Re: [RFC PATCH] ext4: handle wraparound when searching for blocks for indirect mapped blocks

["Theodore Tso" <tytso@mit.edu>]

On Fri, Mar 13, 2026 at 10:00:04AM +0800, Baokun Li wrote:
> IIUC, ngroups/end here only depend on filesystem size and whether the inode
> is extent-based, and both should stay unchanged during block allocation.
> So doing the check once at the beginning should be sufficient. Am I missing
> anything?

The problem here is that case where we use
ext4_get_allocation_groups_count as ngroups and end are different.  In
some places we do this:

        ext4_group_t ngroups = ext4_get_allocation_groups_count(ac);

and in others we do this:

        end = ext4_get_allocation_groups_count(ac);

Now, if start is zero, then these two are equivalent.  But if start is
not zero, but say, is 2**32 - 8, then where we use
ext4_allocaiton_groups_count() as the last block group to search, then
we only will search exactly 8 block groups, and if
there are free blocks in the first 8 block groups, then the scanning
function will fail.

Alternatively, if we just do the check at the beginning, then 2**32 -
8 is a valid starting point, but if we just search forward by ngroups,
then we may end up returning a block group which won't work for
indirect mapped inodes.

Hence, in *every* function where we call
ext4_get_allocaiton_groups_count(), if the goal is to search all block
groups that are valid for indirect mapped inodes, and start might be
greater than 0, we *have* to handle wraparound.

Does that make sense?

						- Ted