[PATCH 3/3] RFC: dax: dax_prepare_freeze

[Boaz Harrosh <boaz@plexistor.com>]

From: Boaz Harrosh <boaz@plexistor.com>

When freezing an FS, we must write protect all IS_DAX()
inodes that have an mmap mapping on an inode. Otherwise
application will be able to modify previously faulted-in
file pages.

I'm actually doing a full unmap_mapping_range because
there is no readily available "mapping_write_protect" like
functionality. I do not think it is worth it to define one
just for here and just for some extra read-faults after an
fs_freeze.

How hot-path is fs_freeze at all?

CC: Jan Kara <jack@suse.cz>
CC: Matthew Wilcox <matthew.r.wilcox@intel.com>
CC: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Boaz Harrosh <boaz@plexistor.com>
---
 fs/dax.c           | 30 ++++++++++++++++++++++++++++++
 fs/super.c         |  3 +++
 include/linux/fs.h |  1 +
 3 files changed, 34 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index d0bd1f4..f3fc28b 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -549,3 +549,33 @@ int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
 	return dax_zero_page_range(inode, from, length, get_block);
 }
 EXPORT_SYMBOL_GPL(dax_truncate_page);
+
+/* This is meant to be called as part of freeze_super. otherwise we might
+ * Need some extra locking before calling here.
+ */
+void dax_prepare_freeze(struct super_block *sb)
+{
+	struct inode *inode;
+
+	/* TODO: each DAX fs has some private mount option to enable DAX. If
+	 * We made that option a generic MS_DAX_ENABLE super_block flag we could
+	 * Avoid the 95% extra unneeded loop-on-all-inodes every freeze.
+	 * if (!(sb->s_flags & MS_DAX_ENABLE))
+	 *	return 0;
+	 */
+
+	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+		/* TODO: For freezing we can actually do with write-protecting
+		 * the page. But I cannot find a ready made function that does
+		 * that for a giving mapping (with all the proper locking).
+		 * How performance sensitive is the all sb_freeze API?
+		 * For now we can just unmap the all mapping, and pay extra
+		 * on read faults.
+		 */
+		/* NOTE: Do not unmap private COW mapped pages it will not
+		 * modify the FS.
+		 */
+		if (IS_DAX(inode))
+			unmap_mapping_range(inode->i_mapping, 0, 0, 0);
+	}
+}
diff --git a/fs/super.c b/fs/super.c
index 2b7dc90..9ef490c 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1329,6 +1329,9 @@ int freeze_super(struct super_block *sb)
 	/* All writers are done so after syncing there won't be dirty data */
 	sync_filesystem(sb);
 
+	/* Need to take care of DAX mmaped inodes */
+	dax_prepare_freeze(sb);
+
 	/* Now wait for internal filesystem counter */
 	sb->s_writers.frozen = SB_FREEZE_FS;
 	smp_wmb();
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 24af817..3b943d4 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2599,6 +2599,7 @@ int dax_truncate_page(struct inode *, loff_t from, get_block_t);
 int dax_fault(struct vm_area_struct *, struct vm_fault *, get_block_t);
 int dax_pfn_mkwrite(struct vm_area_struct *, struct vm_fault *);
 #define dax_mkwrite(vma, vmf, gb)	dax_fault(vma, vmf, gb)
+void dax_prepare_freeze(struct super_block *sb);
 
 #ifdef CONFIG_BLOCK
 typedef void (dio_submit_t)(int rw, struct bio *bio, struct inode *inode,
-- 
1.9.3


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