Re: [PATCH v11 2/2] xfs: add FALLOC_FL_WRITE_ZEROES to XFS code base

[Lukas Herbolt <lukas@herbolt.com>]

On 2026-03-10 11:10, Pankaj Raghav (Samsung) wrote:
>> 	if (xfs_is_always_cow_inode(ip) ||
> 
> Why should we check for this here but we don't do this check in 
> xfs_falloc_zero_range()?
> 
> There is a comment in xfs_falloc_allocate_range():
> 
> 	/*
> 	 * If always_cow mode we can't use preallocations and thus should not
> 	 * create them.
> 	 */
> 	if (xfs_is_always_cow_inode(XFS_I(inode)))
> 		return -EOPNOTSUPP;
> 
> Don't we also use preallocations in xfs_falloc_zero_range()?
> 

For the xfs_falloc_zero_range we return 0 on COW from 
xfs_alloc_file_space and that
results in creating a hole in the file instead of having preallocated 
extents as shown
bellow.

But for the xfs_falloc_write_zero_range(), we want fail as soon as 
possible as possible.

Feel free to correct me if I am wrong.

int
xfs_alloc_file_space(
         struct xfs_inode        *ip,
         xfs_off_t               offset,
         xfs_off_t               len)
{
         xfs_mount_t             *mp = ip->i_mount;
         xfs_off_t               count;
         xfs_filblks_t           allocatesize_fsb;
         xfs_extlen_t            extsz, temp;
         xfs_fileoff_t           startoffset_fsb;
         xfs_fileoff_t           endoffset_fsb;
         int                     rt;
         xfs_trans_t             *tp;
         xfs_bmbt_irec_t         imaps[1], *imapp;
         int                     error;

         if (xfs_is_always_cow_inode(ip))
                 return 0;


root@build-00:/mnt# dd if=/dev/urandom of=/mnt/test.bin bs=10M 
oflag=direct count=1
1+0 records in
1+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0.167452 s, 62.6 MB/s
root@build-00:/mnt# xfs_bmap -vp  test.bin
test.bin:
  EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL FLAGS
    0: [0..20479]:      192..20671        0 (192..20671)     20480 000000
root@build-00:/mnt# strace -e fallocate fallocate -zl 2M  test.bin
fallocate(3, FALLOC_FL_ZERO_RANGE, 0, 2097152) = 0
+++ exited with 0 +++
root@build-00:/mnt# xfs_bmap -vp  test.bin
test.bin:
  EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL FLAGS
    0: [0..4095]:       hole                                  4096
    1: [4096..20479]:   4288..20671       0 (4288..20671)    16384 000000
root@build-00:/mnt# dd if=/dev/urandom of=/mnt/test.bin bs=1M 
oflag=direct count=1 conv=notrunc
1+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.0172151 s, 60.9 MB/s
root@build-00:/mnt# xfs_bmap -vp  test.bin
test.bin:
  EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL FLAGS
    0: [0..2047]:       192..2239         0 (192..2239)       2048 000000
    1: [2048..4095]:    hole                                  2048
    2: [4096..20479]:   4288..20671       0 (4288..20671)    16384 000000
root@build-00:/mnt# cat /sys/fs/xfs/debug/
always_cow          bload_node_slack    larp                mount_delay
bload_leaf_slack    bug_on_assert       log_recovery_delay  
pwork_threads
root@build-00:/mnt# cat /sys/fs/xfs/debug/always_cow
1


-- 
-lhe