linux-kernel - [PATCH RFC 07/19] mm: don't call vm_ops->huge_fault() in wp_huge_pmd()/wp_huge

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Message-Id: <20221107161740.144456-8-david@redhat.com>
Date:   Mon,  7 Nov 2022 17:17:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, etnaviv@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, linux-kselftest@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Nadav Amit <namit@...are.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        David Airlie <airlied@...il.com>,
        Oded Gabbay <ogabbay@...nel.org>, Arnd Bergmann <arnd@...db.de>
Subject: [PATCH RFC 07/19] mm: don't call vm_ops->huge_fault() in wp_huge_pmd()/wp_huge_pud() for private mappings

If we already have a PMD/PUD mapped write-protected in a private mapping
and we want to break COW either due to FAULT_FLAG_WRITE or
FAULT_FLAG_UNSHARE, there is no need to inform the file system just like on
the PTE path.

Let's just split (->zap) + fallback in that case.

This is a preparation for more generic FAULT_FLAG_UNSHARE support in
COW mappings.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/memory.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 41e4c697033a..d2f9673755be 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4791,6 +4791,7 @@ static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
 static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
 {
 	const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
+	vm_fault_t ret;
 
 	if (vma_is_anonymous(vmf->vma)) {
 		if (likely(!unshare) &&
@@ -4798,11 +4799,13 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
 			return handle_userfault(vmf, VM_UFFD_WP);
 		return do_huge_pmd_wp_page(vmf);
 	}
-	if (vmf->vma->vm_ops->huge_fault) {
-		vm_fault_t ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
 
-		if (!(ret & VM_FAULT_FALLBACK))
-			return ret;
+	if (vmf->vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
+		if (vmf->vma->vm_ops->huge_fault) {
+			ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
+			if (!(ret & VM_FAULT_FALLBACK))
+				return ret;
+		}
 	}
 
 	/* COW or write-notify handled on pte level: split pmd. */
@@ -4828,14 +4831,17 @@ static vm_fault_t wp_huge_pud(struct vm_fault *vmf, pud_t orig_pud)
 {
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) &&			\
 	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
+	vm_fault_t ret;
+
 	/* No support for anonymous transparent PUD pages yet */
 	if (vma_is_anonymous(vmf->vma))
 		goto split;
-	if (vmf->vma->vm_ops->huge_fault) {
-		vm_fault_t ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
-
-		if (!(ret & VM_FAULT_FALLBACK))
-			return ret;
+	if (vmf->vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
+		if (vmf->vma->vm_ops->huge_fault) {
+			ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
+			if (!(ret & VM_FAULT_FALLBACK))
+				return ret;
+		}
 	}
 split:
 	/* COW or write-notify not handled on PUD level: split pud.*/
-- 
2.38.1