Re: [RFC PATCH 08/12] khugepaged: Abstract PMD-THP collapse

Dev Jain <dev.jain@xxxxxxx> · Wed, 18 Dec 2024 14:56:35 +0530

On 18/12/24 12:54 am, Ryan Roberts wrote:
On 16/12/2024 16:51, Dev Jain wrote:
Abstract away taking the mmap_lock exclusively, copying page contents, and
setting the PMD, into vma_collapse_anon_folio_pmd().

Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
  mm/khugepaged.c | 119 +++++++++++++++++++++++++++---------------------
  1 file changed, 66 insertions(+), 53 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 078794aa3335..88beebef773e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1111,58 +1111,17 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
  	return SCAN_SUCCEED;
  }
  
-static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
-			      int referenced, int unmapped, int order,
-			      struct collapse_control *cc)
+static int vma_collapse_anon_folio_pmd(struct mm_struct *mm, unsigned long address,
+		struct vm_area_struct *vma, struct collapse_control *cc, pmd_t *pmd,
+		struct folio *folio)
  {
+	struct mmu_notifier_range range;
+	spinlock_t *pmd_ptl, *pte_ptl;
  	LIST_HEAD(compound_pagelist);
-	pmd_t *pmd, _pmd;
-	pte_t *pte;
  	pgtable_t pgtable;
-	struct folio *folio;
-	spinlock_t *pmd_ptl, *pte_ptl;
-	int result = SCAN_FAIL;
-	struct vm_area_struct *vma;
-	struct mmu_notifier_range range;
-
-	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
-
-	/*
-	 * Before allocating the hugepage, release the mmap_lock read lock.
-	 * The allocation can take potentially a long time if it involves
-	 * sync compaction, and we do not need to hold the mmap_lock during
-	 * that. We will recheck the vma after taking it again in write mode.
-	 */
-	mmap_read_unlock(mm);
-
-	result = alloc_charge_folio(&folio, mm, order, cc);
-	if (result != SCAN_SUCCEED)
-		goto out_nolock;
-
-	mmap_read_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, order, cc);
-	if (result != SCAN_SUCCEED) {
-		mmap_read_unlock(mm);
-		goto out_nolock;
-	}
-
-	result = find_pmd_or_thp_or_none(mm, address, &pmd);
-	if (result != SCAN_SUCCEED) {
-		mmap_read_unlock(mm);
-		goto out_nolock;
-	}
-
-	if (unmapped) {
-		/*
-		 * __collapse_huge_page_swapin will return with mmap_lock
-		 * released when it fails. So we jump out_nolock directly in
-		 * that case.  Continuing to collapse causes inconsistency.
-		 */
-		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
-						     referenced, order);
-		if (result != SCAN_SUCCEED)
-			goto out_nolock;
-	}
+	int result;
+	pmd_t _pmd;
+	pte_t *pte;
  
  	mmap_read_unlock(mm);
  	/*
@@ -1174,7 +1133,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
  	 * mmap_lock.
  	 */
  	mmap_write_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, order, cc);
+
+	result = hugepage_vma_revalidate(mm, address, true, &vma, HPAGE_PMD_ORDER, cc);
  	if (result != SCAN_SUCCEED)
  		goto out_up_write;
  	/* check if the pmd is still valid */
@@ -1206,7 +1166,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
  	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
  	if (pte) {
  		result = __collapse_huge_page_isolate(vma, address, pte, cc,
-						      &compound_pagelist, order);
+						      &compound_pagelist, HPAGE_PMD_ORDER);
  		spin_unlock(pte_ptl);
  	} else {
  		result = SCAN_PMD_NULL;
@@ -1262,11 +1222,64 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
  	deferred_split_folio(folio, false);
  	spin_unlock(pmd_ptl);
  
-	folio = NULL;
-
  	result = SCAN_SUCCEED;
  out_up_write:
  	mmap_write_unlock(mm);
+	return result;
+}
+
+static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
+			      int referenced, int unmapped, int order,
+			      struct collapse_control *cc)
+{
+	struct vm_area_struct *vma;
+	int result = SCAN_FAIL;
+	struct folio *folio;
+	pmd_t *pmd;
+
+	/*
+	 * Before allocating the hugepage, release the mmap_lock read lock.
+	 * The allocation can take potentially a long time if it involves
+	 * sync compaction, and we do not need to hold the mmap_lock during
+	 * that. We will recheck the vma after taking it again in write mode.
+	 */
+	mmap_read_unlock(mm);
+
+	result = alloc_charge_folio(&folio, mm, order, cc);
+	if (result != SCAN_SUCCEED)
+		goto out_nolock;
+
+	mmap_read_lock(mm);
+	result = hugepage_vma_revalidate(mm, address, true, &vma, order, cc);
+	if (result != SCAN_SUCCEED) {
+		mmap_read_unlock(mm);
+		goto out_nolock;
+	}
+
+	result = find_pmd_or_thp_or_none(mm, address, &pmd);
+	if (result != SCAN_SUCCEED) {
+		mmap_read_unlock(mm);
+		goto out_nolock;
+	}
+
+	if (unmapped) {
+		/*
+		 * __collapse_huge_page_swapin will return with mmap_lock
+		 * released when it fails. So we jump out_nolock directly in
+		 * that case.  Continuing to collapse causes inconsistency.
+		 */
+		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
+						     referenced, order);
+		if (result != SCAN_SUCCEED)
+			goto out_nolock;
+	}
+
+	if (order == HPAGE_PMD_ORDER)
+		result = vma_collapse_anon_folio_pmd(mm, address, vma, cc, pmd, folio);
I think the locking is broken here? collapse_huge_page() used to enter with the
mmamp read lock and exit without the lock held at all. After the change, this is
only true for order == HPAGE_PMD_ORDER. For other orders, you exit with the mmap
read lock still held. Perhaps:

	else
		mmap_read_unlock(mm);

I have completely messed up sequential building of patches :) I will take care next time.


+
+	if (result == SCAN_SUCCEED)
+		folio = NULL;
+
  out_nolock:
  	if (folio)
  		folio_put(folio);







