linux-kernel - [PATCH v2 1/4] mm: ksm: have KSM VMA checks not require a VMA pointer

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Message-ID: <b7f41a3d8a8538d73610ace3e85f92bb20f8eb42.1747844463.git.lorenzo.stoakes@oracle.com>
Date: Wed, 21 May 2025 19:20:28 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: 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>, David Hildenbrand <david@...hat.com>,
        Xu Xin <xu.xin16@....com.cn>,
        Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Stefan Roesch <shr@...kernel.io>
Subject: [PATCH v2 1/4] mm: ksm: have KSM VMA checks not require a VMA pointer

In subsequent commits we are going to determine KSM eligibility prior to a
VMA being constructed, at which point we will of course not yet have access
to a VMA pointer.

It is trivial to boil down the check logic to be parameterised on
mm_struct, file and VMA flags, so do so.

As a part of this change, additionally expose and use file_is_dax() to
determine whether a file is being mapped under a DAX inode.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Acked-by: David Hildenbrand <david@...hat.com>
Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>
---
 include/linux/fs.h |  7 ++++++-
 mm/ksm.c           | 32 ++++++++++++++++++++------------
 2 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 09c8495dacdb..e1397e2b55ea 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3691,9 +3691,14 @@ void setattr_copy(struct mnt_idmap *, struct inode *inode,
 
 extern int file_update_time(struct file *file);
 
+static inline bool file_is_dax(const struct file *file)
+{
+	return file && IS_DAX(file->f_mapping->host);
+}
+
 static inline bool vma_is_dax(const struct vm_area_struct *vma)
 {
-	return vma->vm_file && IS_DAX(vma->vm_file->f_mapping->host);
+	return file_is_dax(vma->vm_file);
 }
 
 static inline bool vma_is_fsdax(struct vm_area_struct *vma)
diff --git a/mm/ksm.c b/mm/ksm.c
index 8583fb91ef13..08d486f188ff 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -677,28 +677,33 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_v
 	return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
 }
 
-static bool vma_ksm_compatible(struct vm_area_struct *vma)
+static bool ksm_compatible(const struct file *file, vm_flags_t vm_flags)
 {
-	if (vma->vm_flags & (VM_SHARED  | VM_MAYSHARE   | VM_PFNMAP  |
-			     VM_IO      | VM_DONTEXPAND | VM_HUGETLB |
-			     VM_MIXEDMAP| VM_DROPPABLE))
+	if (vm_flags & (VM_SHARED   | VM_MAYSHARE   | VM_PFNMAP  |
+			VM_IO       | VM_DONTEXPAND | VM_HUGETLB |
+			VM_MIXEDMAP | VM_DROPPABLE))
 		return false;		/* just ignore the advice */
 
-	if (vma_is_dax(vma))
+	if (file_is_dax(file))
 		return false;
 
 #ifdef VM_SAO
-	if (vma->vm_flags & VM_SAO)
+	if (vm_flags & VM_SAO)
 		return false;
 #endif
 #ifdef VM_SPARC_ADI
-	if (vma->vm_flags & VM_SPARC_ADI)
+	if (vm_flags & VM_SPARC_ADI)
 		return false;
 #endif
 
 	return true;
 }
 
+static bool vma_ksm_compatible(struct vm_area_struct *vma)
+{
+	return ksm_compatible(vma->vm_file, vma->vm_flags);
+}
+
 static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
 		unsigned long addr)
 {
@@ -2696,14 +2701,17 @@ static int ksm_scan_thread(void *nothing)
 	return 0;
 }
 
-static void __ksm_add_vma(struct vm_area_struct *vma)
+static bool __ksm_should_add_vma(const struct file *file, vm_flags_t vm_flags)
 {
-	unsigned long vm_flags = vma->vm_flags;
-
 	if (vm_flags & VM_MERGEABLE)
-		return;
+		return false;
+
+	return ksm_compatible(file, vm_flags);
+}
 
-	if (vma_ksm_compatible(vma))
+static void __ksm_add_vma(struct vm_area_struct *vma)
+{
+	if (__ksm_should_add_vma(vma->vm_file, vma->vm_flags))
 		vm_flags_set(vma, VM_MERGEABLE);
 }
 
-- 
2.49.0