Le 18/03/2024 à 17:15, Jason Gunthorpe a écrit : > On Thu, Mar 14, 2024 at 01:11:59PM +0000, Christophe Leroy wrote: >> >> >> Le 14/03/2024 à 13:53, Peter Xu a écrit : >>> On Thu, Mar 14, 2024 at 08:45:34AM +0000, Christophe Leroy wrote: >>>> >>>> >>>> Le 13/03/2024 à 22:47, peterx@xxxxxxxxxx a écrit : >>>>> From: Peter Xu <peterx@xxxxxxxxxx> >>>>> >>>>> PowerPC book3s 4K mostly has the same definition on both, except pXd_huge() >>>>> constantly returns 0 for hash MMUs. As Michael Ellerman pointed out [1], >>>>> it is safe to check _PAGE_PTE on hash MMUs, as the bit will never be set so >>>>> it will keep returning false. >>>>> >>>>> As a reference, __p[mu]d_mkhuge() will trigger a BUG_ON trying to create >>>>> such huge mappings for 4K hash MMUs. Meanwhile, the major powerpc hugetlb >>>>> pgtable walker __find_linux_pte() already used pXd_leaf() to check hugetlb >>>>> mappings. >>>>> >>>>> The goal should be that we will have one API pXd_leaf() to detect all kinds >>>>> of huge mappings. AFAICT we need to use the pXd_leaf() impl (rather than >>>>> pXd_huge() ones) to make sure ie. THPs on hash MMU will also return true. >>>> >>>> All kinds of huge mappings ? >>>> >>>> pXd_leaf() will detect only leaf mappings (like pXd_huge() ). There are >>>> also huge mappings through hugepd. On powerpc 8xx we have 8M huge pages >>>> and 512k huge pages. A PGD entry covers 4M so pgd_leaf() won't report >>>> those huge pages. >>> >>> Ah yes, I should always mention this is in the context of leaf huge pages >>> only. Are the examples you provided all fall into hugepd category? If so >>> I can reword the commit message, as: >> >> On powerpc 8xx, only the 8M huge pages fall into the hugepd case. >> >> The 512k hugepages are at PTE level, they are handled more or less like >> CONT_PTE on ARM. see function set_huge_pte_at() for more context. >> >> You can also look at pte_leaf_size() and pgd_leaf_size(). > > IMHO leaf should return false if the thing is pointing to a next level > page table, even if that next level is fully populated with contiguous > pages. > > This seems more aligned with the contig page direction that hugepd > should be moved over to.. Should hugepd be moved to the contig page direction, really ? Would it be acceptable that a 8M hugepage requires 2048 contig entries in 2 page tables, when the hugepd allows a single entry ? Would it be acceptable performancewise ? > >> By the way pgd_leaf_size() looks odd because it is called only when >> pgd_leaf_size() returns true, which never happens for 8M pages. > > Like this, you should reach the actual final leaf that the HW will > load and leaf_size() should say it is greater size than the current > table level. Other levels should return 0. > > If necessary the core MM code should deal with this by iterating over > adjacent tables. > > Jason
