[PATCH uprobe, thp 2/4] uprobe: use original page when all uprobes are removed

[Song Liu <songliubraving@fb.com>]

Currently, uprobe swaps the target page with a anonymous page in both
install_breakpoint() and remove_breakpoint(). When all uprobes on a page
are removed, the given mm is still using an anonymous page (not the
original page).

This patch allows uprobe to use original page when possible (all uprobes
on the page are already removed).

Signed-off-by: Song Liu <songliubraving@fb.com>
---
 kernel/events/uprobes.c | 43 ++++++++++++++++++++++++++++++++---------
 1 file changed, 34 insertions(+), 9 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 78f61bfc6b79..ba49da99d2a2 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -160,16 +160,19 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 	int err;
 	struct mmu_notifier_range range;
 	struct mem_cgroup *memcg;
+	bool orig = new_page->mapping != NULL;  /* new_page == orig_page */
 
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, addr,
 				addr + PAGE_SIZE);
 
 	VM_BUG_ON_PAGE(PageTransHuge(old_page), old_page);
 
-	err = mem_cgroup_try_charge(new_page, vma->vm_mm, GFP_KERNEL, &memcg,
-			false);
-	if (err)
-		return err;
+	if (!orig) {
+		err = mem_cgroup_try_charge(new_page, vma->vm_mm, GFP_KERNEL,
+					    &memcg, false);
+		if (err)
+			return err;
+	}
 
 	/* For try_to_free_swap() and munlock_vma_page() below */
 	lock_page(old_page);
@@ -177,15 +180,22 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 	mmu_notifier_invalidate_range_start(&range);
 	err = -EAGAIN;
 	if (!page_vma_mapped_walk(&pvmw)) {
-		mem_cgroup_cancel_charge(new_page, memcg, false);
+		if (!orig)
+			mem_cgroup_cancel_charge(new_page, memcg, false);
 		goto unlock;
 	}
 	VM_BUG_ON_PAGE(addr != pvmw.address, old_page);
 
 	get_page(new_page);
-	page_add_new_anon_rmap(new_page, vma, addr, false);
-	mem_cgroup_commit_charge(new_page, memcg, false, false);
-	lru_cache_add_active_or_unevictable(new_page, vma);
+	if (orig) {
+		page_add_file_rmap(new_page, false);
+		inc_mm_counter(mm, mm_counter_file(new_page));
+		dec_mm_counter(mm, MM_ANONPAGES);
+	} else {
+		page_add_new_anon_rmap(new_page, vma, addr, false);
+		mem_cgroup_commit_charge(new_page, memcg, false, false);
+		lru_cache_add_active_or_unevictable(new_page, vma);
+	}
 
 	if (!PageAnon(old_page)) {
 		dec_mm_counter(mm, mm_counter_file(old_page));
@@ -461,9 +471,10 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 			unsigned long vaddr, uprobe_opcode_t opcode)
 {
 	struct uprobe *uprobe;
-	struct page *old_page, *new_page;
+	struct page *old_page, *new_page, *orig_page = NULL;
 	struct vm_area_struct *vma;
 	int ret, is_register, ref_ctr_updated = 0;
+	pgoff_t index;
 
 	is_register = is_swbp_insn(&opcode);
 	uprobe = container_of(auprobe, struct uprobe, arch);
@@ -501,6 +512,20 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	copy_highpage(new_page, old_page);
 	copy_to_page(new_page, vaddr, &opcode, UPROBE_SWBP_INSN_SIZE);
 
+	index = vaddr_to_offset(vma, vaddr & PAGE_MASK) >> PAGE_SHIFT;
+	orig_page = find_get_page(vma->vm_file->f_inode->i_mapping, index);
+	if (orig_page) {
+		if (memcmp(page_address(orig_page),
+			   page_address(new_page), PAGE_SIZE) == 0) {
+			/* if new_page matches orig_page, use orig_page */
+			put_page(new_page);
+			new_page = orig_page;
+		} else {
+			put_page(orig_page);
+			orig_page = NULL;
+		}
+	}
+
 	ret = __replace_page(vma, vaddr, old_page, new_page);
 	put_page(new_page);
 put_old:
-- 
2.17.1