linux-kernel - [PATCH v4 06/13] mm: handle_pte_fault() use pte_offset_map_rw

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Message-Id: <eed9571b3d6f260e6e9d571fcac8b1ce8dd3571d.1727148662.git.zhengqi.arch@bytedance.com>
Date: Tue, 24 Sep 2024 14:09:58 +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,
	christophe.leroy2@...soprasteria.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 v4 06/13] mm: handle_pte_fault() use pte_offset_map_rw_nolock()

In handle_pte_fault(), we may modify the vmf->pte after acquiring the
vmf->ptl, so convert it to using pte_offset_map_rw_nolock(). But since we
will do the pte_same() check, so there is no need to get pmdval to do
pmd_same() check, just pass a dummy variable to it.

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
Acked-by: David Hildenbrand <david@...hat.com>
Reviewed-by: Muchun Song <muchun.song@...ux.dev>
---
 mm/memory.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index a245d28787034..6432b636d1ba7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5750,14 +5750,24 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 		vmf->pte = NULL;
 		vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
 	} else {
+		pmd_t dummy_pmdval;
+
 		/*
 		 * A regular pmd is established and it can't morph into a huge
 		 * 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 may be
+		 * modified, but since we will use pte_same() to detect the
+		 * change of the !pte_none() entry, there is no need to recheck
+		 * the pmdval. Here we chooes to pass a dummy variable instead
+		 * of NULL, which helps new user think about why this place is
+		 * special.
 		 */
-		vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
-						 vmf->address, &vmf->ptl);
+		vmf->pte = pte_offset_map_rw_nolock(vmf->vma->vm_mm, vmf->pmd,
+						    vmf->address, &dummy_pmdval,
+						    &vmf->ptl);
 		if (unlikely(!vmf->pte))
 			return 0;
 		vmf->orig_pte = ptep_get_lockless(vmf->pte);
-- 
2.20.1