linux-kernel - [RFC PATCH 31/37] ext4: implement speculative fault handling

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Date:   Tue,  6 Apr 2021 18:44:56 -0700
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Linux-MM <linux-mm@...ck.org>
Cc:     Laurent Dufour <ldufour@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>,
        Linux-Kernel <linux-kernel@...r.kernel.org>,
        Michel Lespinasse <michel@...pinasse.org>
Subject: [RFC PATCH 31/37] ext4: implement speculative fault handling

We just need to make sure ext4_filemap_fault() doesn't block in the
speculative case as it is called with an rcu read lock held.

Signed-off-by: Michel Lespinasse <michel@...pinasse.org>
---
 fs/ext4/file.c  | 1 +
 fs/ext4/inode.c | 7 ++++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 194f5d00fa32..546b9d4aa9d7 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -742,6 +742,7 @@ static const struct vm_operations_struct ext4_file_vm_ops = {
 	.fault		= ext4_filemap_fault,
 	.map_pages	= filemap_map_pages,
 	.page_mkwrite   = ext4_page_mkwrite,
+	.speculative	= true,
 };
 
 static int ext4_file_mmap(struct file *file, struct vm_area_struct *vma)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 0948a43f1b3d..370484403c71 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -6192,7 +6192,12 @@ vm_fault_t ext4_filemap_fault(struct vm_fault *vmf)
 	struct inode *inode = file_inode(vmf->vma->vm_file);
 	vm_fault_t ret;
 
-	down_read(&EXT4_I(inode)->i_mmap_sem);
+	if (vmf->flags & FAULT_FLAG_SPECULATIVE) {
+		if (!down_read_trylock(&EXT4_I(inode)->i_mmap_sem))
+			return VM_FAULT_RETRY;
+	} else {
+		down_read(&EXT4_I(inode)->i_mmap_sem);
+	}
 	ret = filemap_fault(vmf);
 	up_read(&EXT4_I(inode)->i_mmap_sem);
 
-- 
2.20.1