[PATCH v5 0/5] THP aware uprobe

[PATCH v5 0/5] THP aware uprobe

[Song Liu]

This set makes uprobe aware of THPs.

Currently, when uprobe is attached to text on THP, the page is split by
FOLL_SPLIT. As a result, uprobe eliminates the performance benefit of THP.

This set makes uprobe THP-aware. Instead of FOLL_SPLIT, we introduces
FOLL_SPLIT_PMD, which only split PMD for uprobe. After all uprobes within
the THP are removed, the PTEs are regrouped into huge PMD.

Note that, with uprobes attached, the process runs with PTEs for the huge
page. The performance benefit of THP is recovered _after_ all uprobes on
the huge page are detached.

This set (plus a few THP patches) is also available at

   https://github.com/liu-song-6/linux/tree/uprobe-thp

Changes v4 => v5:
1. Propagate pte_alloc() error out of follow_pmd_mask().

Changes since v3:
1. Simplify FOLL_SPLIT_PMD case in follow_pmd_mask(), (Kirill A. Shutemov)
2. Fix try_collapse_huge_pmd() to match change in follow_pmd_mask().

Changes since v2:
1. For FOLL_SPLIT_PMD, populated the page table in follow_pmd_mask().
2. Simplify logic in uprobe_write_opcode. (Oleg Nesterov)
3. Fix page refcount handling with FOLL_SPLIT_PMD.
4. Much more testing, together with THP on ext4 and btrfs (sending in
   separate set).
5. Rebased.

Changes since v1:
1. introduces FOLL_SPLIT_PMD, instead of modifying split_huge_pmd*();
2. reuse pages_identical() from ksm.c;
3. rewrite most of try_collapse_huge_pmd().

Song Liu (5):
  mm: move memcmp_pages() and pages_identical()
  uprobe: use original page when all uprobes are removed
  mm, thp: introduce FOLL_SPLIT_PMD
  uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT
  uprobe: collapse THP pmd after removing all uprobes

 include/linux/huge_mm.h |  7 +++++
 include/linux/mm.h      |  8 +++++
 kernel/events/uprobes.c | 54 +++++++++++++++++++++++++-------
 mm/gup.c                |  8 +++--
 mm/huge_memory.c        | 69 +++++++++++++++++++++++++++++++++++++++++
 mm/ksm.c                | 18 -----------
 mm/util.c               | 13 ++++++++
 7 files changed, 145 insertions(+), 32 deletions(-)

--
2.17.1

[PATCH v5 1/5] mm: move memcmp_pages() and pages_identical()

[Song Liu]

This patch moves memcmp_pages() to mm/util.c and pages_identical() to
mm.h, so that we can use them in other files.

Signed-off-by: Song Liu <songliubraving@fb.com>
---
 include/linux/mm.h |  7 +++++++
 mm/ksm.c           | 18 ------------------
 mm/util.c          | 13 +++++++++++++
 3 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd0b5f4e1e45..0ab8c7d84cd0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2891,5 +2891,12 @@ void __init setup_nr_node_ids(void);
 static inline void setup_nr_node_ids(void) {}
 #endif
 
+extern int memcmp_pages(struct page *page1, struct page *page2);
+
+static inline int pages_identical(struct page *page1, struct page *page2)
+{
+	return !memcmp_pages(page1, page2);
+}
+
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
diff --git a/mm/ksm.c b/mm/ksm.c
index 81c20ed57bf6..6f153f976c4c 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1030,24 +1030,6 @@ static u32 calc_checksum(struct page *page)
 	return checksum;
 }
 
-static int memcmp_pages(struct page *page1, struct page *page2)
-{
-	char *addr1, *addr2;
-	int ret;
-
-	addr1 = kmap_atomic(page1);
-	addr2 = kmap_atomic(page2);
-	ret = memcmp(addr1, addr2, PAGE_SIZE);
-	kunmap_atomic(addr2);
-	kunmap_atomic(addr1);
-	return ret;
-}
-
-static inline int pages_identical(struct page *page1, struct page *page2)
-{
-	return !memcmp_pages(page1, page2);
-}
-
 static int write_protect_page(struct vm_area_struct *vma, struct page *page,
 			      pte_t *orig_pte)
 {
diff --git a/mm/util.c b/mm/util.c
index 9834c4ab7d8e..750e586d50bc 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -755,3 +755,16 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
 out:
 	return res;
 }
+
+int memcmp_pages(struct page *page1, struct page *page2)
+{
+	char *addr1, *addr2;
+	int ret;
+
+	addr1 = kmap_atomic(page1);
+	addr2 = kmap_atomic(page2);
+	ret = memcmp(addr1, addr2, PAGE_SIZE);
+	kunmap_atomic(addr2);
+	kunmap_atomic(addr1);
+	return ret;
+}
-- 
2.17.1

[PATCH v5 2/5] uprobe: use original page when all uprobes are removed

[Song Liu]

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 | 45 +++++++++++++++++++++++++++++++++--------
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 78f61bfc6b79..f7c61a1ef720 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 +180,24 @@ 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) {
+		lock_page(new_page);  /* for page_add_file_rmap() */
+		page_add_file_rmap(new_page, false);
+		unlock_page(new_page);
+		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));
@@ -501,6 +513,23 @@ 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);
 
+	if (!is_register) {
+		struct page *orig_page;
+		pgoff_t index;
+
+		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 (pages_identical(new_page, orig_page)) {
+				put_page(new_page);
+				new_page = orig_page;
+			} else
+				put_page(orig_page);
+		}
+	}
+
 	ret = __replace_page(vma, vaddr, old_page, new_page);
 	put_page(new_page);
 put_old:
-- 
2.17.1

[PATCH v5 3/5] mm, thp: introduce FOLL_SPLIT_PMD

[Song Liu]

This patches introduces a new foll_flag: FOLL_SPLIT_PMD. As the name says
FOLL_SPLIT_PMD splits huge pmd for given mm_struct, the underlining huge
page stays as-is.

FOLL_SPLIT_PMD is useful for cases where we need to use regular pages,
but would switch back to huge page and huge pmd on. One of such example
is uprobe. The following patches use FOLL_SPLIT_PMD in uprobe.

Signed-off-by: Song Liu <songliubraving@fb.com>
---
 include/linux/mm.h | 1 +
 mm/gup.c           | 8 ++++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0ab8c7d84cd0..e605acc4fc81 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2642,6 +2642,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 #define FOLL_COW	0x4000	/* internal GUP flag */
 #define FOLL_ANON	0x8000	/* don't do file mappings */
 #define FOLL_LONGTERM	0x10000	/* mapping lifetime is indefinite: see below */
+#define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
 
 /*
  * NOTE on FOLL_LONGTERM:
diff --git a/mm/gup.c b/mm/gup.c
index ddde097cf9e4..41f2a1fcc6f0 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -398,7 +398,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 		spin_unlock(ptl);
 		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
 	}
-	if (flags & FOLL_SPLIT) {
+	if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
 		int ret;
 		page = pmd_page(*pmd);
 		if (is_huge_zero_page(page)) {
@@ -407,7 +407,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 			split_huge_pmd(vma, pmd, address);
 			if (pmd_trans_unstable(pmd))
 				ret = -EBUSY;
-		} else {
+		} else if (flags & FOLL_SPLIT) {
 			if (unlikely(!try_get_page(page))) {
 				spin_unlock(ptl);
 				return ERR_PTR(-ENOMEM);
@@ -419,6 +419,10 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 			put_page(page);
 			if (pmd_none(*pmd))
 				return no_page_table(vma, flags);
+		} else {  /* flags & FOLL_SPLIT_PMD */
+			spin_unlock(ptl);
+			split_huge_pmd(vma, pmd, address);
+			ret = pte_alloc(mm, pmd);
 		}
 
 		return ret ? ERR_PTR(ret) :
-- 
2.17.1

[PATCH v5 4/5] uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT

[Song Liu]

This patches uses newly added FOLL_SPLIT_PMD in uprobe. This enables easy
regroup of huge pmd after the uprobe is disabled (in next patch).

Signed-off-by: Song Liu <songliubraving@fb.com>
---
 kernel/events/uprobes.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index f7c61a1ef720..a20d7b43a056 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -153,7 +153,7 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 {
 	struct mm_struct *mm = vma->vm_mm;
 	struct page_vma_mapped_walk pvmw = {
-		.page = old_page,
+		.page = compound_head(old_page),
 		.vma = vma,
 		.address = addr,
 	};
@@ -165,8 +165,6 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, addr,
 				addr + PAGE_SIZE);
 
-	VM_BUG_ON_PAGE(PageTransHuge(old_page), old_page);
-
 	if (!orig) {
 		err = mem_cgroup_try_charge(new_page, vma->vm_mm, GFP_KERNEL,
 					    &memcg, false);
@@ -483,7 +481,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 retry:
 	/* Read the page with vaddr into memory */
 	ret = get_user_pages_remote(NULL, mm, vaddr, 1,
-			FOLL_FORCE | FOLL_SPLIT, &old_page, &vma, NULL);
+			FOLL_FORCE | FOLL_SPLIT_PMD, &old_page, &vma, NULL);
 	if (ret <= 0)
 		return ret;
 
-- 
2.17.1

[PATCH v5 5/5] uprobe: collapse THP pmd after removing all uprobes

[Song Liu]

After all uprobes are removed from the huge page (with PTE pgtable), it
is possible to collapse the pmd and benefit from THP again. This patch
does the collapse.

An issue on earlier version was discovered by kbuild test robot.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
---
 include/linux/huge_mm.h |  7 +++++
 kernel/events/uprobes.c |  5 ++-
 mm/huge_memory.c        | 69 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 80 insertions(+), 1 deletion(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 7cd5c150c21d..30669e9a9340 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -250,6 +250,9 @@ static inline bool thp_migration_supported(void)
 	return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);
 }
 
+extern void try_collapse_huge_pmd(struct vm_area_struct *vma,
+				  struct page *page);
+
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
 #define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
@@ -368,6 +371,10 @@ static inline bool thp_migration_supported(void)
 {
 	return false;
 }
+
+static inline void try_collapse_huge_pmd(struct vm_area_struct *vma,
+					 struct page *page) {}
+
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 #endif /* _LINUX_HUGE_MM_H */
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index a20d7b43a056..9bec602bf79e 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -474,6 +474,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	struct page *old_page, *new_page;
 	struct vm_area_struct *vma;
 	int ret, is_register, ref_ctr_updated = 0;
+	struct page *orig_page = NULL;
 
 	is_register = is_swbp_insn(&opcode);
 	uprobe = container_of(auprobe, struct uprobe, arch);
@@ -512,7 +513,6 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	copy_to_page(new_page, vaddr, &opcode, UPROBE_SWBP_INSN_SIZE);
 
 	if (!is_register) {
-		struct page *orig_page;
 		pgoff_t index;
 
 		index = vaddr_to_offset(vma, vaddr & PAGE_MASK) >> PAGE_SHIFT;
@@ -540,6 +540,9 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	if (ret && is_register && ref_ctr_updated)
 		update_ref_ctr(uprobe, mm, -1);
 
+	if (!ret && orig_page && PageTransCompound(orig_page))
+		try_collapse_huge_pmd(vma, orig_page);
+
 	return ret;
 }
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9f8bce9a6b32..cc8464650b72 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2886,6 +2886,75 @@ static struct shrinker deferred_split_shrinker = {
 	.flags = SHRINKER_NUMA_AWARE,
 };
 
+/**
+ * try_collapse_huge_pmd - try collapse pmd for a pte mapped huge page
+ * @vma: vma containing the huge page
+ * @page: any sub page of the huge page
+ */
+void try_collapse_huge_pmd(struct vm_area_struct *vma,
+			   struct page *page)
+{
+	struct page *hpage = compound_head(page);
+	struct mm_struct *mm = vma->vm_mm;
+	struct mmu_notifier_range range;
+	unsigned long haddr;
+	unsigned long addr;
+	pmd_t *pmd, _pmd;
+	spinlock_t *ptl;
+	int i, count = 0;
+
+	VM_BUG_ON_PAGE(!PageCompound(page), page);
+
+	haddr = page_address_in_vma(hpage, vma);
+	pmd = mm_find_pmd(mm, haddr);
+	if (!pmd)
+		return;
+
+	lock_page(hpage);
+	ptl = pmd_lock(mm, pmd);
+
+	/* step 1: check all mapped PTEs */
+	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
+		pte_t *pte = pte_offset_map(pmd, addr);
+
+		if (pte_none(*pte))
+			continue;
+		if (hpage + i != vm_normal_page(vma, addr, *pte)) {
+			spin_unlock(ptl);
+			unlock_page(hpage);
+			return;
+		}
+		count++;
+	}
+
+	/* step 2: adjust rmap */
+	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
+		pte_t *pte = pte_offset_map(pmd, addr);
+		struct page *p;
+
+		if (pte_none(*pte))
+			continue;
+		p = vm_normal_page(vma, addr, *pte);
+		page_remove_rmap(p, false);
+	}
+
+	/* step 3: flip page table */
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL, mm,
+				haddr, haddr + HPAGE_PMD_SIZE);
+	mmu_notifier_invalidate_range_start(&range);
+
+	_pmd = pmdp_collapse_flush(vma, haddr, pmd);
+	spin_unlock(ptl);
+	mmu_notifier_invalidate_range_end(&range);
+
+	/* step 4: free pgtable, set refcount, mm_counters, etc. */
+	page_ref_sub(page, count);
+	unlock_page(hpage);
+	mm_dec_nr_ptes(mm);
+	pte_free(mm, pmd_pgtable(_pmd));
+	add_mm_counter(mm, mm_counter_file(page), -count);
+}
+
 #ifdef CONFIG_DEBUG_FS
 static int split_huge_pages_set(void *data, u64 val)
 {
-- 
2.17.1