[PATCH mm-unstable v4 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

[Nico Pache]

The khugepaged daemon and madvise_collapse have two different
implementations that do almost the same thing. Create collapse_single_pmd
to increase code reuse and create an entry point to these two users.

Refactor madvise_collapse and collapse_scan_mm_slot to use the new
collapse_single_pmd function. To help reduce confusion around the
mmap_locked variable, we rename mmap_locked to lock_dropped in the
collapse_scan_mm_slot() function, and remove the redundant mmap_locked
in madvise_collapse(); this further unifies the code readiblity. the
SCAN_PTE_MAPPED_HUGEPAGE enum is no longer reachable in the
madvise_collapse() function, so we drop it from the list of "continuing"
enums.

This introduces a minor behavioral change that is most likely an
undiscovered bug. The current implementation of khugepaged tests
collapse_test_exit_or_disable() before calling collapse_pte_mapped_thp,
but we weren't doing it in the madvise_collapse case. By unifying these
two callers madvise_collapse now also performs this check. We also modify
the return value to be SCAN_ANY_PROCESS which properly indicates that this
process is no longer valid to operate on.

By moving the madvise_collapse writeback-retry logic into the helper
function we can also avoid having to revalidate the VMA.

We guard the khugepaged_pages_collapsed variable to ensure its only
incremented for khugepaged.

As requested we also convert a VM_BUG_ON to a VM_WARN_ON.

Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@xxxxxxxxxx>
Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
---
 mm/khugepaged.c | 142 ++++++++++++++++++++++++------------------------
 1 file changed, 72 insertions(+), 70 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 3728a2cf133c..d06d84219e1b 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1257,7 +1257,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 
 static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		struct vm_area_struct *vma, unsigned long start_addr,
-		bool *mmap_locked, struct collapse_control *cc)
+		bool *lock_dropped, struct collapse_control *cc)
 {
 	pmd_t *pmd;
 	pte_t *pte, *_pte;
@@ -1432,7 +1432,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		result = collapse_huge_page(mm, start_addr, referenced,
 					    unmapped, cc);
 		/* collapse_huge_page will return with the mmap_lock released */
-		*mmap_locked = false;
+		*lock_dropped = true;
 	}
 out:
 	trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
@@ -2424,6 +2424,67 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 	return result;
 }
 
+/*
+ * Try to collapse a single PMD starting at a PMD aligned addr, and return
+ * the results.
+ */
+static enum scan_result collapse_single_pmd(unsigned long addr,
+		struct vm_area_struct *vma, bool *lock_dropped,
+		struct collapse_control *cc)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	bool triggered_wb = false;
+	enum scan_result result;
+	struct file *file;
+	pgoff_t pgoff;
+
+	mmap_assert_locked(mm);
+
+	if (vma_is_anonymous(vma)) {
+		result = collapse_scan_pmd(mm, vma, addr, lock_dropped, cc);
+		goto end;
+	}
+
+	file = get_file(vma->vm_file);
+	pgoff = linear_page_index(vma, addr);
+
+	mmap_read_unlock(mm);
+	*lock_dropped = true;
+retry:
+	result = collapse_scan_file(mm, addr, file, pgoff, cc);
+
+	/*
+	 * For MADV_COLLAPSE, when encountering dirty pages, try to writeback,
+	 * then retry the collapse one time.
+	 */
+	if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
+	    !triggered_wb && mapping_can_writeback(file->f_mapping)) {
+		const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
+		const loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
+
+		filemap_write_and_wait_range(file->f_mapping, lstart, lend);
+		triggered_wb = true;
+		goto retry;
+	}
+	fput(file);
+
+	if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
+		mmap_read_lock(mm);
+		if (collapse_test_exit_or_disable(mm))
+			result = SCAN_ANY_PROCESS;
+		else
+			result = try_collapse_pte_mapped_thp(mm, addr,
+							     !cc->is_khugepaged);
+		if (result == SCAN_PMD_MAPPED)
+			result = SCAN_SUCCEED;
+		mmap_read_unlock(mm);
+	}
+end:
+	if (cc->is_khugepaged && result == SCAN_SUCCEED)
+		++khugepaged_pages_collapsed;
+	return result;
+}
+
 static void collapse_scan_mm_slot(unsigned int progress_max,
 		enum scan_result *result, struct collapse_control *cc)
 	__releases(&khugepaged_mm_lock)
@@ -2485,46 +2546,21 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
 		VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK);
 
 		while (khugepaged_scan.address < hend) {
-			bool mmap_locked = true;
+			bool lock_dropped = false;
 
 			cond_resched();
 			if (unlikely(collapse_test_exit_or_disable(mm)))
 				goto breakouterloop;
 
-			VM_BUG_ON(khugepaged_scan.address < hstart ||
+			VM_WARN_ON_ONCE(khugepaged_scan.address < hstart ||
 				  khugepaged_scan.address + HPAGE_PMD_SIZE >
 				  hend);
-			if (!vma_is_anonymous(vma)) {
-				struct file *file = get_file(vma->vm_file);
-				pgoff_t pgoff = linear_page_index(vma,
-						khugepaged_scan.address);
-
-				mmap_read_unlock(mm);
-				mmap_locked = false;
-				*result = collapse_scan_file(mm,
-					khugepaged_scan.address, file, pgoff, cc);
-				fput(file);
-				if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
-					mmap_read_lock(mm);
-					if (collapse_test_exit_or_disable(mm))
-						goto breakouterloop;
-					*result = try_collapse_pte_mapped_thp(mm,
-						khugepaged_scan.address, false);
-					if (*result == SCAN_PMD_MAPPED)
-						*result = SCAN_SUCCEED;
-					mmap_read_unlock(mm);
-				}
-			} else {
-				*result = collapse_scan_pmd(mm, vma,
-					khugepaged_scan.address, &mmap_locked, cc);
-			}
-
-			if (*result == SCAN_SUCCEED)
-				++khugepaged_pages_collapsed;
 
+			*result = collapse_single_pmd(khugepaged_scan.address,
+						      vma, &lock_dropped, cc);
 			/* move to next address */
 			khugepaged_scan.address += HPAGE_PMD_SIZE;
-			if (!mmap_locked)
+			if (lock_dropped)
 				/*
 				 * We released mmap_lock so break loop.  Note
 				 * that we drop mmap_lock before all hugepage
@@ -2799,7 +2835,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 	unsigned long hstart, hend, addr;
 	enum scan_result last_fail = SCAN_FAIL;
 	int thps = 0;
-	bool mmap_locked = true;
 
 	BUG_ON(vma->vm_start > start);
 	BUG_ON(vma->vm_end < end);
@@ -2821,13 +2856,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 
 	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
 		enum scan_result result = SCAN_FAIL;
-		bool triggered_wb = false;
 
-retry:
-		if (!mmap_locked) {
+		if (*lock_dropped) {
 			cond_resched();
 			mmap_read_lock(mm);
-			mmap_locked = true;
+			*lock_dropped = false;
 			result = hugepage_vma_revalidate(mm, addr, false, &vma,
 							 cc);
 			if (result  != SCAN_SUCCEED) {
@@ -2837,45 +2870,14 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 
 			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
 		}
-		mmap_assert_locked(mm);
-		if (!vma_is_anonymous(vma)) {
-			struct file *file = get_file(vma->vm_file);
-			pgoff_t pgoff = linear_page_index(vma, addr);
 
-			mmap_read_unlock(mm);
-			mmap_locked = false;
-			*lock_dropped = true;
-			result = collapse_scan_file(mm, addr, file, pgoff, cc);
-
-			if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb &&
-			    mapping_can_writeback(file->f_mapping)) {
-				loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
-				loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
-
-				filemap_write_and_wait_range(file->f_mapping, lstart, lend);
-				triggered_wb = true;
-				fput(file);
-				goto retry;
-			}
-			fput(file);
-		} else {
-			result = collapse_scan_pmd(mm, vma, addr, &mmap_locked, cc);
-		}
-		if (!mmap_locked)
-			*lock_dropped = true;
+		result = collapse_single_pmd(addr, vma, lock_dropped, cc);
 
-handle_result:
 		switch (result) {
 		case SCAN_SUCCEED:
 		case SCAN_PMD_MAPPED:
 			++thps;
 			break;
-		case SCAN_PTE_MAPPED_HUGEPAGE:
-			BUG_ON(mmap_locked);
-			mmap_read_lock(mm);
-			result = try_collapse_pte_mapped_thp(mm, addr, true);
-			mmap_read_unlock(mm);
-			goto handle_result;
 		/* Whitelisted set of results where continuing OK */
 		case SCAN_NO_PTE_TABLE:
 		case SCAN_PTE_NON_PRESENT:
@@ -2898,7 +2900,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 
 out_maybelock:
 	/* Caller expects us to hold mmap_lock on return */
-	if (!mmap_locked)
+	if (*lock_dropped)
 		mmap_read_lock(mm);
 out_nolock:
 	mmap_assert_locked(mm);
-- 
2.53.0