Re: [PATCH 02/21] btrfs: prevent transaction block reserve underflow when starting transaction

[Josef Bacik <josef@toxicpanda.com>]

On Fri, Sep 08, 2023 at 01:09:04PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> When starting a transaction, with a non-zero number of items, we reserve
> metadata space for that number of items and for delayed refs by doing a
> call to btrfs_block_rsv_add(), with the transaction block reserve passed
> as the block reserve argument. This reserves metadata space and adds it
> to the transaction block reserve. Later we migrate the space we reserved
> for delayed references from the transaction block reserve into the delayed
> refs block reserve, by calling btrfs_migrate_to_delayed_refs_rsv().
> 
> btrfs_migrate_to_delayed_refs_rsv() decrements the number of bytes to
> migrate from the source block reserve, and this however may result in an
> underflow in case the space added to the transaction block reserve ended
> up being used by another task that has not reserved enough space for its
> own use - examples are tasks doing reflinks or hole punching because they
> end up calling btrfs_replace_file_extents() -> btrfs_drop_extents() and
> may need to modify/COW a variable number of leaves/paths, so they keep
> trying to use space from the transaction block reserve when they need to
> COW an extent buffer, and may end up trying to use more space then they
> have reserved (1 unit/path only for removing file extent items).
> 
> This can be avoided by simply reserving space first without adding it to
> the transaction block reserve, then add the space for delayed refs to the
> delayed refs block reserve and finally add the remaining reserved space
> to the transaction block reserve. This also makes the code a bit shorter
> and simpler. So just do that.
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef