Le 26/03/2024 à 16:01, Jason Gunthorpe a écrit : > On Mon, Mar 25, 2024 at 07:05:01PM +0000, Christophe Leroy wrote: > >> 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. > > There sure are alot of page tables types here > > I'm a bit wondering about terminology, eg on the first diagram "huge > pte entry" means a PUD entry that is a leaf? Which ones are contiguous > replications? Yes, on the first diagram, a huge pte entry covering the same size as pud entry means a leaf PUD entry. Contiguous replications are only on 8xx for the time being and are displayed as "consecutive entries". > > Just general remarks on the ones with huge pages: > > hash 64k and hugepage 16M/16G > radix 64k/radix hugepage 2M/1G > radix 4k/radix hugepage 2M/1G > nohash 32 > - I think this is just a normal x86 like scheme? PMD/PUD can be a > leaf with the same size as a next level table. > > Do any of these cases need to know the higher level to parse the > lower? eg is there a 2M bit in the PUD indicating that the PMD > is a table of 2M leafs or does each PMD entry have a bit > indicating it is a leaf? For hash and radix there is a bit that tells it is leaf (_PAGE_PTE) For nohash32/e500 I think the drawing is not full right, there is a huge page directory (hugepd) with a single entry. I think it should be possible to change it to a leaf entry, it seems we have bit _PAGE_SW1 available in the PTE. > > hash 4k and hugepage 16M/16G > nohash 64 > - How does this work? I guess since 8xx explicitly calls out > consecutive this is actually the pgd can point to 512 256M > entries or 8 16G entries? Ie the table size at each level is > varable? Or is it the same and the table size is still 512 and > each 16G entry is replicated 64 times? For those it is using the huge page directory (hugepd) which can be hooked at any level and is a directory of huge pages on its own. There is no consecutive entries involved here I think, allthough I'm not completely sure. For hash4k I'm not sure how it works, this was changed by commit e2b3d202d1db ("powerpc: Switch 16GB and 16MB explicit hugepages to a different page table format") For the nohash/64, a PGD entry points either to a regular PUD directory or to a HUGEPD directory. The size of the HUGEPD directory is encoded in the 6 lower bits of the PGD entry. > > Do the offset accessors already abstract this enough? > > 8xx 4K > 8xx 16K > - As this series does? This is how it is prior to the series, ie 16k and 512k pages are implemented as contiguous PTEs in a standard page table while 8M pages are implemented with hugepd and a single entry in it (with two PGD entries pointing to the same huge page directory. Christophe
