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Message-ID: <Z2UaAn6IS5uh5plH@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
Date: Fri, 20 Dec 2024 12:47:22 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Sarthak Kukreti <sarthakkukreti@...gle.com>
Cc: linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
Andreas Dilger <adilger.kernel@...ger.ca>
Subject: Re: [PATCH v2] fallocate: Add support for fixed goal extent
allocations
On Thu, Dec 19, 2024 at 07:46:13PM -0800, Sarthak Kukreti wrote:
> 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>
>
> Changes from v1 (https://lists.openwall.net/linux-ext4/2024/12/12/38):
> - s/EXT2_NEWRANGE_EXACT_GOAL/EXT2_NEWRANGE_FIXED_GOAL
Looks good Sarthak, feel free to add:
Reviewed-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Regards,
ojaswin
>
> ---
> lib/ext2fs/alloc.c | 2 +-
> lib/ext2fs/ext2fs.h | 3 ++-
> lib/ext2fs/fallocate.c | 21 +++++++++++++++++++--
> 3 files changed, 22 insertions(+), 4 deletions(-)
>
> diff --git a/lib/ext2fs/alloc.c b/lib/ext2fs/alloc.c
> index 3fd92167..ba5b1c5e 100644
> --- a/lib/ext2fs/alloc.c
> +++ b/lib/ext2fs/alloc.c
> @@ -390,7 +390,7 @@ no_blocks:
> /*
> * Starting at _goal_, scan around the filesystem to find a run of free blocks
> * that's at least _len_ blocks long. Possible flags:
> - * - EXT2_NEWRANGE_EXACT_GOAL: The range of blocks must start at _goal_.
> + * - EXT2_NEWRANGE_FIXED_GOAL: The range of blocks must start at _goal_.
> * - EXT2_NEWRANGE_MIN_LENGTH: do not return a allocation shorter than _len_.
> * - EXT2_NEWRANGE_ZERO_BLOCKS: Zero blocks pblk to pblk+plen before returning.
> *
> 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.
> --
> 2.47.1.613.gc27f4b7a9f-goog
>
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