linux-kernel - [PATCH 06/14] mm: handle_pte_fault() use pte_offset_map_maywrite

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Message-Id: <239432a0bc56464e58a6baf3622fdc72526c8d57.1724226076.git.zhengqi.arch@bytedance.com>
Date: Wed, 21 Aug 2024 16:18:49 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: david@...hat.com,
	hughd@...gle.com,
	willy@...radead.org,
	muchun.song@...ux.dev,
	vbabka@...nel.org,
	akpm@...ux-foundation.org,
	rppt@...nel.org,
	vishal.moola@...il.com,
	peterx@...hat.com,
	ryan.roberts@....com
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org,
	Qi Zheng <zhengqi.arch@...edance.com>
Subject: [PATCH 06/14] mm: handle_pte_fault() use pte_offset_map_maywrite_nolock()

In handle_pte_fault(), we may modify the vmf->pte after acquiring the
vmf->ptl, so convert it to using pte_offset_map_maywrite_nolock(). But
since we already do the pte_same() check, so there is no need to get
pmdval to do pmd_same() check, just pass NULL to pmdvalp parameter.

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 mm/memory.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 93c0c25433d02..d3378e98faf13 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5504,9 +5504,14 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 		 * pmd by anon khugepaged, since that takes mmap_lock in write
 		 * mode; but shmem or file collapse to THP could still morph
 		 * it into a huge pmd: just retry later if so.
+		 *
+		 * Use the maywrite version to indicate that vmf->pte will be
+		 * modified, but since we will use pte_same() to detect the
+		 * change of the pte entry, there is no need to get pmdval.
 		 */
-		vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
-						 vmf->address, &vmf->ptl);
+		vmf->pte = pte_offset_map_maywrite_nolock(vmf->vma->vm_mm,
+							  vmf->pmd, vmf->address,
+							  NULL, &vmf->ptl);
 		if (unlikely(!vmf->pte))
 			return 0;
 		vmf->orig_pte = ptep_get_lockless(vmf->pte);
-- 
2.20.1