On Fri, May 28, 2021 at 11:48:40AM +1000, Alistair Popple wrote:
[...]
while (page_vma_mapped_walk(&pvmw)) {/* Unexpected PMD-mapped THP? */VM_BUG_ON_PAGE(!pvmw.pte, page);if (!pte_present(*pvmw.pte)) {ret = false;page_vma_mapped_walk_done(&pvmw);break;}subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);I see that all pages passed in should be done after FOLL_SPLIT_PMD, so is this needed? Or say, should subpage==page always be true?
Not always, in the case of a thp there are small ptes which will get device exclusive entries.
FOLL_SPLIT_PMD will first split the huge thp into smaller pages, then do follow_page_pte() on them (in follow_pmd_mask):
if (flags & FOLL_SPLIT_PMD) { int ret; page = pmd_page(*pmd); if (is_huge_zero_page(page)) { spin_unlock(ptl); ret = 0; split_huge_pmd(vma, pmd, address); if (pmd_trans_unstable(pmd)) ret = -EBUSY; } else { spin_unlock(ptl); split_huge_pmd(vma, pmd, address); ret = pte_alloc(mm, pmd) ? -ENOMEM : 0; } return ret ? ERR_PTR(ret) : follow_page_pte(vma, address, pmd, flags,&ctx->pgmap); }
So I thought all pages are small pages?
The page will remain as a transparent huge page though (at least as I understand things). FOLL_SPLIT_PMD turns it into a pte mapped thp by splitting the pmd and creating pte's mapping the subpages but doesn't split the page itself. For comparison FOLL_SPLIT (which has been removed in v5.13 due to lack of use) is what would be used to split the page in the above GUP code by calling split_huge_page() rather than split_huge_pmd().
But shouldn't FOLL_SPLIT_PMD filled in small pfns for each pte? See __split_huge_pmd_locked():
for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) { ... } else { entry = mk_pte(page + i, READ_ONCE(vma->vm_page_prot)); ... } ... set_pte_at(mm, addr, pte, entry); }
Then iiuc the coming follow_page_pte() will directly fetch the small pages?