But if possible, IMHO it is efficient and less
vulnerable to use HW defined PTE attribute bit positions including SW
usable ones than the reserved bits, for a PMD state representation.

Earlier proposal used PTE_SPECIAL (bit 56) instead. Using PMD_TABLE_BIT
helps save bit 56 for later. Thinking about it again, would not these
unused higher bits [59..63] create any problem ? For example while
enabling THP swapping without split via ARCH_WANTS_THP_SWAP or something
else later when these higher bits might be required. I am not sure, just
speculating.

But, do you see any particular problem with PMD_TABLE_BIT ?

> 
> The only doubt I have is that pmd_mkinvalid() is used in other contexts
> when it's not necessarily splitting a pmd (search for the
> pmdp_invalidate() calls). So maybe a better name like PMD_PRESENT with a
> comment that pmd_to_page() is valid (i.e. no migration or swap entry).
> Feel free to suggest a better name.

PMD_INVALID_PRESENT sounds better ?

> 
>> +static inline pmd_t pmd_mksplitting(pmd_t pmd)
>> +{
>> +	unsigned long val = pmd_val(pmd);
>>  
>> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> -#define pmd_trans_huge(pmd)	(pmd_val(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
>> +	return __pmd((val & ~PMD_TYPE_MASK) | PMD_TABLE_BIT);
>> +}
>> +
>> +static inline pmd_t pmd_clrsplitting(pmd_t pmd)
>> +{
>> +	unsigned long val = pmd_val(pmd);
>> +
>> +	return __pmd((val & ~PMD_TYPE_MASK) | PMD_TYPE_SECT);
>> +}
>> +
>> +static inline bool pmd_splitting(pmd_t pmd)
>> +{
>> +	unsigned long val = pmd_val(pmd);
>> +
>> +	if ((val & PMD_TYPE_MASK) == PMD_TABLE_BIT)
>> +		return true;
>> +	return false;
>> +}
>> +
>> +static inline bool pmd_mapped(pmd_t pmd)
>> +{
>> +	return pmd_sect(pmd);
>> +}
>> +
>> +static inline pmd_t pmd_mkinvalid(pmd_t pmd)
>> +{
>> +	/*
>> +	 * Invalidation should not have been invoked on
>> +	 * a PMD table entry. Just warn here otherwise.
>> +	 */
>> +	WARN_ON(pmd_table(pmd));
>> +	return pmd_mksplitting(pmd);
>> +}
> 
> And here we wouldn't need t worry about table checks.> 
This is just a temporary sanity check validating the assumption
that a table entry would never be called with pmdp_invalidate().
This can be dropped later on if required.

>> +static inline int pmd_present(pmd_t pmd);
>> +
>> +static inline int pmd_trans_huge(pmd_t pmd)
>> +{
>> +	if (!pmd_present(pmd))
>> +		return 0;
>> +
>> +	if (!pmd_val(pmd))
>> +		return 0;
>> +
>> +	if (pmd_mapped(pmd))
>> +		return 1;
>> +
>> +	if (pmd_splitting(pmd))
>> +		return 1;
>> +	return 0;
> 
> Doesn't your new pmd_present() already check for splitting? I think

I actually meant pte_present() here instead, my bad.

> checking for bit 0 and the new PMD_PRESENT. That would be similar to
> what we do with PTE_PROT_NONE. Actually, you could use the same bit for
> both.

IIUC PROT NONE is supported at PMD level as well. Hence with valid bit
cleared, there is a chance for misinterpretation between pmd_protnone()
and pmd_splitting() if the same bit (PTE_PROT_NONE) is used.

> 
>> +}
>> +
>> +void set_pmd_at(struct mm_struct *mm, unsigned long addr,
>> +		pmd_t *pmdp, pmd_t pmd);
>>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>>  
>> -#define pmd_present(pmd)	pte_present(pmd_pte(pmd))
>> +static inline int pmd_present(pmd_t pmd)
>> +{
>> +	pte_t pte = pmd_pte(pmd);
>> +
>> +	if (pte_present(pte))
>> +		return 1;
>> +
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +	if (pmd_splitting(pmd))
>> +		return 1;
>> +#endif
>> +	return 0;
>> +}
> 
> [...]
> 
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 990929c8837e..337519031115 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -22,6 +22,8 @@
>>  #include <linux/io.h>
>>  #include <linux/mm.h>
>>  #include <linux/vmalloc.h>
>> +#include <linux/swap.h>
>> +#include <linux/swapops.h>
>>  
>>  #include <asm/barrier.h>
>>  #include <asm/cputype.h>
>> @@ -1483,3 +1485,21 @@ static int __init prevent_bootmem_remove_init(void)
>>  }
>>  device_initcall(prevent_bootmem_remove_init);
>>  #endif
>> +
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +void set_pmd_at(struct mm_struct *mm, unsigned long addr,
>> +		pmd_t *pmdp, pmd_t pmd)
>> +{
>> +	/*
>> +	 * PMD migration entries need to retain splitting PMD
>> +	 * representation created with pmdp_invalidate(). But
>> +	 * any non-migration entry which just might have been
>> +	 * invalidated previously, still need be a normal huge
>> +	 * page. Hence selectively clear splitting entries.
>> +	 */
>> +	if (!is_migration_entry(pmd_to_swp_entry(pmd)))
>> +		pmd = pmd_clrsplitting(pmd);
>> +
>> +	set_pte_at(mm, addr, (pte_t *)pmdp, pmd_pte(pmd));
>> +}
>> +#endif
> 
> So a pmdp_invalidate() returns the old pmd. Do we ever need to rebuild a
> pmd based on the actual bits in the new invalidated pmdp? Wondering how
> the table bit ends up here that we need to pmd_clrsplitting().

Yes, a pmd is always rebuilt via set_pmd_at() with the old value as
returned from an earlier pmdp_invalidate() but which may have been
changed with standard page table entry transformations. Basically,
it will not be created afresh from the pfn and VMA flags.

Some example here:

1. dax_entry_mkclean (fs/dax.c)

	pmd = pmdp_invalidate(vma, address, pmdp);
	pmd = pmd_wrprotect(pmd);
	pmd = pmd_mkclean(pmd);
	set_pmd_at(vma->vm_mm, address, pmdp, pmd);

2. clear_soft_dirty_pmd (fs/proc/task_mmu.c)

	old = pmdp_invalidate(vma, addr, pmdp);
	if (pmd_dirty(old))
		pmd = pmd_mkdirty(pmd);
	if (pmd_young(old))
		pmd = pmd_mkyoung(pmd);
	pmd = pmd_wrprotect(pmd);
	pmd = pmd_clear_soft_dirty(pmd);
	set_pmd_at(vma->vm_mm, addr, pmdp, pmd);

3. madvise_free_huge_pmd (mm/huge_memory.c)

	orig_pmd = *pmd;
	....
	pmdp_invalidate(vma, addr, pmd);
	orig_pmd = pmd_mkold(orig_pmd);
	orig_pmd = pmd_mkclean(orig_pmd);
        set_pmd_at(mm, addr, pmd, orig_pmd);

4. page_mkclean_one (mm/rmap.c)

	entry = pmdp_invalidate(vma, address, pmd);
	entry = pmd_wrprotect(entry);
	entry = pmd_mkclean(entry);
	set_pmd_at(vma->vm_mm, address, pmd, entry);

Any additional bit set in PMD via pmdp_invalidate() needs to be
cleared off in set_pmd_at(), unless it is a migration entry.