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
