[PATCH v2 18/46] hugetlb: add HGM support to __unmap_hugepage_range

[James Houghton]

Enlighten __unmap_hugepage_range to deal with high-granularity mappings.
This doesn't change its API; it still must be called with hugepage
alignment, but it will correctly unmap hugepages that have been mapped
at high granularity.

Eventually, functionality here can be expanded to allow users to call
MADV_DONTNEED on PAGE_SIZE-aligned sections of a hugepage, but that is
not done here.

Introduce hugetlb_remove_rmap to properly decrement mapcount for
high-granularity-mapped HugeTLB pages.

Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index b46617207c93..31267471760e 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -598,9 +598,9 @@ static inline void tlb_flush_p4d_range(struct mmu_gather *tlb,
 		__tlb_remove_tlb_entry(tlb, ptep, address);	\
 	} while (0)
 
-#define tlb_remove_huge_tlb_entry(h, tlb, ptep, address)	\
+#define tlb_remove_huge_tlb_entry(tlb, hpte, address)	\
 	do {							\
-		unsigned long _sz = huge_page_size(h);		\
+		unsigned long _sz = hugetlb_pte_size(&hpte);	\
 		if (_sz >= P4D_SIZE)				\
 			tlb_flush_p4d_range(tlb, address, _sz);	\
 		else if (_sz >= PUD_SIZE)			\
@@ -609,7 +609,7 @@ static inline void tlb_flush_p4d_range(struct mmu_gather *tlb,
 			tlb_flush_pmd_range(tlb, address, _sz);	\
 		else						\
 			tlb_flush_pte_range(tlb, address, _sz);	\
-		__tlb_remove_tlb_entry(tlb, ptep, address);	\
+		__tlb_remove_tlb_entry(tlb, hpte.ptep, address);\
 	} while (0)
 
 /**
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index b767b6889dea..1a1a71868dfd 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -160,6 +160,9 @@ struct hugepage_subpool *hugepage_new_subpool(struct hstate *h, long max_hpages,
 						long min_hpages);
 void hugepage_put_subpool(struct hugepage_subpool *spool);
 
+void hugetlb_remove_rmap(struct page *subpage, unsigned long shift,
+			 struct hstate *h, struct vm_area_struct *vma);
+
 void hugetlb_dup_vma_private(struct vm_area_struct *vma);
 void clear_vma_resv_huge_pages(struct vm_area_struct *vma);
 int hugetlb_sysctl_handler(struct ctl_table *, int, void *, size_t *, loff_t *);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ed1d806020de..ecf1a28dbaaa 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -120,6 +120,28 @@ enum hugetlb_level hpage_size_to_level(unsigned long sz)
 	return HUGETLB_LEVEL_PGD;
 }
 
+void hugetlb_remove_rmap(struct page *subpage, unsigned long shift,
+			 struct hstate *h, struct vm_area_struct *vma)
+{
+	struct page *hpage = compound_head(subpage);
+
+	if (shift == huge_page_shift(h)) {
+		VM_BUG_ON_PAGE(subpage != hpage, subpage);
+		page_remove_rmap(hpage, vma, true);
+	} else {
+		unsigned long nr_subpages = 1UL << (shift - PAGE_SHIFT);
+		struct page *final_page = &subpage[nr_subpages];
+
+		VM_BUG_ON_PAGE(HPageVmemmapOptimized(hpage), hpage);
+		/*
+		 * Decrement the mapcount on each page that is getting
+		 * unmapped.
+		 */
+		for (; subpage < final_page; ++subpage)
+			page_remove_rmap(subpage, vma, false);
+	}
+}
+
 static inline bool subpool_is_free(struct hugepage_subpool *spool)
 {
 	if (spool->count)
@@ -5466,10 +5488,10 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
 {
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long address;
-	pte_t *ptep;
+	struct hugetlb_pte hpte;
 	pte_t pte;
 	spinlock_t *ptl;
-	struct page *page;
+	struct page *hpage, *subpage;
 	struct hstate *h = hstate_vma(vma);
 	unsigned long sz = huge_page_size(h);
 	unsigned long last_addr_mask;
@@ -5479,35 +5501,33 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
 	BUG_ON(start & ~huge_page_mask(h));
 	BUG_ON(end & ~huge_page_mask(h));
 
-	/*
-	 * This is a hugetlb vma, all the pte entries should point
-	 * to huge page.
-	 */
-	tlb_change_page_size(tlb, sz);
 	tlb_start_vma(tlb, vma);
 
 	last_addr_mask = hugetlb_mask_last_page(h);
 	address = start;
-	for (; address < end; address += sz) {
-		ptep = hugetlb_walk(vma, address, sz);
-		if (!ptep) {
-			address |= last_addr_mask;
+
+	while (address < end) {
+		if (hugetlb_full_walk(&hpte, vma, address)) {
+			address = (address | last_addr_mask) + sz;
 			continue;
 		}
 
-		ptl = huge_pte_lock(h, mm, ptep);
-		if (huge_pmd_unshare(mm, vma, address, ptep)) {
+		ptl = hugetlb_pte_lock(&hpte);
+		if (hugetlb_pte_size(&hpte) == sz &&
+		    huge_pmd_unshare(mm, vma, address, hpte.ptep)) {
 			spin_unlock(ptl);
 			tlb_flush_pmd_range(tlb, address & PUD_MASK, PUD_SIZE);
 			force_flush = true;
 			address |= last_addr_mask;
+			address += sz;
 			continue;
 		}
 
-		pte = huge_ptep_get(ptep);
+		pte = huge_ptep_get(hpte.ptep);
+
 		if (huge_pte_none(pte)) {
 			spin_unlock(ptl);
-			continue;
+			goto next_hpte;
 		}
 
 		/*
@@ -5523,24 +5543,35 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
 			 */
 			if (pte_swp_uffd_wp_any(pte) &&
 			    !(zap_flags & ZAP_FLAG_DROP_MARKER))
-				set_huge_pte_at(mm, address, ptep,
+				set_huge_pte_at(mm, address, hpte.ptep,
 						make_pte_marker(PTE_MARKER_UFFD_WP));
 			else
-				huge_pte_clear(mm, address, ptep, sz);
+				huge_pte_clear(mm, address, hpte.ptep,
+						hugetlb_pte_size(&hpte));
+			spin_unlock(ptl);
+			goto next_hpte;
+		}
+
+		if (unlikely(!hugetlb_pte_present_leaf(&hpte, pte))) {
+			/*
+			 * We raced with someone splitting out from under us.
+			 * Retry the walk.
+			 */
 			spin_unlock(ptl);
 			continue;
 		}
 
-		page = pte_page(pte);
+		subpage = pte_page(pte);
+		hpage = compound_head(subpage);
 		/*
 		 * If a reference page is supplied, it is because a specific
 		 * page is being unmapped, not a range. Ensure the page we
 		 * are about to unmap is the actual page of interest.
 		 */
 		if (ref_page) {
-			if (page != ref_page) {
+			if (hpage != ref_page) {
 				spin_unlock(ptl);
-				continue;
+				goto next_hpte;
 			}
 			/*
 			 * Mark the VMA as having unmapped its page so that
@@ -5550,25 +5581,32 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
 			set_vma_resv_flags(vma, HPAGE_RESV_UNMAPPED);
 		}
 
-		pte = huge_ptep_get_and_clear(mm, address, ptep);
-		tlb_remove_huge_tlb_entry(h, tlb, ptep, address);
+		pte = huge_ptep_get_and_clear(mm, address, hpte.ptep);
+		tlb_change_page_size(tlb, hugetlb_pte_size(&hpte));
+		tlb_remove_huge_tlb_entry(tlb, hpte, address);
 		if (huge_pte_dirty(pte))
-			set_page_dirty(page);
+			set_page_dirty(hpage);
 		/* Leave a uffd-wp pte marker if needed */
 		if (huge_pte_uffd_wp(pte) &&
 		    !(zap_flags & ZAP_FLAG_DROP_MARKER))
-			set_huge_pte_at(mm, address, ptep,
+			set_huge_pte_at(mm, address, hpte.ptep,
 					make_pte_marker(PTE_MARKER_UFFD_WP));
-		hugetlb_count_sub(pages_per_huge_page(h), mm);
-		page_remove_rmap(page, vma, true);
+		hugetlb_count_sub(hugetlb_pte_size(&hpte)/PAGE_SIZE, mm);
+		hugetlb_remove_rmap(subpage, hpte.shift, h, vma);
 
 		spin_unlock(ptl);
-		tlb_remove_page_size(tlb, page, huge_page_size(h));
 		/*
-		 * Bail out after unmapping reference page if supplied
+		 * Lower the reference count on the head page.
+		 */
+		tlb_remove_page_size(tlb, hpage, sz);
+		/*
+		 * Bail out after unmapping reference page if supplied,
+		 * and there's only one PTE mapping this page.
 		 */
-		if (ref_page)
+		if (ref_page && hugetlb_pte_size(&hpte) == sz)
 			break;
+next_hpte:
+		address += hugetlb_pte_size(&hpte);
 	}
 	tlb_end_vma(tlb, vma);
 
@@ -5846,7 +5884,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
 		/* Break COW or unshare */
 		huge_ptep_clear_flush(vma, haddr, ptep);
 		mmu_notifier_invalidate_range(mm, range.start, range.end);
-		page_remove_rmap(old_page, vma, true);
+		hugetlb_remove_rmap(old_page, huge_page_shift(h), h, vma);
 		hugepage_add_new_anon_rmap(new_folio, vma, haddr);
 		set_huge_pte_at(mm, haddr, ptep,
 				make_huge_pte(vma, &new_folio->page, !unshare));
-- 
2.39.2.637.g21b0678d19-goog