linux-kernel - [PATCH v2] mm: fix finish_fault() handling for large folios

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Message-ID: <20250226162341.915535-1-bgeffon@google.com>
Date: Wed, 26 Feb 2025 11:23:41 -0500
From: Brian Geffon <bgeffon@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Zi Yan <ziy@...dia.com>, Kefeng Wang <wangkefeng.wang@...wei.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Matthew Wilcox <willy@...radead.org>, David Hildenbrand <david@...hat.com>, 
	Brian Geffon <bgeffon@...gle.com>, stable@...r.kernel.org, 
	Hugh Dickins <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Marek Maslanka <mmaslanka@...gle.com>
Subject: [PATCH v2] mm: fix finish_fault() handling for large folios

When handling faults for anon shmem finish_fault() will attempt to install
ptes for the entire folio. Unfortunately if it encounters a single
non-pte_none entry in that range it will bail, even if the pte that
triggered the fault is still pte_none. When this situation happens the
fault will be retried endlessly never making forward progress.

This patch fixes this behavior and if it detects that a pte in the range
is not pte_none it will fall back to setting a single pte.

Cc: stable@...r.kernel.org
Cc: Hugh Dickins <hughd@...gle.com>
Fixes: 43e027e41423 ("mm: memory: extend finish_fault() to support large folio")
Suggested-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
Reported-by: Marek Maslanka <mmaslanka@...gle.com>
Signed-off-by: Brian Geffon <bgeffon@...gle.com>
---
 mm/memory.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index b4d3d4893267..b6c467fdbfa4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5183,7 +5183,11 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 	bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
 		      !(vma->vm_flags & VM_SHARED);
 	int type, nr_pages;
-	unsigned long addr = vmf->address;
+	unsigned long addr;
+	bool needs_fallback = false;
+
+fallback:
+	addr = vmf->address;
 
 	/* Did we COW the page? */
 	if (is_cow)
@@ -5222,7 +5226,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 	 * approach also applies to non-anonymous-shmem faults to avoid
 	 * inflating the RSS of the process.
 	 */
-	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
+	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
+			unlikely(needs_fallback)) {
 		nr_pages = 1;
 	} else if (nr_pages > 1) {
 		pgoff_t idx = folio_page_idx(folio, page);
@@ -5258,9 +5263,9 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 		ret = VM_FAULT_NOPAGE;
 		goto unlock;
 	} else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
-		update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
-		ret = VM_FAULT_NOPAGE;
-		goto unlock;
+		needs_fallback = true;
+		pte_unmap_unlock(vmf->pte, vmf->ptl);
+		goto fallback;
 	}
 
 	folio_ref_add(folio, nr_pages - 1);
-- 
2.48.1.711.g2feabab25a-goog