linux-kernel - Re: [PATCH v7 6/8] fsdax: Dedup file range to use a compare function

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Date:   Mon, 23 Aug 2021 15:16:42 +0200
From:   Christoph Hellwig <hch@....de>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     djwong@...nel.org, hch@....de, linux-xfs@...r.kernel.org,
        dan.j.williams@...el.com, david@...morbit.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        nvdimm@...ts.linux.dev, rgoldwyn@...e.de, viro@...iv.linux.org.uk,
        willy@...radead.org, Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH v7 6/8] fsdax: Dedup file range to use a compare
 function

On Mon, Aug 16, 2021 at 02:03:57PM +0800, Shiyang Ruan wrote:
> +	id = dax_read_lock();
> +	while ((ret = iomap_iter2(&it_src, &it_dest, ops)) > 0) {
> +		it_src.processed = it_dest.processed =
> +			dax_range_compare_iter(&it_src, &it_dest, same);
> +	}
> +	dax_read_unlock(id);

I think it would be better to move the DAX locking into
dax_range_compare_iter to avoid very long hold times.

> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(dax_dedupe_file_range_compare);

No need for the export.

> +EXPORT_SYMBOL(dax_remap_file_range_prep);

EXPORT_SYMBOL_GPL, please.

Attached is a totally untested patch that just has two levels of
iterations instead of the new iter2 helper:


diff --git a/fs/dax.c b/fs/dax.c
index 0e0536765a7efc..2b65471785290d 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1885,6 +1885,7 @@ static loff_t dax_range_compare_iter(struct iomap_iter *it_src,
 	loff_t len = min(smap->length, dmap->length);
 	void *saddr, *daddr;
 	int ret;
+	int id;
 
 	if (smap->type == IOMAP_HOLE && dmap->type == IOMAP_HOLE) {
 		*same = true;
@@ -1896,47 +1897,56 @@ static loff_t dax_range_compare_iter(struct iomap_iter *it_src,
 		return 0;
 	}
 
+	id = dax_read_lock();
 	ret = dax_iomap_direct_access(smap, pos1, ALIGN(pos1 + len, PAGE_SIZE),
 				      &saddr, NULL);
 	if (ret < 0)
-		return -EIO;
+		goto out_unlock;
 
 	ret = dax_iomap_direct_access(dmap, pos2, ALIGN(pos2 + len, PAGE_SIZE),
 				      &daddr, NULL);
 	if (ret < 0)
-		return -EIO;
+		goto out_unlock;
 
 	*same = !memcmp(saddr, daddr, len);
 	if (!*same)
-		return 0;
+		len = 0;
+	dax_read_unlock(id);
 	return len;
+
+out_unlock:
+	dax_read_unlock(id);
+	return -EIO;
 }
 
 int dax_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
-		struct inode *dest, loff_t destoff, loff_t len, bool *same,
+		struct inode *dst, loff_t dstoff, loff_t len, bool *same,
 		const struct iomap_ops *ops)
 {
-	struct iomap_iter it_src = {
+	struct iomap_iter src_iter = {
 		.inode		= src,
 		.pos		= srcoff,
 		.len		= len,
 	};
-	struct iomap_iter it_dest = {
-		.inode		= dest,
-		.pos		= destoff,
+	struct iomap_iter dst_iter = {
+		.inode		= dst,
+		.pos		= dstoff,
 		.len		= len,
 	};
-	int id, ret;
+	int ret;
 
-	id = dax_read_lock();
-	while ((ret = iomap_iter2(&it_src, &it_dest, ops)) > 0) {
-		it_src.processed = it_dest.processed =
-			dax_range_compare_iter(&it_src, &it_dest, same);
+	while ((ret = iomap_iter(&src_iter, ops)) > 0) {
+		while ((ret = iomap_iter(&dst_iter, ops)) > 0) {
+			dst_iter.processed = dax_range_compare_iter(&src_iter,
+					&dst_iter, same);
+			if (dst_iter.processed > 0)
+				src_iter.processed += dst_iter.processed;
+			else if (!src_iter.processed)
+				src_iter.processed = dst_iter.processed;
+		}
 	}
-	dax_read_unlock(id);
 	return ret;
 }
-EXPORT_SYMBOL_GPL(dax_dedupe_file_range_compare);
 
 int dax_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 			      struct file *file_out, loff_t pos_out,
@@ -1946,4 +1956,4 @@ int dax_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 	return __generic_remap_file_range_prep(file_in, pos_in, file_out,
 					       pos_out, len, remap_flags, ops);
 }
-EXPORT_SYMBOL(dax_remap_file_range_prep);
+EXPORT_SYMBOL_GPL(dax_remap_file_range_prep);