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But it seems that's xfs specific and there is a > >> separate mechanism to deal with looking up ->mapping/index for that. So > >> I guess we should still be able to safely store this on the folio > >> head. I will double check and update this change. > >> > > > > I think there is path to store this information only on the folio head. > > However, ugh, I think this is potentially another "head" of the > > pmd_devmap() hydra. > > > > pmd_devmap() taught the core-mm to treat dax_pmds indentically to > > thp_pmds *except* for the __split_huge_pmd() case: > > > > 5c7fb56e5e3f mm, dax: dax-pmd vs thp-pmd vs hugetlbfs-pmd > > > > Later on pmd migration entries joined pmd_devmap() in skipping splits: > > > > 84c3fc4e9c56 mm: thp: check pmd migration entry in common path > > > > Unfortunately, pmd_devmap() stopped being considered for skipping > > splits here: > > > > 7f7609175ff2 mm/huge_memory: remove stale locking logic from __split_huge_pmd() > > > > Likely __split_huge_pmd_locked() grew support for pmd migration handling > > and forgot about the pmd_devmap() case. > > > > So now Linux has been allowing FSDAX pmd splits since v5.18... > > From what I see we currently (in v6.6) have this in > __split_huge_pmd_locked(): > > if (!vma_is_anonymous(vma)) { > old_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd); > /* > * We are going to unmap this huge page. So > * just go ahead and zap it > */ > if (arch_needs_pgtable_deposit()) > zap_deposited_table(mm, pmd); > if (vma_is_special_huge(vma)) > return; > > Where vma_is_special_huge(vma) returns true for vma_is_dax(). So AFAICT > we're still skipping the split right? In all versions we just zap the > PMD and continue. What am I missing? Ah, good point I missed that. One more dragon vanquished.