On 03/29/2016 01:35 AM, Ingo Molnar wrote:
>
> * Mike Kravetz <[email protected]> wrote:
>
>> When creating a hugetlb mapping, attempt PUD_SIZE alignment if the
>> following conditions are met:
>> - Address passed to mmap or shmat is NULL
>> - The mapping is flaged as shared
>> - The mapping is at least PUD_SIZE in length
>> If a PUD_SIZE aligned mapping can not be created, then fall back to a
>> huge page size mapping.
>>
>> Signed-off-by: Mike Kravetz <[email protected]>
>> ---
>> arch/x86/mm/hugetlbpage.c | 64
>> ++++++++++++++++++++++++++++++++++++++++++++---
>> 1 file changed, 61 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
>> index 42982b2..4f53af5 100644
>> --- a/arch/x86/mm/hugetlbpage.c
>> +++ b/arch/x86/mm/hugetlbpage.c
>> @@ -78,14 +78,39 @@ static unsigned long
>> hugetlb_get_unmapped_area_bottomup(struct file *file,
>> {
>> struct hstate *h = hstate_file(file);
>> struct vm_unmapped_area_info info;
>> + bool pud_size_align = false;
>> + unsigned long ret_addr;
>> +
>> + /*
>> + * If PMD sharing is enabled, align to PUD_SIZE to facilitate
>> + * sharing. Only attempt alignment if no address was passed in,
>> + * flags indicate sharing and size is big enough.
>> + */
>> + if (IS_ENABLED(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) &&
>> + !addr && flags & MAP_SHARED && len >= PUD_SIZE)
>> + pud_size_align = true;
>>
>> info.flags = 0;
>> info.length = len;
>> info.low_limit = current->mm->mmap_legacy_base;
>> info.high_limit = TASK_SIZE;
>> - info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>> + if (pud_size_align)
>> + info.align_mask = PAGE_MASK & (PUD_SIZE - 1);
>> + else
>> + info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>> info.align_offset = 0;
>> - return vm_unmapped_area(&info);
>> + ret_addr = vm_unmapped_area(&info);
>> +
>> + /*
>> + * If failed with PUD_SIZE alignment, try again with huge page
>> + * size alignment.
>> + */
>> + if ((ret_addr & ~PAGE_MASK) && pud_size_align) {
>> + info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>> + ret_addr = vm_unmapped_area(&info);
>> + }
>
> So AFAICS 'ret_addr' is either page aligned, or is an error code. Wouldn't it
> be a
> lot easier to read to say:
>
> if ((long)ret_addr > 0 && pud_size_align) {
> info.align_mask = PAGE_MASK & ~huge_page_mask(h);
> ret_addr = vm_unmapped_area(&info);
> }
>
> return ret_addr;
>
> to make it clear that it's about error handling, not some alignment
> requirement/restriction?
Yes, I agree that is easier to read. However, it assumes that process
virtual addresses can never evaluate to a negative long value. This may
be the case for x86_64 today. But, there are other architectures where
this is not the case. I know this is x86 specific code, but might it be
possible that x86 virtual addresses could be negative longs in the future?
It appears that all callers of vm_unmapped_area() are using the page aligned
check to determine error. I would prefer to do the same, and can add
comments to make that more clear.
Thanks,
--
Mike Kravetz
>
>> /*
>> + * If failed with PUD_SIZE alignment, try again with huge page
>> + * size alignment.
>> + */
>> + if ((addr & ~PAGE_MASK) && pud_size_align) {
>> + info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>> + addr = vm_unmapped_area(&info);
>> + }
>
> Ditto.
>
>> addr = vm_unmapped_area(&info);
>> +
>> + /*
>> + * If failed again with PUD_SIZE alignment, finally try with
>> + * huge page size alignment.
>> + */
>> + if (addr & ~PAGE_MASK) {
>> + info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>> + addr = vm_unmapped_area(&info);
>
> Ditto.
>
> Thanks,
>
> Ingo
>
