Re: [PATCH v5 0/8] Support for contiguous pte hugepages

[Punit Agrawal]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Tue, 20 Jun 2017 14:39:57 +0100 Punit Agrawal <punit.agrawal@xxxxxxx> wrote:
>
>> 
>> The architecture supports two flavours of hugepages -
>> 
>> * Block mappings at the pud/pmd level
>> 
>>   These are regular hugepages where a pmd or a pud page table entry
>>   points to a block of memory. Depending on the PAGE_SIZE in use the
>>   following size of block mappings are supported -
>> 
>>           PMD	PUD
>>           ---	---
>>   4K:      2M	 1G
>>   16K:    32M
>>   64K:   512M
>> 
>>   For certain applications/usecases such as HPC and large enterprise
>>   workloads, folks are using 64k page size but the minimum hugepage size
>>   of 512MB isn't very practical.
>> 
>> To overcome this ...
>> 
>> * Using the Contiguous bit
>> 
>>   The architecture provides a contiguous bit in the translation table
>>   entry which acts as a hint to the mmu to indicate that it is one of a
>>   contiguous set of entries that can be cached in a single TLB entry.
>> 
>>   We use the contiguous bit in Linux to increase the mapping size at the
>>   pmd and pte (last) level.
>> 
>>   The number of supported contiguous entries varies by page size and
>>   level of the page table.
>> 
>>   Using the contiguous bit allows additional hugepage sizes -
>> 
>>            CONT PTE    PMD    CONT PMD    PUD
>>            --------    ---    --------    ---
>>     4K:         64K     2M         32M     1G
>>     16K:         2M    32M          1G
>>     64K:         2M   512M         16G
>> 
>>   Of these, 64K with 4K and 2M with 64K pages have been explicitly
>>   requested by a few different users.
>> 
>> Entries with the contiguous bit set are required to be modified all
>> together - which makes things like memory poisoning and migration
>> impossible to do correctly without knowing the size of hugepage being
>> dealt with - the reason for adding size parameter to a few of the
>> hugepage helpers in this series.
>> 
>
> Thanks, I added the above to the 1/n changelog.  Perhaps it's worth
> adding something like this to Documentation/vm/hugetlbpage.txt.

Yes, it would be useful to have this documented.

I'll send a patch once the architecture bits for re-enabling contiguous
hugepages are merged.

Thanks,
Punit