linux-kernel - [PATCH v4 03/10] mm/mremap: put VMA check and prep logic into helper function

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Message-ID: <15efa3c57935f7f8894094b94c1803c2f322c511.1752770784.git.lorenzo.stoakes@oracle.com>
Date: Thu, 17 Jul 2025 17:55:53 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Peter Xu <peterx@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Rik van Riel <riel@...riel.com>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: [PATCH v4 03/10] mm/mremap: put VMA check and prep logic into helper function

Rather than lumping everything together in do_mremap(), add a new helper
function, check_prep_vma(), to do the work relating to each VMA.

This further lays groundwork for subsequent patches which will allow for
batched VMA mremap().

Additionally, if we set vrm->new_addr == vrm->addr when prepping the VMA,
this avoids us needing to do so in the expand VMA mlocked case.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/mremap.c | 58 ++++++++++++++++++++++++++---------------------------
 1 file changed, 28 insertions(+), 30 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 9ce20c238ffd..60eb0ac8634b 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1634,7 +1634,6 @@ static bool align_hugetlb(struct vma_remap_struct *vrm)
 static unsigned long expand_vma(struct vma_remap_struct *vrm)
 {
 	unsigned long err;
-	unsigned long addr = vrm->addr;
 
 	err = remap_is_valid(vrm);
 	if (err)
@@ -1649,16 +1648,8 @@ static unsigned long expand_vma(struct vma_remap_struct *vrm)
 		if (err)
 			return err;
 
-		/*
-		 * We want to populate the newly expanded portion of the VMA to
-		 * satisfy the expectation that mlock()'ing a VMA maintains all
-		 * of its pages in memory.
-		 */
-		if (vrm->mlocked)
-			vrm->new_addr = addr;
-
 		/* OK we're done! */
-		return addr;
+		return vrm->addr;
 	}
 
 	/*
@@ -1714,10 +1705,33 @@ static unsigned long mremap_at(struct vma_remap_struct *vrm)
 	return -EINVAL;
 }
 
+static int check_prep_vma(struct vma_remap_struct *vrm)
+{
+	struct vm_area_struct *vma = vrm->vma;
+
+	if (!vma)
+		return -EFAULT;
+
+	/* If mseal()'d, mremap() is prohibited. */
+	if (!can_modify_vma(vma))
+		return -EPERM;
+
+	/* Align to hugetlb page size, if required. */
+	if (is_vm_hugetlb_page(vma) && !align_hugetlb(vrm))
+		return -EINVAL;
+
+	vrm_set_delta(vrm);
+	vrm->remap_type = vrm_remap_type(vrm);
+	/* For convenience, we set new_addr even if VMA won't move. */
+	if (!vrm_implies_new_addr(vrm))
+		vrm->new_addr = vrm->addr;
+
+	return 0;
+}
+
 static unsigned long do_mremap(struct vma_remap_struct *vrm)
 {
 	struct mm_struct *mm = current->mm;
-	struct vm_area_struct *vma;
 	unsigned long res;
 
 	vrm->old_len = PAGE_ALIGN(vrm->old_len);
@@ -1731,26 +1745,10 @@ static unsigned long do_mremap(struct vma_remap_struct *vrm)
 		return -EINTR;
 	vrm->mmap_locked = true;
 
-	vma = vrm->vma = vma_lookup(mm, vrm->addr);
-	if (!vma) {
-		res = -EFAULT;
-		goto out;
-	}
-
-	/* If mseal()'d, mremap() is prohibited. */
-	if (!can_modify_vma(vma)) {
-		res = -EPERM;
-		goto out;
-	}
-
-	/* Align to hugetlb page size, if required. */
-	if (is_vm_hugetlb_page(vma) && !align_hugetlb(vrm)) {
-		res = -EINVAL;
+	vrm->vma = vma_lookup(current->mm, vrm->addr);
+	res = check_prep_vma(vrm);
+	if (res)
 		goto out;
-	}
-
-	vrm_set_delta(vrm);
-	vrm->remap_type = vrm_remap_type(vrm);
 
 	/* Actually execute mremap. */
 	res = vrm_implies_new_addr(vrm) ? mremap_to(vrm) : mremap_at(vrm);
-- 
2.50.1