Le 25/03/2024 à 17:19, Jason Gunthorpe a écrit : > On Mon, Mar 25, 2024 at 03:55:54PM +0100, Christophe Leroy wrote: >> Unlike many architectures, powerpc 8xx hardware tablewalk requires >> a two level process for all page sizes, allthough second level only >> has one entry when pagesize is 8M. >> >> To fit with Linux page table topology and without requiring special >> page directory layout like hugepd, the page entry will be replicated >> 1024 times in the standard page table. However for large pages it is >> necessary to set bits in the level-1 (PMD) entry. At the time being, >> for 512k pages the flag is kept in the PTE and inserted in the PMD >> entry at TLB miss exception, that is necessary because we can have >> pages of different sizes in a page table. However the 12 PTE bits are >> fully used and there is no room for an additional bit for page size. >> >> For 8M pages, there will be only one page per PMD entry, it is >> therefore possible to flag the pagesize in the PMD entry, with the >> advantage that the information will already be at the right place for >> the hardware. >> >> To do so, add a new helper called pmd_populate_size() which takes the >> page size as an additional argument, and modify __pte_alloc() to also >> take that argument. pte_alloc() is left unmodified in order to >> reduce churn on callers, and a pte_alloc_size() is added for use by >> pte_alloc_huge(). >> >> When an architecture doesn't provide pmd_populate_size(), >> pmd_populate() is used as a fallback. > > I think it would be a good idea to document what the semantic is > supposed to be for sz? > > Just a general remark, probably nothing for this, but with these new > arguments the historical naming seems pretty tortured for > pte_alloc_size().. Something like pmd_populate_leaf(size) as a naming > scheme would make this more intuitive. Ie pmd_populate_leaf() gives > you a PMD entry where the entry points to a leaf page table able to > store folios of at least size. > > Anyhow, I thought the edits to the mm helpers were fine, certainly > much nicer than hugepd. Do you see a path to remove hugepd entirely > from here? Not looked into details yet, but I guess so. By the way there is a wiki dedicated to huge pages on powerpc, you can have a look at it here : https://github.com/linuxppc/wiki/wiki/Huge-pages , maybe you'll find good ideas there to help me. Christophe
