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

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

On Wed, Mar 11, 2026 at 10:38:20AM +0800, Baokun Li wrote:
> 
> Good spotting! ext4_find_goal() ensures that the goal block obtained for
> indirect-block-based files will not exceed EXT4_MAX_BLOCK_FILE_PHYS.
> However, on an ext4 filesystem where the two file formats are mixed,
> it is indeed possible to get an excessively large goal group via stream
> allocation.

Well, I didn't spot it; an LLM AI noticed.  :-) Arguably I should have
noticed it when doing my review, but I didn't.

> Since the mixed-format case is quite rare, I think we can simply validate
> start in ext4_mb_scan_groups() and reset it to 0 when it exceeds the limit,
> like this:

No, I don't think that's enough.  It's not just that we could have an
excessively large goal group.  The goal group could also just be, say,
2**32 - 5.  If the next 5 groups are full, then when we do an
optimized scan, we will end up beyond the 2**32 limit.  That's why we
need to add some kidn of wraparound logic to *any* caller to
ext4_get_allocation_groups_count().

							- Ted