Re: [PATCH v6 11/12] xfs: add xfs_compute_atomic_write_unit_max()

Dave Chinner
2025-04-08T22:47:12Z

On Tue, Apr 08, 2025 at 10:42:08AM +0000, John Garry wrote:
> Now that CoW-based atomic writes are supported, update the max size of an
> atomic write for the data device.
>
> The limit of a CoW-based atomic write will be the limit of the number of
> logitems which can fit into a single transaction.

I still think this is the wrong way to define the maximum
size of a COW-based atomic write because it is going to change from
filesystem to filesystem and that variability in supported maximum
length will be exposed to userspace...

i.e. Maximum supported atomic write size really should be defined as
a well documented fixed size (e.g. 16MB). Then the transaction
reservations sizes needed to perform that conversion can be
calculated directly from that maximum size and optimised directly
for the conversion operation that atomic writes need to perform.

.....

> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index b2dd0c0bf509..42b2b7540507 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -615,6 +615,28 @@ xfs_init_mount_workqueues(
> return -ENOMEM;
> }
>
> +unsigned int
> +xfs_atomic_write_logitems(
> + struct xfs_mount *mp)
> +{
> + unsigned int efi = xfs_efi_item_overhead(1);
> + unsigned int rui = xfs_rui_item_overhead(1);
> + unsigned int cui = xfs_cui_item_overhead(1);
> + unsigned int bui = xfs_bui_item_overhead(1);
> + unsigned int logres = M_RES(mp)->tr_write.tr_logres;
> +
> + /*
> + * Maximum overhead to complete an atomic write ioend in software:
> + * remove data fork extent + remove cow fork extent +
> + * map extent into data fork
> + */
> + unsigned int atomic_logitems =
> + (bui + cui + rui + efi) + (cui + rui) + (bui + rui);

This seems wrong. Unmap from the data fork only logs a (bui + cui)
pair, we don't log a RUI or an EFI until the transaction that
processes the BUI or CUI actually frees an extent from the the BMBT
or removes a block from the refcount btree.

We also need to be able to relog all the intents and everything that
was modified, so we effectively have at least one
xfs_allocfree_block_count() reservation needed here as well. Even
finishing an invalidation BUI can result in BMBT block allocation
occurring if the operation splits an existing extent record and the
insert of the new record causes a BMBT block split....

> +
> + /* atomic write limits are always a power-of-2 */
> + return rounddown_pow_of_two(logres / (2 * atomic_logitems));

What is the magic 2 in that division?

> +}

Also this function does not belong in xfs_super.c - that file is for
interfacing with the VFS layer. Calculating log reservation
constants at mount time is done in xfs_trans_resv.c - I suspect most
of the code in this patch should probably be moved there and run
from xfs_trans_resv_calc()...

-Dave.
--
Dave Chinner
david@fromorbit.com