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.
