Re: [PATCH 2/6] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS

[Nitin Gupta]

On Mon, Oct 16, 2017 at 7:44 AM, Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
> On Fri, Oct 13, 2017 at 05:00:12PM -0700, Nitin Gupta wrote:
>> On Fri, Sep 29, 2017 at 7:08 AM, Kirill A. Shutemov
>> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
>> > With boot-time switching between paging mode we will have variable
>> > MAX_PHYSMEM_BITS.
>> >
>> > Let's use the maximum variable possible for CONFIG_X86_5LEVEL=y
>> > configuration to define zsmalloc data structures.
>> >
>> > The patch introduces MAX_POSSIBLE_PHYSMEM_BITS to cover such case.
>> > It also suits well to handle PAE special case.
>> >
>> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
>> > Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
>> > Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
>> > ---
>> >  arch/x86/include/asm/pgtable-3level_types.h |  1 +
>> >  arch/x86/include/asm/pgtable_64_types.h     |  2 ++
>> >  mm/zsmalloc.c                               | 13 +++++++------
>> >  3 files changed, 10 insertions(+), 6 deletions(-)
>> >
>> > diff --git a/arch/x86/include/asm/pgtable-3level_types.h b/arch/x86/include/asm/pgtable-3level_types.h
>> > index b8a4341faafa..3fe1d107a875 100644
>> > --- a/arch/x86/include/asm/pgtable-3level_types.h
>> > +++ b/arch/x86/include/asm/pgtable-3level_types.h
>> > @@ -43,5 +43,6 @@ typedef union {
>> >   */
>> >  #define PTRS_PER_PTE   512
>> >
>> > +#define MAX_POSSIBLE_PHYSMEM_BITS      36
>> >
>> >  #endif /* _ASM_X86_PGTABLE_3LEVEL_DEFS_H */
>> > diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
>> > index 06470da156ba..39075df30b8a 100644
>> > --- a/arch/x86/include/asm/pgtable_64_types.h
>> > +++ b/arch/x86/include/asm/pgtable_64_types.h
>> > @@ -39,6 +39,8 @@ typedef struct { pteval_t pte; } pte_t;
>> >  #define P4D_SIZE       (_AC(1, UL) << P4D_SHIFT)
>> >  #define P4D_MASK       (~(P4D_SIZE - 1))
>> >
>> > +#define MAX_POSSIBLE_PHYSMEM_BITS      52
>> > +
>> >  #else /* CONFIG_X86_5LEVEL */
>> >
>> >  /*
>> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>> > index 7c38e850a8fc..7bde01c55c90 100644
>> > --- a/mm/zsmalloc.c
>> > +++ b/mm/zsmalloc.c
>> > @@ -82,18 +82,19 @@
>> >   * This is made more complicated by various memory models and PAE.
>> >   */
>> >
>> > -#ifndef MAX_PHYSMEM_BITS
>> > -#ifdef CONFIG_HIGHMEM64G
>> > -#define MAX_PHYSMEM_BITS 36
>> > -#else /* !CONFIG_HIGHMEM64G */
>> > +#ifndef MAX_POSSIBLE_PHYSMEM_BITS
>> > +#ifdef MAX_PHYSMEM_BITS
>> > +#define MAX_POSSIBLE_PHYSMEM_BITS MAX_PHYSMEM_BITS
>> > +#else
>> >  /*
>> >   * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
>> >   * be PAGE_SHIFT
>> >   */
>> > -#define MAX_PHYSMEM_BITS BITS_PER_LONG
>> > +#define MAX_POSSIBLE_PHYSMEM_BITS BITS_PER_LONG
>> >  #endif
>> >  #endif
>> > -#define _PFN_BITS              (MAX_PHYSMEM_BITS - PAGE_SHIFT)
>> > +
>> > +#define _PFN_BITS              (MAX_POSSIBLE_PHYSMEM_BITS - PAGE_SHIFT)
>> >
>>
>>
>> I think we can avoid using this new constant in zsmalloc.
>>
>> The reason for trying to save on MAX_PHYSMEM_BITS is just to gain more
>> bits for OBJ_INDEX_BITS which would reduce ZS_MIN_ALLOC_SIZE. However,
>> for all practical values of ZS_MAX_PAGES_PER_ZSPAGE, this min size
>> would remain 32 bytes.
>>
>> So, we can unconditionally use MAX_PHYSMEM_BITS = BITS_PER_LONG and
>> thus OBJ_INDEX_BITS = PAGE_SHIFT.
>
> As you understand the topic better than me, could you prepare the patch?
>


Actually no changes are necessary.

As long as physical address bits <= BITS_PER_LONG, then setting
_PFN_BITS to the most conservative value of BITS_PER_LONG is
fine. AFAIK, this condition does not hold on x86 PAE where PA
bits (36) > BITS_PER_LONG (32), so only that case need special
handling to make sure PFN bits are not lost when encoding
allocated object location in an unsigned long.

Thanks,
Nitin