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

From: Andrew Morton
Date: Tue Jun 20 2017 - 17:08:42 EST

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 -
> --- ---
> 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 -
> -------- --- -------- ---
> 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.