On 11/10/22 15:56, Peter Xu wrote: > Hi, Mike, > > Sorry to be late, took me quite some time working on another bug.. > > On Mon, Nov 07, 2022 at 05:19:09PM -0800, Mike Kravetz wrote: > > madvise(MADV_DONTNEED) ends up calling zap_page_range() to clear page > > tables associated with the address range. For hugetlb vmas, > > zap_page_range will call __unmap_hugepage_range_final. However, > > __unmap_hugepage_range_final assumes the passed vma is about to be removed > > and deletes the vma_lock to prevent pmd sharing as the vma is on the way > > out. In the case of madvise(MADV_DONTNEED) the vma remains, but the > > missing vma_lock prevents pmd sharing and could potentially lead to issues > > with truncation/fault races. > > > > This issue was originally reported here [1] as a BUG triggered in > > page_try_dup_anon_rmap. Prior to the introduction of the hugetlb > > vma_lock, __unmap_hugepage_range_final cleared the VM_MAYSHARE flag to > > prevent pmd sharing. Subsequent faults on this vma were confused as > > VM_MAYSHARE indicates a sharable vma, but was not set so page_mapping was > > not set in new pages added to the page table. This resulted in pages that > > appeared anonymous in a VM_SHARED vma and triggered the BUG. > > > > Address issue by: > > - Add a new zap flag ZAP_FLAG_UNMAP to indicate an unmap call from > > unmap_vmas(). This is used to indicate the 'final' unmapping of a vma. > > When called via MADV_DONTNEED, this flag is not set and the vm_lock is > > not deleted. > > - mmu notification is removed from __unmap_hugepage_range to avoid > > duplication, and notification is added to the other calling routine > > (unmap_hugepage_range). > > - notification range in zap_page_range_single is updated to take into > > account the possibility of hugetlb pmd sharing. > > - zap_page_range_single renamed to __zap_page_range_single to be used > > internally within mm/memory.c > > - zap_vma_range() interface created to zap a range within a single vma. > > - madvise_dontneed_single_vma is updated to call zap_vma_range instead of > > zap_page_range as it only operates on a range within a single vma > > > > [1] https://lore.kernel.org/lkml/CAO4mrfdLMXsao9RF4fUE8-Wfde8xmjsKrTNMNC9wjUb6JudD0g@xxxxxxxxxxxxxx/ > > Fixes: 90e7e7f5ef3f ("mm: enable MADV_DONTNEED for hugetlb mappings") > > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> > > Reported-by: Wei Chen <harperchen1110@xxxxxxxxx> > > Cc: <stable@xxxxxxxxxxxxxxx> > > --- > > include/linux/mm.h | 5 +++++ > > mm/hugetlb.c | 45 +++++++++++++++++++++++++++------------------ > > mm/madvise.c | 4 ++-- > > mm/memory.c | 17 +++++++++++++---- > > 4 files changed, 47 insertions(+), 24 deletions(-) > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 978c17df053e..d205bcd9cd2e 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -1840,6 +1840,8 @@ void zap_vma_ptes(struct vm_area_struct *vma, unsigned long address, > > unsigned long size); > > void zap_page_range(struct vm_area_struct *vma, unsigned long address, > > unsigned long size); > > +void zap_vma_range(struct vm_area_struct *vma, unsigned long address, > > + unsigned long size); > > void unmap_vmas(struct mmu_gather *tlb, struct maple_tree *mt, > > struct vm_area_struct *start_vma, unsigned long start, > > unsigned long end); > > @@ -3464,4 +3466,7 @@ madvise_set_anon_name(struct mm_struct *mm, unsigned long start, > > */ > > #define ZAP_FLAG_DROP_MARKER ((__force zap_flags_t) BIT(0)) > > > > +/* Set in unmap_vmas() to indicate an unmap call. Only used by hugetlb */ > > +#define ZAP_FLAG_UNMAP ((__force zap_flags_t) BIT(1)) > > It seems this is not set anywhere in the patch? Correct. Should be set in unmap_vmas. > > + > > #endif /* _LINUX_MM_H */ > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > > index ceb47c4e183a..7c8fbce4441e 100644 > > --- a/mm/hugetlb.c > > +++ b/mm/hugetlb.c > > @@ -5072,7 +5072,6 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct > > struct page *page; > > struct hstate *h = hstate_vma(vma); > > unsigned long sz = huge_page_size(h); > > - struct mmu_notifier_range range; > > unsigned long last_addr_mask; > > bool force_flush = false; > > > > @@ -5087,13 +5086,6 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct > > tlb_change_page_size(tlb, sz); > > tlb_start_vma(tlb, vma); > > > > - /* > > - * If sharing possible, alert mmu notifiers of worst case. > > - */ > > - mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, mm, start, > > - end); > > - adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end); > > - mmu_notifier_invalidate_range_start(&range); > > last_addr_mask = hugetlb_mask_last_page(h); > > address = start; > > for (; address < end; address += sz) { > > @@ -5178,7 +5170,6 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct > > if (ref_page) > > break; > > } > > - mmu_notifier_invalidate_range_end(&range); > > tlb_end_vma(tlb, vma); > > > > /* > > @@ -5203,32 +5194,50 @@ void __unmap_hugepage_range_final(struct mmu_gather *tlb, > > unsigned long end, struct page *ref_page, > > zap_flags_t zap_flags) > > { > > + bool final = zap_flags & ZAP_FLAG_UNMAP; > > + > > hugetlb_vma_lock_write(vma); > > i_mmap_lock_write(vma->vm_file->f_mapping); > > > > __unmap_hugepage_range(tlb, vma, start, end, ref_page, zap_flags); > > > > /* > > - * Unlock and free the vma lock before releasing i_mmap_rwsem. When > > - * the vma_lock is freed, this makes the vma ineligible for pmd > > - * sharing. And, i_mmap_rwsem is required to set up pmd sharing. > > - * This is important as page tables for this unmapped range will > > - * be asynchrously deleted. If the page tables are shared, there > > - * will be issues when accessed by someone else. > > + * When called via zap_vma_range (MADV_DONTNEED), this is not the > > + * final unmap of the vma, and we do not want to delete the vma_lock. > > */ > > - __hugetlb_vma_unlock_write_free(vma); > > - > > - i_mmap_unlock_write(vma->vm_file->f_mapping); > > + if (final) { > > + /* > > + * Unlock and free the vma lock before releasing i_mmap_rwsem. > > + * When the vma_lock is freed, this makes the vma ineligible > > + * for pmd sharing. And, i_mmap_rwsem is required to set up > > + * pmd sharing. This is important as page tables for this > > + * unmapped range will be asynchrously deleted. If the page > > + * tables are shared, there will be issues when accessed by > > + * someone else. > > + */ > > + __hugetlb_vma_unlock_write_free(vma); > > + i_mmap_unlock_write(vma->vm_file->f_mapping); > > + } else { > > + i_mmap_unlock_write(vma->vm_file->f_mapping); > > + hugetlb_vma_unlock_write(vma); > > + } > > } > > > > void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start, > > unsigned long end, struct page *ref_page, > > zap_flags_t zap_flags) > > { > > + struct mmu_notifier_range range; > > struct mmu_gather tlb; > > > > + mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm, > > Should this be s/UNMAP/CLEAR/? As IIUC the unmap path was only happening > in __unmap_hugepage_range_final(). Right, unmap_hugepage_range is employed in the truncate and hole punch path where we should be using the CLEAR notifier. > > + start, end); > > + adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end); > > tlb_gather_mmu(&tlb, vma->vm_mm); > > + > > __unmap_hugepage_range(&tlb, vma, start, end, ref_page, zap_flags); > > + > > + mmu_notifier_invalidate_range_end(&range); > > tlb_finish_mmu(&tlb); > > } > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > index c7105ec6d08c..9d2625b8029a 100644 > > --- a/mm/madvise.c > > +++ b/mm/madvise.c > > @@ -772,7 +772,7 @@ static int madvise_free_single_vma(struct vm_area_struct *vma, > > * Application no longer needs these pages. If the pages are dirty, > > * it's OK to just throw them away. The app will be more careful about > > * data it wants to keep. Be sure to free swap resources too. The > > - * zap_page_range call sets things up for shrink_active_list to actually free > > + * zap_vma_range call sets things up for shrink_active_list to actually free > > * these pages later if no one else has touched them in the meantime, > > * although we could add these pages to a global reuse list for > > * shrink_active_list to pick up before reclaiming other pages. > > @@ -790,7 +790,7 @@ static int madvise_free_single_vma(struct vm_area_struct *vma, > > static long madvise_dontneed_single_vma(struct vm_area_struct *vma, > > unsigned long start, unsigned long end) > > { > > - zap_page_range(vma, start, end - start); > > + zap_vma_range(vma, start, end - start); > > I'd rather just call zap_page_range_single() directly with NULL passed > over, considering that this is for stable, but no strong opinions. As mentioned in reply to Nadav, if we expose zap_page_range_single we need to expose 'struct zap_details'. Any concerns there? That was the primary reason I did not just call zap_page_range_single directly. -- Mike Kravetz
