On 22/09/2023 03:54, Qi Zheng wrote:
Hi Ryan,
On 2023/9/22 00:20, Ryan Roberts wrote:
When called with a swap entry that does not embed a PFN (e.g. PTE_MARKER_POISONED or PTE_MARKER_UFFD_WP), the previous implementation of set_huge_pte_at() would either cause a BUG() to fire (if CONFIG_DEBUG_VM is enabled) or cause a dereference of an invalid address and subsequent panic.
arm64's huge pte implementation supports multiple huge page sizes, some of which are implemented in the page table with contiguous mappings. So set_huge_pte_at() needs to work out how big the logical pte is, so that it can also work out how many physical ptes (or pmds) need to be written. It does this by grabbing the folio out of the pte and querying its size.
However, there are cases when the pte being set is actually a swap entry. But this also used to work fine, because for huge ptes, we only ever saw migration entries and hwpoison entries. And both of these types of swap entries have a PFN embedded, so the code would grab that and everything still worked out.
But over time, more calls to set_huge_pte_at() have been added that set swap entry types that do not embed a PFN. And this causes the code to go bang. The triggering case is for the uffd poison test, commit 99aa77215ad0 ("selftests/mm: add uffd unit test for UFFDIO_POISON"), which sets a PTE_MARKER_POISONED swap entry. But review shows there are other places too (PTE_MARKER_UFFD_WP).
So the root cause is due to commit 18f3962953e4 ("mm: hugetlb: kill set_huge_swap_pte_at()"), which aimed to simplify the interface to the core code by removing set_huge_swap_pte_at() (which took a page size parameter) and replacing it with calls to set_huge_swap_pte_at() where the size was inferred from the folio, as descibed above. While that commit didn't break anything at the time,
If it didn't break anything at that time, then shouldn't the Fixes tag be added to this commit?
it did break the interface because it couldn't handle swap entries without PFNs. And since then new callers have come along which rely on this working.
So the Fixes tag should be added only to the commit that introduces the first new callers?
Well I guess it's a matter of point of view; My view is that 18f3962953e4 is the buggy change because it broke the interface to not be able to handle swap entries which do not contain PFNs. The fact that there were no callers that used the interface in this way at the time of the commit is irrelevant in my view. But I already added 2 fixes tags; one for the buggy commit, and the other for the commit containing the new user of the interface.
Other than that, LGTM.
Thanks!
Thanks, Qi
Now that we have modified the set_huge_pte_at() interface to pass the vma, we can extract the huge page size from it and fix this issue.
I'm tagging the commit that added the uffd poison feature, since that is what exposed the problem, as well as the original change that broke the interface. Hopefully this is valuable for people doing bisect.
Signed-off-by: Ryan Roberts ryan.roberts@arm.com Fixes: 18f3962953e4 ("mm: hugetlb: kill set_huge_swap_pte_at()") Fixes: 8a13897fb0da ("mm: userfaultfd: support UFFDIO_POISON for hugetlbfs")
arch/arm64/mm/hugetlbpage.c | 17 +++-------------- 1 file changed, 3 insertions(+), 14 deletions(-)
diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c index 844832511c1e..a08601a14689 100644 --- a/arch/arm64/mm/hugetlbpage.c +++ b/arch/arm64/mm/hugetlbpage.c @@ -241,13 +241,6 @@ static void clear_flush(struct mm_struct *mm, flush_tlb_range(&vma, saddr, addr); } -static inline struct folio *hugetlb_swap_entry_to_folio(swp_entry_t entry) -{ - VM_BUG_ON(!is_migration_entry(entry) && !is_hwpoison_entry(entry));
- return page_folio(pfn_to_page(swp_offset_pfn(entry))); -}
void set_huge_pte_at(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep, pte_t pte) { @@ -258,13 +251,10 @@ void set_huge_pte_at(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn, dpfn; pgprot_t hugeprot; - if (!pte_present(pte)) { - struct folio *folio;
- folio = hugetlb_swap_entry_to_folio(pte_to_swp_entry(pte)); - ncontig = num_contig_ptes(folio_size(folio), &pgsize); + ncontig = num_contig_ptes(huge_page_size(hstate_vma(vma)), &pgsize); - for (i = 0; i < ncontig; i++, ptep++) + if (!pte_present(pte)) { + for (i = 0; i < ncontig; i++, ptep++, addr += pgsize) set_pte_at(mm, addr, ptep, pte); return; } @@ -274,7 +264,6 @@ void set_huge_pte_at(struct vm_area_struct *vma, unsigned long addr, return; } - ncontig = find_num_contig(mm, addr, ptep, &pgsize); pfn = pte_pfn(pte); dpfn = pgsize >> PAGE_SHIFT; hugeprot = pte_pgprot(pte);