[PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[Mark Harmstone]

Add a new unprivileged BTRFS_IOC_GET_CSUMS ioctl, which can be used to
query the on-disk csums for a file.

This is done by userspace passing a struct btrfs_ioctl_get_csums_args to
the kernel, which details the offset and length we're interested in, and
a buffer for the kernel to write its results into. The kernel writes a
struct btrfs_ioctl_get_csums_entry into the buffer, followed by the
csums if available.

If the extent is an uncompressed, non-nodatasum extent, the kernel sets
the entry type to BTRFS_GET_CSUMS_HAS_CSUMS and follows it with the
csums. If it is sparse, preallocated, or beyond the EOF, it sets the
type to BTRFS_GET_CSUMS_SPARSE - this is so userspace knows it can use
the precomputed hash of the zero sector. Otherwise, it sets the type to
BTRFS_GET_CSUMS_NO_CSUMS.

We do store the csums of compressed extents, but we deliberately don't
return them here: they're hashed over the compressed data, not the
uncompressed data that's returned to userspace.

The main use case for this is for speeding up mkfs.btrfs --rootdir. For
the case when the source FS is btrfs and using the same csum algorithm,
we can avoid having to recalculate the csums - in my synthetic
benchmarks (16GB file on a spinning-rust drive), this resulted in a ~11%
speed-up (218s to 196s).

When using the --reflink option added in btrfs-progs v6.16.1, we can forgo
reading the data entirely, resulting a ~2200% speed-up on the same test
(128s to 6s).

    # mkdir rootdir
    # dd if=/dev/urandom of=rootdir/file bs=4096 count=4194304

    (without ioctl)
    # echo 3 > /proc/sys/vm/drop_caches
    # time mkfs.btrfs --rootdir rootdir testimg
    ...
    real    3m37.965s
    user    0m5.496s
    sys     0m6.125s

    # echo 3 > /proc/sys/vm/drop_caches
    # time mkfs.btrfs --rootdir rootdir --reflink testimg
    ...
    real    2m8.342s
    user    0m5.472s
    sys     0m1.667s

    (with ioctl)
    # echo 3 > /proc/sys/vm/drop_caches
    # time mkfs.btrfs --rootdir rootdir testimg
    ...
    real    3m15.865s
    user    0m4.258s
    sys     0m6.261s

    # echo 3 > /proc/sys/vm/drop_caches
    # time mkfs.btrfs --rootdir rootdir --reflink testimg
    ...
    real    0m5.847s
    user    0m2.899s
    sys     0m0.097s

Signed-off-by: Mark Harmstone <mark@harmstone.com>
---
 fs/btrfs/ioctl.c           | 330 +++++++++++++++++++++++++++++++++++++
 include/uapi/linux/btrfs.h |  21 +++
 2 files changed, 351 insertions(+)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index a4d715bbed57ba..b7c8bfb90fed29 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -56,6 +56,7 @@
 #include "uuid-tree.h"
 #include "ioctl.h"
 #include "file.h"
+#include "file-item.h"
 #include "scrub.h"
 #include "super.h"
 
@@ -5138,6 +5139,333 @@ static int btrfs_ioctl_shutdown(struct btrfs_fs_info *fs_info, unsigned long arg
 }
 #endif
 
+#define GET_CSUMS_BUF_MAX	(16 * 1024 * 1024)
+
+static int copy_csums_to_user(struct btrfs_fs_info *fs_info, u64 disk_bytenr,
+			      u64 len, u8 __user *buf)
+{
+	struct btrfs_root *csum_root;
+	struct btrfs_ordered_sum *sums;
+	LIST_HEAD(list);
+	const u32 csum_size = fs_info->csum_size;
+	int ret;
+
+	csum_root = btrfs_csum_root(fs_info, disk_bytenr);
+
+	ret = btrfs_lookup_csums_list(csum_root, disk_bytenr,
+				      disk_bytenr + len - 1, &list, false);
+	if (ret < 0)
+		return ret;
+
+	/* Clear the output buffer to handle potential gaps in csum coverage. */
+	if (clear_user(buf, (len >> fs_info->sectorsize_bits) * csum_size)) {
+		ret = -EFAULT;
+		goto out;
+	}
+
+	ret = 0;
+	while (!list_empty(&list)) {
+		u64 offset;
+		size_t copy_size;
+
+		sums = list_first_entry(&list, struct btrfs_ordered_sum, list);
+		list_del(&sums->list);
+
+		offset = ((sums->logical - disk_bytenr) >> fs_info->sectorsize_bits) * csum_size;
+		copy_size = (sums->len >> fs_info->sectorsize_bits) * csum_size;
+
+		if (copy_to_user(buf + offset, sums->sums, copy_size)) {
+			kfree(sums);
+			ret = -EFAULT;
+			goto out;
+		}
+
+		kfree(sums);
+	}
+
+out:
+	while (!list_empty(&list)) {
+		sums = list_first_entry(&list, struct btrfs_ordered_sum, list);
+		list_del(&sums->list);
+		kfree(sums);
+	}
+	return ret;
+}
+
+static int btrfs_ioctl_get_csums(struct file *file, void __user *argp)
+{
+	struct inode *inode = file_inode(file);
+	struct btrfs_inode *bi = BTRFS_I(inode);
+	struct btrfs_fs_info *fs_info = bi->root->fs_info;
+	struct btrfs_root *root = bi->root;
+	struct btrfs_ioctl_get_csums_args args;
+	BTRFS_PATH_AUTO_FREE(path);
+	const u64 ino = btrfs_ino(bi);
+	const u32 sectorsize = fs_info->sectorsize;
+	const u32 csum_size = fs_info->csum_size;
+	u8 __user *ubuf;
+	u64 buf_limit;
+	u64 buf_used = 0;
+	u64 cur_offset;
+	u64 end_offset;
+	u64 prev_extent_end;
+	struct btrfs_key key;
+	int ret;
+
+	if (!(file->f_mode & FMODE_READ))
+		return -EBADF;
+
+	if (!S_ISREG(inode->i_mode))
+		return -EINVAL;
+
+	if (copy_from_user(&args, argp, sizeof(args)))
+		return -EFAULT;
+
+	if (!IS_ALIGNED(args.offset, sectorsize) ||
+	    !IS_ALIGNED(args.length, sectorsize))
+		return -EINVAL;
+	if (args.length == 0)
+		return -EINVAL;
+	if (args.offset + args.length < args.offset)
+		return -EOVERFLOW;
+	if (args.buf_size < sizeof(struct btrfs_ioctl_get_csums_entry))
+		return -EINVAL;
+
+	buf_limit = min_t(u64, args.buf_size, GET_CSUMS_BUF_MAX);
+	ubuf = (u8 __user *)(argp + offsetof(struct btrfs_ioctl_get_csums_args, buf));
+	cur_offset = args.offset;
+	end_offset = args.offset + args.length;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	ret = btrfs_wait_ordered_range(bi, cur_offset, args.length);
+	if (ret)
+		return ret;
+
+	btrfs_inode_lock(bi, BTRFS_ILOCK_SHARED);
+
+	ret = btrfs_wait_ordered_range(bi, cur_offset, args.length);
+	if (ret)
+		goto out_unlock;
+
+	/* NODATASUM early exit. */
+	if (bi->flags & BTRFS_INODE_NODATASUM) {
+		struct btrfs_ioctl_get_csums_entry entry = {
+			.offset = cur_offset,
+			.length = end_offset - cur_offset,
+			.type = BTRFS_GET_CSUMS_NO_CSUMS,
+		};
+
+		if (copy_to_user(ubuf, &entry, sizeof(entry))) {
+			ret = -EFAULT;
+			goto out_unlock;
+		}
+
+		buf_used = sizeof(entry);
+		cur_offset = end_offset;
+		goto done;
+	}
+
+	prev_extent_end = cur_offset;
+
+	while (cur_offset < end_offset) {
+		struct btrfs_file_extent_item *ei;
+		struct extent_buffer *leaf;
+		struct btrfs_ioctl_get_csums_entry entry;
+		u64 extent_end;
+		u64 disk_bytenr = 0;
+		u64 extent_offset = 0;
+		u64 range_start, range_len;
+		u64 entry_csum_size;
+		u64 key_offset;
+		int extent_type;
+		u8 compression;
+
+		/* Search for the extent at or before cur_offset. */
+		key.objectid = ino;
+		key.type = BTRFS_EXTENT_DATA_KEY;
+		key.offset = cur_offset;
+
+		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+		if (ret < 0)
+			goto out_unlock;
+
+		if (ret > 0 && path->slots[0] > 0) {
+			btrfs_item_key_to_cpu(path->nodes[0], &key,
+					      path->slots[0] - 1);
+			if (key.objectid == ino &&
+			    key.type == BTRFS_EXTENT_DATA_KEY) {
+				path->slots[0]--;
+				if (btrfs_file_extent_end(path) <= cur_offset)
+					path->slots[0]++;
+			}
+		}
+
+		if (path->slots[0] >= btrfs_header_nritems(path->nodes[0])) {
+			ret = btrfs_next_leaf(root, path);
+			if (ret < 0)
+				goto out_unlock;
+			if (ret > 0) {
+				ret = 0;
+				btrfs_release_path(path);
+				break;
+			}
+		}
+
+		leaf = path->nodes[0];
+
+		btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
+		if (key.objectid != ino || key.type != BTRFS_EXTENT_DATA_KEY) {
+			btrfs_release_path(path);
+			break;
+		}
+
+		extent_end = btrfs_file_extent_end(path);
+		key_offset = key.offset;
+
+		/* Read extent fields before releasing the path. */
+		ei = btrfs_item_ptr(leaf, path->slots[0],
+				    struct btrfs_file_extent_item);
+		extent_type = btrfs_file_extent_type(leaf, ei);
+		compression = btrfs_file_extent_compression(leaf, ei);
+
+		if (extent_type != BTRFS_FILE_EXTENT_INLINE) {
+			disk_bytenr = btrfs_file_extent_disk_bytenr(leaf, ei);
+			if (disk_bytenr && compression == BTRFS_COMPRESS_NONE)
+				extent_offset = btrfs_file_extent_offset(leaf, ei);
+		}
+
+		btrfs_release_path(path);
+
+		/* Implicit hole (NO_HOLES feature). */
+		if (prev_extent_end < key_offset) {
+			u64 hole_end = min(key_offset, end_offset);
+			u64 hole_len = hole_end - prev_extent_end;
+
+			if (prev_extent_end >= cur_offset) {
+				memset(&entry, 0, sizeof(entry));
+				entry.offset = prev_extent_end;
+				entry.length = hole_len;
+				entry.type = BTRFS_GET_CSUMS_SPARSE;
+
+				if (buf_used + sizeof(entry) > buf_limit)
+					goto done;
+				if (copy_to_user(ubuf + buf_used, &entry,
+						 sizeof(entry))) {
+					ret = -EFAULT;
+					goto out_unlock;
+				}
+				buf_used += sizeof(entry);
+				cur_offset = hole_end;
+			}
+
+			if (key_offset >= end_offset) {
+				cur_offset = end_offset;
+				break;
+			}
+		}
+
+		/* Clamp to our query range. */
+		range_start = max(cur_offset, key_offset);
+		range_len = min(extent_end, end_offset) - range_start;
+
+		memset(&entry, 0, sizeof(entry));
+		entry.offset = range_start;
+		entry.length = range_len;
+
+		if (extent_type == BTRFS_FILE_EXTENT_INLINE) {
+			entry.type = BTRFS_GET_CSUMS_NO_CSUMS;
+			entry_csum_size = 0;
+		} else if (extent_type == BTRFS_FILE_EXTENT_PREALLOC) {
+			entry.type = BTRFS_GET_CSUMS_SPARSE;
+			entry_csum_size = 0;
+		} else {
+			/* BTRFS_FILE_EXTENT_REG */
+			if (disk_bytenr == 0) {
+				/* Explicit hole. */
+				entry.type = BTRFS_GET_CSUMS_SPARSE;
+				entry_csum_size = 0;
+			} else if (compression != BTRFS_COMPRESS_NONE) {
+				entry.type = BTRFS_GET_CSUMS_NO_CSUMS;
+				entry_csum_size = 0;
+			} else {
+				entry.type = BTRFS_GET_CSUMS_HAS_CSUMS;
+				entry_csum_size = (range_len >> fs_info->sectorsize_bits) * csum_size;
+			}
+		}
+
+		/* Check if this entry (+ csum data) fits in the buffer. */
+		if (buf_used + sizeof(entry) + entry_csum_size > buf_limit) {
+			if (buf_used == 0) {
+				ret = -EOVERFLOW;
+				goto out_unlock;
+			}
+			goto done;
+		}
+
+		if (copy_to_user(ubuf + buf_used, &entry, sizeof(entry))) {
+			ret = -EFAULT;
+			goto out_unlock;
+		}
+		buf_used += sizeof(entry);
+
+		if (entry.type == BTRFS_GET_CSUMS_HAS_CSUMS) {
+			ret = copy_csums_to_user(fs_info,
+				disk_bytenr + extent_offset + (range_start - key_offset),
+				range_len, ubuf + buf_used);
+			if (ret)
+				goto out_unlock;
+			buf_used += entry_csum_size;
+		}
+
+		cur_offset = range_start + range_len;
+		prev_extent_end = extent_end;
+
+		if (fatal_signal_pending(current)) {
+			if (buf_used == 0) {
+				ret = -EINTR;
+				goto out_unlock;
+			}
+			goto done;
+		}
+
+		cond_resched();
+	}
+
+	/* Handle trailing implicit hole. */
+	if (cur_offset < end_offset) {
+		struct btrfs_ioctl_get_csums_entry entry = {
+			.offset = prev_extent_end,
+			.length = end_offset - prev_extent_end,
+			.type = BTRFS_GET_CSUMS_SPARSE,
+		};
+
+		if (buf_used + sizeof(entry) <= buf_limit) {
+			if (copy_to_user(ubuf + buf_used, &entry,
+					 sizeof(entry))) {
+				ret = -EFAULT;
+				goto out_unlock;
+			}
+			buf_used += sizeof(entry);
+			cur_offset = end_offset;
+		}
+	}
+
+done:
+	args.offset = cur_offset;
+	args.length = (cur_offset < end_offset) ? end_offset - cur_offset : 0;
+	args.buf_size = buf_used;
+
+	if (copy_to_user(argp, &args, sizeof(args)))
+		ret = -EFAULT;
+
+out_unlock:
+	btrfs_inode_unlock(bi, BTRFS_ILOCK_SHARED);
+	return ret;
+}
+
 long btrfs_ioctl(struct file *file, unsigned int
 		cmd, unsigned long arg)
 {
@@ -5293,6 +5621,8 @@ long btrfs_ioctl(struct file *file, unsigned int
 #endif
 	case BTRFS_IOC_SUBVOL_SYNC_WAIT:
 		return btrfs_ioctl_subvol_sync(fs_info, argp);
+	case BTRFS_IOC_GET_CSUMS:
+		return btrfs_ioctl_get_csums(file, argp);
 #ifdef CONFIG_BTRFS_EXPERIMENTAL
 	case BTRFS_IOC_SHUTDOWN:
 		return btrfs_ioctl_shutdown(fs_info, arg);
diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index 9165154a274d94..db1374c892f825 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -1100,6 +1100,25 @@ enum btrfs_err_code {
 	BTRFS_ERROR_DEV_RAID1C4_MIN_NOT_MET,
 };
 
+/* Types for struct btrfs_ioctl_get_csums_entry::type */
+#define BTRFS_GET_CSUMS_HAS_CSUMS	0
+#define BTRFS_GET_CSUMS_SPARSE		1
+#define BTRFS_GET_CSUMS_NO_CSUMS	2
+
+struct btrfs_ioctl_get_csums_entry {
+	__u64 offset;		/* file offset of this range */
+	__u64 length;		/* length in bytes */
+	__u32 type;		/* BTRFS_GET_CSUMS_* type */
+	__u32 reserved;		/* padding, must be 0 */
+};
+
+struct btrfs_ioctl_get_csums_args {
+	__u64 offset;		/* in/out: file offset */
+	__u64 length;		/* in/out: range length */
+	__u64 buf_size;		/* in/out: buffer capacity / bytes written */
+	__u8 buf[];		/* out: entries + csum data */
+};
+
 /* Flags for IOC_SHUTDOWN, must match XFS_FSOP_GOING_FLAGS_* flags. */
 #define BTRFS_SHUTDOWN_FLAGS_DEFAULT			0x0
 #define BTRFS_SHUTDOWN_FLAGS_LOGFLUSH			0x1
@@ -1226,6 +1245,8 @@ enum btrfs_err_code {
 				     struct btrfs_ioctl_encoded_io_args)
 #define BTRFS_IOC_SUBVOL_SYNC_WAIT _IOW(BTRFS_IOCTL_MAGIC, 65, \
 					struct btrfs_ioctl_subvol_wait)
+#define BTRFS_IOC_GET_CSUMS _IOWR(BTRFS_IOCTL_MAGIC, 66, \
+				  struct btrfs_ioctl_get_csums_args)
 
 /* Shutdown ioctl should follow XFS's interfaces, thus not using btrfs magic. */
 #define BTRFS_IOC_SHUTDOWN	_IOR('X', 125, __u32)
-- 
2.52.0

Re: [PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[Mark Harmstone]

mkfs patches for this: https://github.com/kdave/btrfs-progs/pull/1096

On 20/03/2026 12.50 pm, Mark Harmstone wrote:
> Add a new unprivileged BTRFS_IOC_GET_CSUMS ioctl, which can be used to
> query the on-disk csums for a file.
> 
> This is done by userspace passing a struct btrfs_ioctl_get_csums_args to
> the kernel, which details the offset and length we're interested in, and
> a buffer for the kernel to write its results into. The kernel writes a
> struct btrfs_ioctl_get_csums_entry into the buffer, followed by the
> csums if available.
> 
> If the extent is an uncompressed, non-nodatasum extent, the kernel sets
> the entry type to BTRFS_GET_CSUMS_HAS_CSUMS and follows it with the
> csums. If it is sparse, preallocated, or beyond the EOF, it sets the
> type to BTRFS_GET_CSUMS_SPARSE - this is so userspace knows it can use
> the precomputed hash of the zero sector. Otherwise, it sets the type to
> BTRFS_GET_CSUMS_NO_CSUMS.
> 
> We do store the csums of compressed extents, but we deliberately don't
> return them here: they're hashed over the compressed data, not the
> uncompressed data that's returned to userspace.
> 
> The main use case for this is for speeding up mkfs.btrfs --rootdir. For
> the case when the source FS is btrfs and using the same csum algorithm,
> we can avoid having to recalculate the csums - in my synthetic
> benchmarks (16GB file on a spinning-rust drive), this resulted in a ~11%
> speed-up (218s to 196s).
> 
> When using the --reflink option added in btrfs-progs v6.16.1, we can forgo
> reading the data entirely, resulting a ~2200% speed-up on the same test
> (128s to 6s).
> 
>      # mkdir rootdir
>      # dd if=/dev/urandom of=rootdir/file bs=4096 count=4194304
> 
>      (without ioctl)
>      # echo 3 > /proc/sys/vm/drop_caches
>      # time mkfs.btrfs --rootdir rootdir testimg
>      ...
>      real    3m37.965s
>      user    0m5.496s
>      sys     0m6.125s
> 
>      # echo 3 > /proc/sys/vm/drop_caches
>      # time mkfs.btrfs --rootdir rootdir --reflink testimg
>      ...
>      real    2m8.342s
>      user    0m5.472s
>      sys     0m1.667s
> 
>      (with ioctl)
>      # echo 3 > /proc/sys/vm/drop_caches
>      # time mkfs.btrfs --rootdir rootdir testimg
>      ...
>      real    3m15.865s
>      user    0m4.258s
>      sys     0m6.261s
> 
>      # echo 3 > /proc/sys/vm/drop_caches
>      # time mkfs.btrfs --rootdir rootdir --reflink testimg
>      ...
>      real    0m5.847s
>      user    0m2.899s
>      sys     0m0.097s
> 
> Signed-off-by: Mark Harmstone <mark@harmstone.com>
> ---
>   fs/btrfs/ioctl.c           | 330 +++++++++++++++++++++++++++++++++++++
>   include/uapi/linux/btrfs.h |  21 +++
>   2 files changed, 351 insertions(+)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index a4d715bbed57ba..b7c8bfb90fed29 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -56,6 +56,7 @@
>   #include "uuid-tree.h"
>   #include "ioctl.h"
>   #include "file.h"
> +#include "file-item.h"
>   #include "scrub.h"
>   #include "super.h"
>   
> @@ -5138,6 +5139,333 @@ static int btrfs_ioctl_shutdown(struct btrfs_fs_info *fs_info, unsigned long arg
>   }
>   #endif
>   
> +#define GET_CSUMS_BUF_MAX	(16 * 1024 * 1024)
> +
> +static int copy_csums_to_user(struct btrfs_fs_info *fs_info, u64 disk_bytenr,
> +			      u64 len, u8 __user *buf)
> +{
> +	struct btrfs_root *csum_root;
> +	struct btrfs_ordered_sum *sums;
> +	LIST_HEAD(list);
> +	const u32 csum_size = fs_info->csum_size;
> +	int ret;
> +
> +	csum_root = btrfs_csum_root(fs_info, disk_bytenr);
> +
> +	ret = btrfs_lookup_csums_list(csum_root, disk_bytenr,
> +				      disk_bytenr + len - 1, &list, false);
> +	if (ret < 0)
> +		return ret;
> +
> +	/* Clear the output buffer to handle potential gaps in csum coverage. */
> +	if (clear_user(buf, (len >> fs_info->sectorsize_bits) * csum_size)) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
> +
> +	ret = 0;
> +	while (!list_empty(&list)) {
> +		u64 offset;
> +		size_t copy_size;
> +
> +		sums = list_first_entry(&list, struct btrfs_ordered_sum, list);
> +		list_del(&sums->list);
> +
> +		offset = ((sums->logical - disk_bytenr) >> fs_info->sectorsize_bits) * csum_size;
> +		copy_size = (sums->len >> fs_info->sectorsize_bits) * csum_size;
> +
> +		if (copy_to_user(buf + offset, sums->sums, copy_size)) {
> +			kfree(sums);
> +			ret = -EFAULT;
> +			goto out;
> +		}
> +
> +		kfree(sums);
> +	}
> +
> +out:
> +	while (!list_empty(&list)) {
> +		sums = list_first_entry(&list, struct btrfs_ordered_sum, list);
> +		list_del(&sums->list);
> +		kfree(sums);
> +	}
> +	return ret;
> +}
> +
> +static int btrfs_ioctl_get_csums(struct file *file, void __user *argp)
> +{
> +	struct inode *inode = file_inode(file);
> +	struct btrfs_inode *bi = BTRFS_I(inode);
> +	struct btrfs_fs_info *fs_info = bi->root->fs_info;
> +	struct btrfs_root *root = bi->root;
> +	struct btrfs_ioctl_get_csums_args args;
> +	BTRFS_PATH_AUTO_FREE(path);
> +	const u64 ino = btrfs_ino(bi);
> +	const u32 sectorsize = fs_info->sectorsize;
> +	const u32 csum_size = fs_info->csum_size;
> +	u8 __user *ubuf;
> +	u64 buf_limit;
> +	u64 buf_used = 0;
> +	u64 cur_offset;
> +	u64 end_offset;
> +	u64 prev_extent_end;
> +	struct btrfs_key key;
> +	int ret;
> +
> +	if (!(file->f_mode & FMODE_READ))
> +		return -EBADF;
> +
> +	if (!S_ISREG(inode->i_mode))
> +		return -EINVAL;
> +
> +	if (copy_from_user(&args, argp, sizeof(args)))
> +		return -EFAULT;
> +
> +	if (!IS_ALIGNED(args.offset, sectorsize) ||
> +	    !IS_ALIGNED(args.length, sectorsize))
> +		return -EINVAL;
> +	if (args.length == 0)
> +		return -EINVAL;
> +	if (args.offset + args.length < args.offset)
> +		return -EOVERFLOW;
> +	if (args.buf_size < sizeof(struct btrfs_ioctl_get_csums_entry))
> +		return -EINVAL;
> +
> +	buf_limit = min_t(u64, args.buf_size, GET_CSUMS_BUF_MAX);
> +	ubuf = (u8 __user *)(argp + offsetof(struct btrfs_ioctl_get_csums_args, buf));
> +	cur_offset = args.offset;
> +	end_offset = args.offset + args.length;
> +
> +	path = btrfs_alloc_path();
> +	if (!path)
> +		return -ENOMEM;
> +
> +	ret = btrfs_wait_ordered_range(bi, cur_offset, args.length);
> +	if (ret)
> +		return ret;
> +
> +	btrfs_inode_lock(bi, BTRFS_ILOCK_SHARED);
> +
> +	ret = btrfs_wait_ordered_range(bi, cur_offset, args.length);
> +	if (ret)
> +		goto out_unlock;
> +
> +	/* NODATASUM early exit. */
> +	if (bi->flags & BTRFS_INODE_NODATASUM) {
> +		struct btrfs_ioctl_get_csums_entry entry = {
> +			.offset = cur_offset,
> +			.length = end_offset - cur_offset,
> +			.type = BTRFS_GET_CSUMS_NO_CSUMS,
> +		};
> +
> +		if (copy_to_user(ubuf, &entry, sizeof(entry))) {
> +			ret = -EFAULT;
> +			goto out_unlock;
> +		}
> +
> +		buf_used = sizeof(entry);
> +		cur_offset = end_offset;
> +		goto done;
> +	}
> +
> +	prev_extent_end = cur_offset;
> +
> +	while (cur_offset < end_offset) {
> +		struct btrfs_file_extent_item *ei;
> +		struct extent_buffer *leaf;
> +		struct btrfs_ioctl_get_csums_entry entry;
> +		u64 extent_end;
> +		u64 disk_bytenr = 0;
> +		u64 extent_offset = 0;
> +		u64 range_start, range_len;
> +		u64 entry_csum_size;
> +		u64 key_offset;
> +		int extent_type;
> +		u8 compression;
> +
> +		/* Search for the extent at or before cur_offset. */
> +		key.objectid = ino;
> +		key.type = BTRFS_EXTENT_DATA_KEY;
> +		key.offset = cur_offset;
> +
> +		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
> +		if (ret < 0)
> +			goto out_unlock;
> +
> +		if (ret > 0 && path->slots[0] > 0) {
> +			btrfs_item_key_to_cpu(path->nodes[0], &key,
> +					      path->slots[0] - 1);
> +			if (key.objectid == ino &&
> +			    key.type == BTRFS_EXTENT_DATA_KEY) {
> +				path->slots[0]--;
> +				if (btrfs_file_extent_end(path) <= cur_offset)
> +					path->slots[0]++;
> +			}
> +		}
> +
> +		if (path->slots[0] >= btrfs_header_nritems(path->nodes[0])) {
> +			ret = btrfs_next_leaf(root, path);
> +			if (ret < 0)
> +				goto out_unlock;
> +			if (ret > 0) {
> +				ret = 0;
> +				btrfs_release_path(path);
> +				break;
> +			}
> +		}
> +
> +		leaf = path->nodes[0];
> +
> +		btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
> +		if (key.objectid != ino || key.type != BTRFS_EXTENT_DATA_KEY) {
> +			btrfs_release_path(path);
> +			break;
> +		}
> +
> +		extent_end = btrfs_file_extent_end(path);
> +		key_offset = key.offset;
> +
> +		/* Read extent fields before releasing the path. */
> +		ei = btrfs_item_ptr(leaf, path->slots[0],
> +				    struct btrfs_file_extent_item);
> +		extent_type = btrfs_file_extent_type(leaf, ei);
> +		compression = btrfs_file_extent_compression(leaf, ei);
> +
> +		if (extent_type != BTRFS_FILE_EXTENT_INLINE) {
> +			disk_bytenr = btrfs_file_extent_disk_bytenr(leaf, ei);
> +			if (disk_bytenr && compression == BTRFS_COMPRESS_NONE)
> +				extent_offset = btrfs_file_extent_offset(leaf, ei);
> +		}
> +
> +		btrfs_release_path(path);
> +
> +		/* Implicit hole (NO_HOLES feature). */
> +		if (prev_extent_end < key_offset) {
> +			u64 hole_end = min(key_offset, end_offset);
> +			u64 hole_len = hole_end - prev_extent_end;
> +
> +			if (prev_extent_end >= cur_offset) {
> +				memset(&entry, 0, sizeof(entry));
> +				entry.offset = prev_extent_end;
> +				entry.length = hole_len;
> +				entry.type = BTRFS_GET_CSUMS_SPARSE;
> +
> +				if (buf_used + sizeof(entry) > buf_limit)
> +					goto done;
> +				if (copy_to_user(ubuf + buf_used, &entry,
> +						 sizeof(entry))) {
> +					ret = -EFAULT;
> +					goto out_unlock;
> +				}
> +				buf_used += sizeof(entry);
> +				cur_offset = hole_end;
> +			}
> +
> +			if (key_offset >= end_offset) {
> +				cur_offset = end_offset;
> +				break;
> +			}
> +		}
> +
> +		/* Clamp to our query range. */
> +		range_start = max(cur_offset, key_offset);
> +		range_len = min(extent_end, end_offset) - range_start;
> +
> +		memset(&entry, 0, sizeof(entry));
> +		entry.offset = range_start;
> +		entry.length = range_len;
> +
> +		if (extent_type == BTRFS_FILE_EXTENT_INLINE) {
> +			entry.type = BTRFS_GET_CSUMS_NO_CSUMS;
> +			entry_csum_size = 0;
> +		} else if (extent_type == BTRFS_FILE_EXTENT_PREALLOC) {
> +			entry.type = BTRFS_GET_CSUMS_SPARSE;
> +			entry_csum_size = 0;
> +		} else {
> +			/* BTRFS_FILE_EXTENT_REG */
> +			if (disk_bytenr == 0) {
> +				/* Explicit hole. */
> +				entry.type = BTRFS_GET_CSUMS_SPARSE;
> +				entry_csum_size = 0;
> +			} else if (compression != BTRFS_COMPRESS_NONE) {
> +				entry.type = BTRFS_GET_CSUMS_NO_CSUMS;
> +				entry_csum_size = 0;
> +			} else {
> +				entry.type = BTRFS_GET_CSUMS_HAS_CSUMS;
> +				entry_csum_size = (range_len >> fs_info->sectorsize_bits) * csum_size;
> +			}
> +		}
> +
> +		/* Check if this entry (+ csum data) fits in the buffer. */
> +		if (buf_used + sizeof(entry) + entry_csum_size > buf_limit) {
> +			if (buf_used == 0) {
> +				ret = -EOVERFLOW;
> +				goto out_unlock;
> +			}
> +			goto done;
> +		}
> +
> +		if (copy_to_user(ubuf + buf_used, &entry, sizeof(entry))) {
> +			ret = -EFAULT;
> +			goto out_unlock;
> +		}
> +		buf_used += sizeof(entry);
> +
> +		if (entry.type == BTRFS_GET_CSUMS_HAS_CSUMS) {
> +			ret = copy_csums_to_user(fs_info,
> +				disk_bytenr + extent_offset + (range_start - key_offset),
> +				range_len, ubuf + buf_used);
> +			if (ret)
> +				goto out_unlock;
> +			buf_used += entry_csum_size;
> +		}
> +
> +		cur_offset = range_start + range_len;
> +		prev_extent_end = extent_end;
> +
> +		if (fatal_signal_pending(current)) {
> +			if (buf_used == 0) {
> +				ret = -EINTR;
> +				goto out_unlock;
> +			}
> +			goto done;
> +		}
> +
> +		cond_resched();
> +	}
> +
> +	/* Handle trailing implicit hole. */
> +	if (cur_offset < end_offset) {
> +		struct btrfs_ioctl_get_csums_entry entry = {
> +			.offset = prev_extent_end,
> +			.length = end_offset - prev_extent_end,
> +			.type = BTRFS_GET_CSUMS_SPARSE,
> +		};
> +
> +		if (buf_used + sizeof(entry) <= buf_limit) {
> +			if (copy_to_user(ubuf + buf_used, &entry,
> +					 sizeof(entry))) {
> +				ret = -EFAULT;
> +				goto out_unlock;
> +			}
> +			buf_used += sizeof(entry);
> +			cur_offset = end_offset;
> +		}
> +	}
> +
> +done:
> +	args.offset = cur_offset;
> +	args.length = (cur_offset < end_offset) ? end_offset - cur_offset : 0;
> +	args.buf_size = buf_used;
> +
> +	if (copy_to_user(argp, &args, sizeof(args)))
> +		ret = -EFAULT;
> +
> +out_unlock:
> +	btrfs_inode_unlock(bi, BTRFS_ILOCK_SHARED);
> +	return ret;
> +}
> +
>   long btrfs_ioctl(struct file *file, unsigned int
>   		cmd, unsigned long arg)
>   {
> @@ -5293,6 +5621,8 @@ long btrfs_ioctl(struct file *file, unsigned int
>   #endif
>   	case BTRFS_IOC_SUBVOL_SYNC_WAIT:
>   		return btrfs_ioctl_subvol_sync(fs_info, argp);
> +	case BTRFS_IOC_GET_CSUMS:
> +		return btrfs_ioctl_get_csums(file, argp);
>   #ifdef CONFIG_BTRFS_EXPERIMENTAL
>   	case BTRFS_IOC_SHUTDOWN:
>   		return btrfs_ioctl_shutdown(fs_info, arg);
> diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
> index 9165154a274d94..db1374c892f825 100644
> --- a/include/uapi/linux/btrfs.h
> +++ b/include/uapi/linux/btrfs.h
> @@ -1100,6 +1100,25 @@ enum btrfs_err_code {
>   	BTRFS_ERROR_DEV_RAID1C4_MIN_NOT_MET,
>   };
>   
> +/* Types for struct btrfs_ioctl_get_csums_entry::type */
> +#define BTRFS_GET_CSUMS_HAS_CSUMS	0
> +#define BTRFS_GET_CSUMS_SPARSE		1
> +#define BTRFS_GET_CSUMS_NO_CSUMS	2
> +
> +struct btrfs_ioctl_get_csums_entry {
> +	__u64 offset;		/* file offset of this range */
> +	__u64 length;		/* length in bytes */
> +	__u32 type;		/* BTRFS_GET_CSUMS_* type */
> +	__u32 reserved;		/* padding, must be 0 */
> +};
> +
> +struct btrfs_ioctl_get_csums_args {
> +	__u64 offset;		/* in/out: file offset */
> +	__u64 length;		/* in/out: range length */
> +	__u64 buf_size;		/* in/out: buffer capacity / bytes written */
> +	__u8 buf[];		/* out: entries + csum data */
> +};
> +
>   /* Flags for IOC_SHUTDOWN, must match XFS_FSOP_GOING_FLAGS_* flags. */
>   #define BTRFS_SHUTDOWN_FLAGS_DEFAULT			0x0
>   #define BTRFS_SHUTDOWN_FLAGS_LOGFLUSH			0x1
> @@ -1226,6 +1245,8 @@ enum btrfs_err_code {
>   				     struct btrfs_ioctl_encoded_io_args)
>   #define BTRFS_IOC_SUBVOL_SYNC_WAIT _IOW(BTRFS_IOCTL_MAGIC, 65, \
>   					struct btrfs_ioctl_subvol_wait)
> +#define BTRFS_IOC_GET_CSUMS _IOWR(BTRFS_IOCTL_MAGIC, 66, \
> +				  struct btrfs_ioctl_get_csums_args)
>   
>   /* Shutdown ioctl should follow XFS's interfaces, thus not using btrfs magic. */
>   #define BTRFS_IOC_SHUTDOWN	_IOR('X', 125, __u32)

Re: [PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[Qu Wenruo]

在 2026/3/20 23:20, Mark Harmstone 写道:
> Add a new unprivileged BTRFS_IOC_GET_CSUMS ioctl, which can be used to
> query the on-disk csums for a file.
> 
> This is done by userspace passing a struct btrfs_ioctl_get_csums_args to
> the kernel, which details the offset and length we're interested in, and
> a buffer for the kernel to write its results into. The kernel writes a
> struct btrfs_ioctl_get_csums_entry into the buffer, followed by the
> csums if available.
> 
> If the extent is an uncompressed, non-nodatasum extent, the kernel sets
> the entry type to BTRFS_GET_CSUMS_HAS_CSUMS and follows it with the
> csums. If it is sparse, preallocated, or beyond the EOF, it sets the
> type to BTRFS_GET_CSUMS_SPARSE - this is so userspace knows it can use
> the precomputed hash of the zero sector. Otherwise, it sets the type to
> BTRFS_GET_CSUMS_NO_CSUMS.

I'm not sure if it's a good idea to put hole and preallocated range into 
the same BTRFS_GET_CSUMS_SPARSE.

Although both means there is no csum, hole case means there is really no 
data extent, thus we should not create any extent instead of writing zero.

For preallocated, indicating it has no CSUM can allow mkfs to 
distinguish hole and preallocated, thus change to zero writes to 
prealloc, which is faster and make the resulted fs more aligned to the 
source dir.


And for EOF checks, I think we don't need to bother that much, aka, just 
let it return the regular results.

My assumption is, the mkfs shouldn't pass a range completely beyond the 
round_up(i_size), as non-reflink rootdir population would always read 
out the content of the inode from the host fs.
Thus we won't really read beyond the inode size.

> 
> We do store the csums of compressed extents, but we deliberately don't
> return them here: they're hashed over the compressed data, not the
> uncompressed data that's returned to userspace.

I agree with the skip of compressed extents, but I'd prefer to have a 
special flag to indicate that, other than NO_CSUMS.

Or mkfs is unable to distinguish hole and compressed extents.

[...]
> +/* Types for struct btrfs_ioctl_get_csums_entry::type */
> +#define BTRFS_GET_CSUMS_HAS_CSUMS	0
> +#define BTRFS_GET_CSUMS_SPARSE		1
> +#define BTRFS_GET_CSUMS_NO_CSUMS	2
> +
> +struct btrfs_ioctl_get_csums_entry {
> +	__u64 offset;		/* file offset of this range */
> +	__u64 length;		/* length in bytes */
> +	__u32 type;		/* BTRFS_GET_CSUMS_* type */
> +	__u32 reserved;		/* padding, must be 0 */
> +};
> +
> +struct btrfs_ioctl_get_csums_args {
> +	__u64 offset;		/* in/out: file offset */
> +	__u64 length;		/* in/out: range length */
> +	__u64 buf_size;		/* in/out: buffer capacity / bytes written */
> +	__u8 buf[];		/* out: entries + csum data */
> +};

 From the progs usage, it is always a single btrfs_ioctl_get_csums_entry 
at the beginning of buf[], then real buffer for csum, can we just 
combine both structures into one?

Furthermore, since we only query one extent at one time, the 
offset/length are more or less duplicated between args and entry structure.

We can just save the length into the args without the need for entry 
members (except the type).

Thanks,
Qu

> +
>   /* Flags for IOC_SHUTDOWN, must match XFS_FSOP_GOING_FLAGS_* flags. */
>   #define BTRFS_SHUTDOWN_FLAGS_DEFAULT			0x0
>   #define BTRFS_SHUTDOWN_FLAGS_LOGFLUSH			0x1
> @@ -1226,6 +1245,8 @@ enum btrfs_err_code {
>   				     struct btrfs_ioctl_encoded_io_args)
>   #define BTRFS_IOC_SUBVOL_SYNC_WAIT _IOW(BTRFS_IOCTL_MAGIC, 65, \
>   					struct btrfs_ioctl_subvol_wait)
> +#define BTRFS_IOC_GET_CSUMS _IOWR(BTRFS_IOCTL_MAGIC, 66, \
> +				  struct btrfs_ioctl_get_csums_args)
>   
>   /* Shutdown ioctl should follow XFS's interfaces, thus not using btrfs magic. */
>   #define BTRFS_IOC_SHUTDOWN	_IOR('X', 125, __u32)

Re: [PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[Qu Wenruo]

在 2026/3/21 08:48, Qu Wenruo 写道:
> 
> 
> 在 2026/3/20 23:20, Mark Harmstone 写道:
>> Add a new unprivileged BTRFS_IOC_GET_CSUMS ioctl, which can be used to
>> query the on-disk csums for a file.
>>
>> This is done by userspace passing a struct btrfs_ioctl_get_csums_args to
>> the kernel, which details the offset and length we're interested in, and
>> a buffer for the kernel to write its results into. The kernel writes a
>> struct btrfs_ioctl_get_csums_entry into the buffer, followed by the
>> csums if available.
>>
>> If the extent is an uncompressed, non-nodatasum extent, the kernel sets
>> the entry type to BTRFS_GET_CSUMS_HAS_CSUMS and follows it with the
>> csums. If it is sparse, preallocated, or beyond the EOF, it sets the
>> type to BTRFS_GET_CSUMS_SPARSE - this is so userspace knows it can use
>> the precomputed hash of the zero sector. Otherwise, it sets the type to
>> BTRFS_GET_CSUMS_NO_CSUMS.
> 
> I'm not sure if it's a good idea to put hole and preallocated range into 
> the same BTRFS_GET_CSUMS_SPARSE.
> 
> Although both means there is no csum, hole case means there is really no 
> data extent, thus we should not create any extent instead of writing zero.
> 
> For preallocated, indicating it has no CSUM can allow mkfs to 
> distinguish hole and preallocated, thus change to zero writes to 
> prealloc, which is faster and make the resulted fs more aligned to the 
> source dir.
> 
> 
> And for EOF checks, I think we don't need to bother that much, aka, just 
> let it return the regular results.
> 
> My assumption is, the mkfs shouldn't pass a range completely beyond the 
> round_up(i_size), as non-reflink rootdir population would always read 
> out the content of the inode from the host fs.
> Thus we won't really read beyond the inode size.

After more investigation, I think we can put the 
hole/preallocation/compression detection into the user space.

The hole detection is already pending for merge, mostly through 
SEEK_DATA/SEEK_HOLE flags of lseek():

  https://github.com/kdave/btrfs-progs/pull/1097

I'm planning to implement preallocation detection through fiemap, which 
also allows us to detect compressed range and skip them for your case.

With all those features implemented in progs, we can further simplify 
the get csum ioctl, to something more aligned to 
btrfs_lookup_csums_bitmap().

We do not need to bother why there is no checksum for some ranges, that 
will be handled by progs first, we only need to return all the checksums 
found for the specified range.

And as an extra safenet, use some bitmap inthe ioctl structure to 
indicate which ranges have checksum and which doesn't.

This will definitely simplify the ioctl as we only need to do csum tree 
lookup, no need to bother anything in the subvolume tree.

Thanks,
Qu

> 
>>
>> We do store the csums of compressed extents, but we deliberately don't
>> return them here: they're hashed over the compressed data, not the
>> uncompressed data that's returned to userspace.
> 
> I agree with the skip of compressed extents, but I'd prefer to have a 
> special flag to indicate that, other than NO_CSUMS.
> 
> Or mkfs is unable to distinguish hole and compressed extents.
> 
> [...]
>> +/* Types for struct btrfs_ioctl_get_csums_entry::type */
>> +#define BTRFS_GET_CSUMS_HAS_CSUMS    0
>> +#define BTRFS_GET_CSUMS_SPARSE        1
>> +#define BTRFS_GET_CSUMS_NO_CSUMS    2
>> +
>> +struct btrfs_ioctl_get_csums_entry {
>> +    __u64 offset;        /* file offset of this range */
>> +    __u64 length;        /* length in bytes */
>> +    __u32 type;        /* BTRFS_GET_CSUMS_* type */
>> +    __u32 reserved;        /* padding, must be 0 */
>> +};
>> +
>> +struct btrfs_ioctl_get_csums_args {
>> +    __u64 offset;        /* in/out: file offset */
>> +    __u64 length;        /* in/out: range length */
>> +    __u64 buf_size;        /* in/out: buffer capacity / bytes written */
>> +    __u8 buf[];        /* out: entries + csum data */
>> +};
> 
>  From the progs usage, it is always a single btrfs_ioctl_get_csums_entry 
> at the beginning of buf[], then real buffer for csum, can we just 
> combine both structures into one?
> 
> Furthermore, since we only query one extent at one time, the offset/ 
> length are more or less duplicated between args and entry structure.
> 
> We can just save the length into the args without the need for entry 
> members (except the type).
> 
> Thanks,
> Qu
> 
>> +
>>   /* Flags for IOC_SHUTDOWN, must match XFS_FSOP_GOING_FLAGS_* flags. */
>>   #define BTRFS_SHUTDOWN_FLAGS_DEFAULT            0x0
>>   #define BTRFS_SHUTDOWN_FLAGS_LOGFLUSH            0x1
>> @@ -1226,6 +1245,8 @@ enum btrfs_err_code {
>>                        struct btrfs_ioctl_encoded_io_args)
>>   #define BTRFS_IOC_SUBVOL_SYNC_WAIT _IOW(BTRFS_IOCTL_MAGIC, 65, \
>>                       struct btrfs_ioctl_subvol_wait)
>> +#define BTRFS_IOC_GET_CSUMS _IOWR(BTRFS_IOCTL_MAGIC, 66, \
>> +                  struct btrfs_ioctl_get_csums_args)
>>   /* Shutdown ioctl should follow XFS's interfaces, thus not using 
>> btrfs magic. */
>>   #define BTRFS_IOC_SHUTDOWN    _IOR('X', 125, __u32)
> 
> 

Re: [PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[Mark Harmstone]

On 25/03/2026 7.34 am, Qu Wenruo wrote:
> 
> 
> 在 2026/3/21 08:48, Qu Wenruo 写道:
>>
>>
>> 在 2026/3/20 23:20, Mark Harmstone 写道:
>>> Add a new unprivileged BTRFS_IOC_GET_CSUMS ioctl, which can be used to
>>> query the on-disk csums for a file.
>>>
>>> This is done by userspace passing a struct btrfs_ioctl_get_csums_args to
>>> the kernel, which details the offset and length we're interested in, and
>>> a buffer for the kernel to write its results into. The kernel writes a
>>> struct btrfs_ioctl_get_csums_entry into the buffer, followed by the
>>> csums if available.
>>>
>>> If the extent is an uncompressed, non-nodatasum extent, the kernel sets
>>> the entry type to BTRFS_GET_CSUMS_HAS_CSUMS and follows it with the
>>> csums. If it is sparse, preallocated, or beyond the EOF, it sets the
>>> type to BTRFS_GET_CSUMS_SPARSE - this is so userspace knows it can use
>>> the precomputed hash of the zero sector. Otherwise, it sets the type to
>>> BTRFS_GET_CSUMS_NO_CSUMS.
>>
>> I'm not sure if it's a good idea to put hole and preallocated range 
>> into the same BTRFS_GET_CSUMS_SPARSE.
>>
>> Although both means there is no csum, hole case means there is really 
>> no data extent, thus we should not create any extent instead of 
>> writing zero.

Thanks Qu.

"SPARSE" is probably a bad name for it. It probably should be "ZERO" or 
somesuch. The point is to tell userspace not to waste time calculating 
csums, but use the precomputed values because the data would be zero 
(for whatever reason).

>> For preallocated, indicating it has no CSUM can allow mkfs to 
>> distinguish hole and preallocated, thus change to zero writes to 
>> prealloc, which is faster and make the resulted fs more aligned to the 
>> source dir.
>>
>>
>> And for EOF checks, I think we don't need to bother that much, aka, 
>> just let it return the regular results.
>>
>> My assumption is, the mkfs shouldn't pass a range completely beyond 
>> the round_up(i_size), as non-reflink rootdir population would always 
>> read out the content of the inode from the host fs.
>> Thus we won't really read beyond the inode size.
> 
> After more investigation, I think we can put the hole/preallocation/ 
> compression detection into the user space.
> 
> The hole detection is already pending for merge, mostly through 
> SEEK_DATA/SEEK_HOLE flags of lseek():
> 
>   https://github.com/kdave/btrfs-progs/pull/1097
> 
> I'm planning to implement preallocation detection through fiemap, which 
> also allows us to detect compressed range and skip them for your case.
> 
> With all those features implemented in progs, we can further simplify 
> the get csum ioctl, to something more aligned to 
> btrfs_lookup_csums_bitmap().
> 
> We do not need to bother why there is no checksum for some ranges, that 
> will be handled by progs first, we only need to return all the checksums 
> found for the specified range.
> 
> And as an extra safenet, use some bitmap inthe ioctl structure to 
> indicate which ranges have checksum and which doesn't.
> 
> This will definitely simplify the ioctl as we only need to do csum tree 
> lookup, no need to bother anything in the subvolume tree.

Unfortunately this won't work. You have to explicitly filter out 
compressed extents, and identifying these requires checking the FS tree. 
The reason is that they may be "bookended", and you would leak 
information about other files if you returned the whole of the csums for 
the compressed extent. This is the reason why encoded read needs root.

> Thanks,
> Qu
> 
>>
>>>
>>> We do store the csums of compressed extents, but we deliberately don't
>>> return them here: they're hashed over the compressed data, not the
>>> uncompressed data that's returned to userspace.
>>
>> I agree with the skip of compressed extents, but I'd prefer to have a 
>> special flag to indicate that, other than NO_CSUMS.
>>
>> Or mkfs is unable to distinguish hole and compressed extents.

Compressed extents result in NO_CSUMS, a hole results in SPARSE.

>> [...]
>>> +/* Types for struct btrfs_ioctl_get_csums_entry::type */
>>> +#define BTRFS_GET_CSUMS_HAS_CSUMS    0
>>> +#define BTRFS_GET_CSUMS_SPARSE        1
>>> +#define BTRFS_GET_CSUMS_NO_CSUMS    2
>>> +
>>> +struct btrfs_ioctl_get_csums_entry {
>>> +    __u64 offset;        /* file offset of this range */
>>> +    __u64 length;        /* length in bytes */
>>> +    __u32 type;        /* BTRFS_GET_CSUMS_* type */
>>> +    __u32 reserved;        /* padding, must be 0 */
>>> +};
>>> +
>>> +struct btrfs_ioctl_get_csums_args {
>>> +    __u64 offset;        /* in/out: file offset */
>>> +    __u64 length;        /* in/out: range length */
>>> +    __u64 buf_size;        /* in/out: buffer capacity / bytes 
>>> written */
>>> +    __u8 buf[];        /* out: entries + csum data */
>>> +};
>>
>>  From the progs usage, it is always a single 
>> btrfs_ioctl_get_csums_entry at the beginning of buf[], then real 
>> buffer for csum, can we just combine both structures into one?
>>
>> Furthermore, since we only query one extent at one time, the offset/ 
>> length are more or less duplicated between args and entry structure.
>>
>> We can just save the length into the args without the need for entry 
>> members (except the type).
>>
>> Thanks,
>> Qu
>>
>>> +
>>>   /* Flags for IOC_SHUTDOWN, must match XFS_FSOP_GOING_FLAGS_* flags. */
>>>   #define BTRFS_SHUTDOWN_FLAGS_DEFAULT            0x0
>>>   #define BTRFS_SHUTDOWN_FLAGS_LOGFLUSH            0x1
>>> @@ -1226,6 +1245,8 @@ enum btrfs_err_code {
>>>                        struct btrfs_ioctl_encoded_io_args)
>>>   #define BTRFS_IOC_SUBVOL_SYNC_WAIT _IOW(BTRFS_IOCTL_MAGIC, 65, \
>>>                       struct btrfs_ioctl_subvol_wait)
>>> +#define BTRFS_IOC_GET_CSUMS _IOWR(BTRFS_IOCTL_MAGIC, 66, \
>>> +                  struct btrfs_ioctl_get_csums_args)
>>>   /* Shutdown ioctl should follow XFS's interfaces, thus not using 
>>> btrfs magic. */
>>>   #define BTRFS_IOC_SHUTDOWN    _IOR('X', 125, __u32)
>>
>>
> 
> 

Re: [PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[Qu Wenruo]

在 2026/3/26 01:13, Mark Harmstone 写道:
> On 25/03/2026 7.34 am, Qu Wenruo wrote:
>>
>>
>> 在 2026/3/21 08:48, Qu Wenruo 写道:
>>>
>>>
>>> 在 2026/3/20 23:20, Mark Harmstone 写道:
>>>> Add a new unprivileged BTRFS_IOC_GET_CSUMS ioctl, which can be used to
>>>> query the on-disk csums for a file.
>>>>
>>>> This is done by userspace passing a struct 
>>>> btrfs_ioctl_get_csums_args to
>>>> the kernel, which details the offset and length we're interested in, 
>>>> and
>>>> a buffer for the kernel to write its results into. The kernel writes a
>>>> struct btrfs_ioctl_get_csums_entry into the buffer, followed by the
>>>> csums if available.
>>>>
>>>> If the extent is an uncompressed, non-nodatasum extent, the kernel sets
>>>> the entry type to BTRFS_GET_CSUMS_HAS_CSUMS and follows it with the
>>>> csums. If it is sparse, preallocated, or beyond the EOF, it sets the
>>>> type to BTRFS_GET_CSUMS_SPARSE - this is so userspace knows it can use
>>>> the precomputed hash of the zero sector. Otherwise, it sets the type to
>>>> BTRFS_GET_CSUMS_NO_CSUMS.
>>>
>>> I'm not sure if it's a good idea to put hole and preallocated range 
>>> into the same BTRFS_GET_CSUMS_SPARSE.
>>>
>>> Although both means there is no csum, hole case means there is really 
>>> no data extent, thus we should not create any extent instead of 
>>> writing zero.
> 
> Thanks Qu.
> 
> "SPARSE" is probably a bad name for it. It probably should be "ZERO" or 
> somesuch. The point is to tell userspace not to waste time calculating 
> csums, but use the precomputed values because the data would be zero 
> (for whatever reason).
> 
>>> For preallocated, indicating it has no CSUM can allow mkfs to 
>>> distinguish hole and preallocated, thus change to zero writes to 
>>> prealloc, which is faster and make the resulted fs more aligned to 
>>> the source dir.
>>>
>>>
>>> And for EOF checks, I think we don't need to bother that much, aka, 
>>> just let it return the regular results.
>>>
>>> My assumption is, the mkfs shouldn't pass a range completely beyond 
>>> the round_up(i_size), as non-reflink rootdir population would always 
>>> read out the content of the inode from the host fs.
>>> Thus we won't really read beyond the inode size.
>>
>> After more investigation, I think we can put the hole/preallocation/ 
>> compression detection into the user space.
>>
>> The hole detection is already pending for merge, mostly through 
>> SEEK_DATA/SEEK_HOLE flags of lseek():
>>
>>   https://github.com/kdave/btrfs-progs/pull/1097
>>
>> I'm planning to implement preallocation detection through fiemap, 
>> which also allows us to detect compressed range and skip them for your 
>> case.
>>
>> With all those features implemented in progs, we can further simplify 
>> the get csum ioctl, to something more aligned to 
>> btrfs_lookup_csums_bitmap().
>>
>> We do not need to bother why there is no checksum for some ranges, 
>> that will be handled by progs first, we only need to return all the 
>> checksums found for the specified range.
>>
>> And as an extra safenet, use some bitmap inthe ioctl structure to 
>> indicate which ranges have checksum and which doesn't.
>>
>> This will definitely simplify the ioctl as we only need to do csum 
>> tree lookup, no need to bother anything in the subvolume tree.
> 
> Unfortunately this won't work. You have to explicitly filter out 
> compressed extents, and identifying these requires checking the FS tree.

That's done by progs through fiemap. There will be a flag ENCODED for 
compressed file extents.

> The reason is that they may be "bookended", and you would leak 
> information about other files if you returned the whole of the csums for 
> the compressed extent. This is the reason why encoded read needs root.

Nope, for the fiemap call, we will never reach any bookend extents.

Thanks,
Qu

> 
>> Thanks,
>> Qu
>>
>>>
>>>>
>>>> We do store the csums of compressed extents, but we deliberately don't
>>>> return them here: they're hashed over the compressed data, not the
>>>> uncompressed data that's returned to userspace.
>>>
>>> I agree with the skip of compressed extents, but I'd prefer to have a 
>>> special flag to indicate that, other than NO_CSUMS.
>>>
>>> Or mkfs is unable to distinguish hole and compressed extents.
> 
> Compressed extents result in NO_CSUMS, a hole results in SPARSE.
> 
>>> [...]
>>>> +/* Types for struct btrfs_ioctl_get_csums_entry::type */
>>>> +#define BTRFS_GET_CSUMS_HAS_CSUMS    0
>>>> +#define BTRFS_GET_CSUMS_SPARSE        1
>>>> +#define BTRFS_GET_CSUMS_NO_CSUMS    2
>>>> +
>>>> +struct btrfs_ioctl_get_csums_entry {
>>>> +    __u64 offset;        /* file offset of this range */
>>>> +    __u64 length;        /* length in bytes */
>>>> +    __u32 type;        /* BTRFS_GET_CSUMS_* type */
>>>> +    __u32 reserved;        /* padding, must be 0 */
>>>> +};
>>>> +
>>>> +struct btrfs_ioctl_get_csums_args {
>>>> +    __u64 offset;        /* in/out: file offset */
>>>> +    __u64 length;        /* in/out: range length */
>>>> +    __u64 buf_size;        /* in/out: buffer capacity / bytes 
>>>> written */
>>>> +    __u8 buf[];        /* out: entries + csum data */
>>>> +};
>>>
>>>  From the progs usage, it is always a single 
>>> btrfs_ioctl_get_csums_entry at the beginning of buf[], then real 
>>> buffer for csum, can we just combine both structures into one?
>>>
>>> Furthermore, since we only query one extent at one time, the offset/ 
>>> length are more or less duplicated between args and entry structure.
>>>
>>> We can just save the length into the args without the need for entry 
>>> members (except the type).
>>>
>>> Thanks,
>>> Qu
>>>
>>>> +
>>>>   /* Flags for IOC_SHUTDOWN, must match XFS_FSOP_GOING_FLAGS_* 
>>>> flags. */
>>>>   #define BTRFS_SHUTDOWN_FLAGS_DEFAULT            0x0
>>>>   #define BTRFS_SHUTDOWN_FLAGS_LOGFLUSH            0x1
>>>> @@ -1226,6 +1245,8 @@ enum btrfs_err_code {
>>>>                        struct btrfs_ioctl_encoded_io_args)
>>>>   #define BTRFS_IOC_SUBVOL_SYNC_WAIT _IOW(BTRFS_IOCTL_MAGIC, 65, \
>>>>                       struct btrfs_ioctl_subvol_wait)
>>>> +#define BTRFS_IOC_GET_CSUMS _IOWR(BTRFS_IOCTL_MAGIC, 66, \
>>>> +                  struct btrfs_ioctl_get_csums_args)
>>>>   /* Shutdown ioctl should follow XFS's interfaces, thus not using 
>>>> btrfs magic. */
>>>>   #define BTRFS_IOC_SHUTDOWN    _IOR('X', 125, __u32)
>>>
>>>
>>
>>
>