linux-ext4 - [PATCH] fallocate: Add support for fixed goal extent allocations

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Message-ID: <20241212224958.62905-1-sarthakkukreti@google.com>
Date: Thu, 12 Dec 2024 14:49:58 -0800
From: Sarthak Kukreti <sarthakkukreti@...gle.com>
To: linux-ext4@...r.kernel.org
Cc: Andreas Dilger <adilger.kernel@...ger.ca>, "Theodore Ts'o" <tytso@....edu>, 
	Sarthak Kukreti <sarthakkukreti@...gle.com>
Subject: [PATCH] fallocate: Add support for fixed goal extent allocations

Add a new flag to add support for fixed goal allocations in
ext_falloc_helper. For fixed goal allocations, omit merging extents and
return an error unless the exact extent is found.

Use case:
On ChromiumOS, we'd like to add the capability of resetting a filesystem
while preserving a set of files in-place. This will be used during
filesystem reset flows where everything apart from select files (which
contain system applications) should be removed: the combined size of the
files can exceed the amount of available space in other
partitions/memory. The reset process will look something like:

1. Reset code dumps the FIEMAP of the set of preserved files into a
file.
2. Mkfs.ext4 is called on the filesystem with -E nodiscard.
3. Post mkfs, the reset code will utilize ext2fs_fallocate w/
EXT2_FALLOCATE_FIXED_GOAL | EXT2_FALLOCATE_FORCE_INIT on the extent list
created in step 1.

Signed-off-by: Sarthak Kukreti <sarthakkukreti@...gle.com>
---
 lib/ext2fs/ext2fs.h    |  3 ++-
 lib/ext2fs/fallocate.c | 21 +++++++++++++++++++--
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/lib/ext2fs/ext2fs.h b/lib/ext2fs/ext2fs.h
index 6e87829f..313c5981 100644
--- a/lib/ext2fs/ext2fs.h
+++ b/lib/ext2fs/ext2fs.h
@@ -1446,7 +1446,8 @@ extern errcode_t ext2fs_decode_extent(struct ext2fs_extent *to, void *from,
 #define EXT2_FALLOCATE_FORCE_INIT	(0x2)
 #define EXT2_FALLOCATE_FORCE_UNINIT	(0x4)
 #define EXT2_FALLOCATE_INIT_BEYOND_EOF	(0x8)
-#define EXT2_FALLOCATE_ALL_FLAGS	(0xF)
+#define EXT2_FALLOCATE_FIXED_GOAL	(0x10)
+#define EXT2_FALLOCATE_ALL_FLAGS	(0x1F)
 errcode_t ext2fs_fallocate(ext2_filsys fs, int flags, ext2_ino_t ino,
 			   struct ext2_inode *inode, blk64_t goal,
 			   blk64_t start, blk64_t len);
diff --git a/lib/ext2fs/fallocate.c b/lib/ext2fs/fallocate.c
index 5cde7d5c..20aa9c9f 100644
--- a/lib/ext2fs/fallocate.c
+++ b/lib/ext2fs/fallocate.c
@@ -103,7 +103,7 @@ static errcode_t ext_falloc_helper(ext2_filsys fs,
 				   blk64_t alloc_goal)
 {
 	struct ext2fs_extent	newex, ex;
-	int			op;
+	int			op, new_range_flags = 0;
 	blk64_t			fillable, pblk, plen, x, y;
 	blk64_t			eof_blk = 0, cluster_fill = 0;
 	errcode_t		err;
@@ -132,6 +132,9 @@ static errcode_t ext_falloc_helper(ext2_filsys fs,
 	max_uninit_len = EXT_UNINIT_MAX_LEN & ~EXT2FS_CLUSTER_MASK(fs);
 	max_init_len = EXT_INIT_MAX_LEN & ~EXT2FS_CLUSTER_MASK(fs);
 
+	if (flags & EXT2_FALLOCATE_FIXED_GOAL)
+		goto no_implied;
+
 	/* We must lengthen the left extent to the end of the cluster */
 	if (left_ext && EXT2FS_CLUSTER_RATIO(fs) > 1) {
 		/* How many more blocks can be attached to left_ext? */
@@ -605,12 +608,15 @@ no_implied:
 		max_extent_len = max_uninit_len;
 		newex.e_flags = EXT2_EXTENT_FLAGS_UNINIT;
 	}
+
+	if (flags & EXT2_FALLOCATE_FIXED_GOAL)
+		new_range_flags = EXT2_NEWRANGE_FIXED_GOAL | EXT2_NEWRANGE_MIN_LENGTH;
 	pblk = alloc_goal;
 	y = range_len;
 	for (x = 0; x < y;) {
 		cluster_fill = newex.e_lblk & EXT2FS_CLUSTER_MASK(fs);
 		fillable = min(range_len + cluster_fill, max_extent_len);
-		err = ext2fs_new_range(fs, 0, pblk & ~EXT2FS_CLUSTER_MASK(fs),
+		err = ext2fs_new_range(fs, new_range_flags, pblk & ~EXT2FS_CLUSTER_MASK(fs),
 				       fillable,
 				       NULL, &pblk, &plen);
 		if (err)
@@ -681,6 +687,16 @@ static errcode_t extent_fallocate(ext2_filsys fs, int flags, ext2_ino_t ino,
 	if (err)
 		return err;
 
+	/*
+	 * For fixed goal allocations, let the allocations fail iff we can't
+	 * find the exact goal extent.
+	 */
+	if (flags & EXT2_FALLOCATE_FIXED_GOAL) {
+		err = ext_falloc_helper(fs, flags, ino, inode, handle, NULL,
+					NULL, start, len, goal);
+		goto errout;
+	}
+
 	/*
 	 * Find the extent closest to the start of the alloc range.  We don't
 	 * check the return value because _goto() sets the current node to the
@@ -796,6 +812,7 @@ errout:
  * - EXT2_FALLOCATE_FORCE_INIT: Create only initialized extents.
  * - EXT2_FALLOCATE_FORCE_UNINIT: Create only uninitialized extents.
  * - EXT2_FALLOCATE_INIT_BEYOND_EOF: Create extents beyond EOF.
+ * - EXT2_FALLOCATE_FIXED_GOAL: Ensure range starts at goal.
  *
  * If neither FORCE_INIT nor FORCE_UNINIT are specified, this function will
  * try to expand any extents it finds, zeroing blocks as necessary.
-- 
2.47.0.rc1.288.g06298d1525-goog