Joao Martins <joao.m.martins@xxxxxxxxxx> writes:
> On 16/06/2023 12:08, Aneesh Kumar K.V wrote:
>> dax vmemmap optimization requires a minimum of 2 PAGE_SIZE area within
>> vmemmap such that tail page mapping can point to the second PAGE_SIZE area.
>> Enforce that in vmemmap_can_optimize() function.
>>
>> Architectures like powerpc also want to enable vmemmap optimization
>> conditionally (only with radix MMU translation). Hence allow architecture
>> override.
>>
> This makes sense. The enforcing here is not just for correctness but because you
> want to use VMEMMAP_RESERVE_NR supposedly?
>
> I would suggest having two patches one for the refactor and another one for the
> override, but I don't feel particularly strongly about it.
>
I will wait for feedback from others. If we have others also suggesting
for the split patch I will do that.
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
>> ---
>> include/linux/mm.h | 30 ++++++++++++++++++++++++++----
>> mm/mm_init.c | 2 +-
>> 2 files changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 27ce77080c79..9a45e61cd83f 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -31,6 +31,8 @@
>> #include <linux/memremap.h>
>> #include <linux/slab.h>
>>
>> +#include <asm/page.h>
>> +
>
> Why is this include needed?
That was for PAGE_SHIFT. But then we do pull that through other include
dependencies. I will see if i can drop that.
>
>> struct mempolicy;
>> struct anon_vma;
>> struct anon_vma_chain;
>> @@ -3550,13 +3552,33 @@ void vmemmap_free(unsigned long start, unsigned long end,
>> struct vmem_altmap *altmap);
>> #endif
>>
>> +#define VMEMMAP_RESERVE_NR 2
>
> see below
>
>> #ifdef CONFIG_ARCH_WANT_OPTIMIZE_VMEMMAP
>> -static inline bool vmemmap_can_optimize(struct vmem_altmap *altmap,
>> - struct dev_pagemap *pgmap)
>> +static inline bool __vmemmap_can_optimize(struct vmem_altmap *altmap,
>> + struct dev_pagemap *pgmap)
>> {
>> - return is_power_of_2(sizeof(struct page)) &&
>> - pgmap && (pgmap_vmemmap_nr(pgmap) > 1) && !altmap;
>> + if (pgmap) {
>> + unsigned long nr_pages;
>> + unsigned long nr_vmemmap_pages;
>> +
>> + nr_pages = pgmap_vmemmap_nr(pgmap);
>> + nr_vmemmap_pages = ((nr_pages * sizeof(struct page)) >> PAGE_SHIFT);
>> + /*
>> + * For vmemmap optimization with DAX we need minimum 2 vmemmap
>
>
>
>> + * pages. See layout diagram in Documentation/mm/vmemmap_dedup.rst
>> + */
>> + return is_power_of_2(sizeof(struct page)) &&
>> + (nr_vmemmap_pages > VMEMMAP_RESERVE_NR) && !altmap;
>> + }
>
> It would be more readable (i.e. less identation) if you just reverse this:
>
> unsigned long nr_vmemmap_pages;
>
> if (!pgmap || !is_power_of_2(sizeof(struct page))
> return false;
>
> nr_vmemmap_pages = ((pgmap_vmemmap_nr(pgmap) *
> sizeof(struct page)) >> PAGE_SHIFT);
>
> /*
> * For vmemmap optimization with DAX we need minimum 2 vmemmap
> * pages. See layout diagram in Documentation/mm/vmemmap_dedup.rst
> */
> return (nr_vmemmap_pages > VMEMMAP_RESERVE_NR) && !altmap;
>
>
Will update
>> + return false;
>> }
>> +/*
>> + * If we don't have an architecture override, use the generic rule
>> + */
>> +#ifndef vmemmap_can_optimize
>> +#define vmemmap_can_optimize __vmemmap_can_optimize
>> +#endif
>> +
>
> sparse-vmemmap code is trivial to change to use dedup a single vmemmap page
> (e.g. to align with hugetlb), hopefully the architecture override to do. this is
> to say whether VMEMMAP_RESERVE_NR should have similar to above?
VMEMMAP_RESERVE_NR was added to avoid the usage of `2` in the below code.
The reason we need the arch override was to add an additional check on
ppc64 as shown by patch
https://lore.kernel.org/linux-mm/20230616110826.344417-16-aneesh.kumar@xxxxxxxxxxxxx/
bool vmemmap_can_optimize(struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
{
if (radix_enabled())
return __vmemmap_can_optimize(altmap, pgmap);
return false;
}
>
>> #else
>> static inline bool vmemmap_can_optimize(struct vmem_altmap *altmap,
>> struct dev_pagemap *pgmap)
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 7f7f9c677854..d1676afc94f1 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -1020,7 +1020,7 @@ static inline unsigned long compound_nr_pages(struct vmem_altmap *altmap,
>> if (!vmemmap_can_optimize(altmap, pgmap))
>> return pgmap_vmemmap_nr(pgmap);
>>
>> - return 2 * (PAGE_SIZE / sizeof(struct page));
>> + return VMEMMAP_RESERVE_NR * (PAGE_SIZE / sizeof(struct page));
>> }
>>
>> static void __ref memmap_init_compound(struct page *head,
