Re: [PATCH v5 03/10] xfs: Refactor xfs_reflink_end_cow_extent()

[John Garry]

On 12/03/2025 15:46, Darrick J. Wong wrote:
> On Wed, Mar 12, 2025 at 01:35:23AM -0700, Christoph Hellwig wrote:
> > On Wed, Mar 12, 2025 at 08:27:05AM +0000, John Garry wrote:
> > > On 12/03/2025 07:24, Christoph Hellwig wrote:
> > > > On Mon, Mar 10, 2025 at 06:39:39PM +0000, John Garry wrote:
> > > > > Refactor xfs_reflink_end_cow_extent() into separate parts which process
> > > > > the CoW range and commit the transaction.
> > > > >
> > > > > This refactoring will be used in future for when it is required to commit
> > > > > a range of extents as a single transaction, similar to how it was done
> > > > > pre-commit d6f215f359637.
> > > >
> > > > Darrick pointed out that if you do more than just a tiny number
> > > > of extents per transactions you run out of log reservations very
> > > > quickly here:
> > > >
> > > > https://urldefense.com/v3/__https://lore.kernel.org/all/20240329162936.GI6390@frogsfrogsfrogs/__;!!ACWV5N9M2RV99hQ!PWLcBof1tKimKUObvCj4vOhljWjFmjtzVHLx9apcU5Rah1xZnmp_3PIq6eSwx6TdEXzMLYYyBfmZLgvj$
> > > >
> > > > how does your scheme deal with that?
> > > The resblks calculation in xfs_reflink_end_atomic_cow() takes care of this,
> > > right? Or does the log reservation have a hard size limit, regardless of
> > > that calculation?
> >
> > The resblks calculated there are the reserved disk blocks and have
> > nothing to do with the log reservations, which comes from the
> > tr_write field passed in.  There is some kind of upper limited to it
> > obviously by the log size, although I'm not sure if we've formalized
> > that somewhere.  Dave might be the right person to ask about that.
>
> The (very very rough) upper limit for how many intent items you can
> attach to a tr_write transaction is:
>
> per_extent_cost = (cui_size + rui_size + bui_size + efi_size + ili_size)
> max_blocks = tr_write::tr_logres / per_extent_cost
>
> (ili_size is the inode log item size)

So will it be something like this:

static size_t
xfs_compute_awu_max_extents(
	struct xfs_mount	*mp)
{
	struct xfs_trans_res	*resp = &M_RES(mp)->tr_write;
	size_t			logtotal = xfs_bui_log_format_sizeof(1)+
				xfs_cui_log_format_sizeof(1) +
				xfs_efi_log_format_sizeof(1) +
				xfs_rui_log_format_sizeof(1) +
				sizeof(struct xfs_inode_log_format);

	return rounddown_pow_of_two(resp->tr_logres / logtotal);
}

static inline void
xfs_compute_awu_max(
	struct xfs_mount	*mp, int jjcount)
{
....
	mp->m_awu_max =
	min_t(unsigned int, awu_max, xfs_compute_awu_max_extents(mp));
}

> ((I would halve that for the sake of paranoia))
>
> since you have to commit all those intent items into the first
> transaction in the chain.  The difficulty we've always had is computing
> the size of an intent item in the ondisk log, since that's a (somewhat
> minor) layering violation -- it's xfs_cui_log_format_sizeof() for a CUI,
> but then there' could be overhead for the ondisk log headers themselves.
>
> Maybe we ought to formalize the computation of that since reap.c also
> has a handwavy XREAP_MAX_DEFER_CHAIN that it uses to roll the scrub
> transaction periodically... because I'd prefer we not add another
> hardcoded limit.  My guess is that the software fallback can probably
> support any awu_max that a hardware wants to throw at us, but let's
> actually figure out the min(sw, hw) that we can support and cap it at
> that.
>
> --D