Re: [PATCH 09/12] xfs: Introduce delayed logging core code

Dave Chinner
2010-05-10T12:14:22Z

On Mon, May 10, 2010 at 07:44:35AM -0400, Christoph Hellwig wrote:
> Looks good to me,
>
>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
>
> A couple comments below anyway:
>
> > +int
> > +xlog_cil_init_post_recovery(
> > + struct log *log)
> > +{
> > + if (!log->l_cilp)
> > + return 0;
> > +
> > + log->l_cilp->xc_ctx->ticket = xlog_cil_ticket_alloc(log);
> > + log->l_cilp->xc_ctx->sequence = 1;
> > + log->l_cilp->xc_ctx->commit_lsn = xlog_assign_lsn(log->l_curr_cycle,
> > + log->l_curr_block);
> > + return 0;
> > +}
>
> This should return void.

OK.

> > +static void
> > +xlog_cil_insert(
> > + struct log *log,
> > + struct xlog_ticket *ticket,
> > + struct xfs_log_item *item,
> > + struct xfs_log_vec *lv)
> > +{
> > + struct xfs_cil *cil = log->l_cilp;
> > + struct xfs_log_vec *old = lv->lv_item->li_lv;
> > + struct xfs_cil_ctx *ctx = cil->xc_ctx;
> > + int len;
> > + int diff_iovecs;
> > + int iclog_space;
> > +
> > + if (old) {
> > + /* existing lv on log item, space used is a delta */
> > + ASSERT(!list_empty(&item->li_cil));
> > + ASSERT(old->lv_buf && old->lv_buf_len && old->lv_niovecs);
> > +
> > + len = lv->lv_buf_len - old->lv_buf_len;
> > + diff_iovecs = lv->lv_niovecs - old->lv_niovecs;
>
> Add asserts that len and diff_iovecs aren't negative here?

Actually, they can be negative here - a previously logged buffer
that is now stale will go from ((N dirty regions * 128 bytes) +
format header) to (zero dirty regions + format header), and
effectively free up space as what was previously logged is now
ignored due to the XFS_BLI_CANCEL flag in the format header.

> > + for (lv = log_vector; lv; lv = lv->lv_next) {
> > + void *ptr;
> > + int index;
> > + int offset = 0;
> > + int len = 0;
> > +
> > + for (index = 0; index < lv->lv_niovecs; index++)
> > + len += lv->lv_iovecp[index].i_len;
> > +
> > + lv->lv_buf_len = len;
> > + lv->lv_buf = kmem_zalloc(lv->lv_buf_len, KM_SLEEP|KM_NOFS);
> > + ptr = lv->lv_buf;
> > +
> > + for (index = 0; index < lv->lv_niovecs; index++) {
> > + struct xfs_log_iovec *vec = &lv->lv_iovecp[index];
> > +
> > + memcpy(ptr, vec->i_addr, vec->i_len);
> > + vec->i_addr = ptr;
> > + xlog_write_adv_cnt(&ptr, &len, &offset, vec->i_len);
> > + }
> > + ASSERT(len == 0);
> > +
> > + xlog_cil_insert(log, ticket, lv->lv_item, lv);
>
> The use of xlog_write_adv_cnt doesn't seem quite optimal to me. The
> offset variable is entirely unused, and len is only used for an asswer
> that could easily be reformulated as
>
> ASSERT(ptr == lv->lv_buf + len);
>
> if we replace the xlog_write_adv_cnt with a simple
>
> ptr += vec->i_len;

Good idea - xlog_write_adv_cnt() was left over from the original
code that was copied from the log code.

>
> > +/*
> > + * Push the Committed Item List to the log. If the push_now flag is not set,
> > + * then it is a background flush and so we can chose to ignore it.
> > + */
> > +int
> > +xlog_cil_push(
> > + struct log *log,
> > + int push_now)
> > +{
> > + struct xfs_cil *cil = log->l_cilp;
>
> The variables don't line up here. There's another instance of that
> in xlog_cil_insert, btw.

Ok, will fix.

>
> > + /* check if we've anything to push */
> > + if (list_empty(&cil->xc_cil)) {
> > + up_write(&cil->xc_ctx_lock);
> > + xfs_log_ticket_put(new_ctx->ticket);
> > + kmem_free(new_ctx);
> > + return 0;
> > + }
>
> Please add a out_skip label for this cleanup code, as it would be
> duplicated by the background flushing check added in a later patch.

OK.

> > + new_lv = kmem_zalloc(sizeof(*new_lv) +
> > + lidp->lid_size * sizeof(struct xfs_log_iovec),
> > + KM_SLEEP);
> > +
> > + /* The allocated iovec region lies beyond the log vector. */
> > + new_lv->lv_iovecp = (struct xfs_log_iovec *)&new_lv[1];
> > + if (!ret_lv)
> > + ret_lv = new_lv;
> > + else
> > + lv->lv_next = new_lv;
> > + lv = new_lv;
>
> I'd suggest already setting up lv->lv_niovecs and lv->lv_item here
> instead of in xfs_trans_fill_log_vecs. That way xfs_trans_fill_log_vecs
> can be simplified to:
>
> STATIC void
> xfs_trans_fill_log_vecs(
> struct xfs_trans *tp,
> struct xfs_log_vec *log_vector)
> {
> struct xfs_log_vec *lv;
>
> for (lv = log_vector; lv = lv->lv_next; lv)
> IOP_FORMAT(lidp->lid_item, lv->lv_iovecp);
> }
>
> Or just inlined into the caller or even xfs_log_commit_cil given how simple
> it is now. Moving it to xfs_log_commit_cil would also help avoiding the
> locking imbalance where xfs_log_commit_cil is called with xc_ctx_lock
> held but returns without it after the last patch in the series. That
> again might allow merging the IOP_FORMAT loop into xlog_cil_format_items.
>
> Btw, I wonder if xfs_log_commit_cil should simply move to xfs_trans.c?
> That would avoid having to export xfs_trans_unreserve_and_mod_sb,
> xfs_trans_free_items and xfs_trans_free from there, and only require
> exporting xlog_cil_format_items (if we didn't move that one as well,
> then xlog_cil_insert), while keeping things a lot more symmetric with
> the traditional commit path.

I did find it a bit hard trying to draw the line between the trans
subsystem and the logging subsystem because of the interactions and
the way they changed as I developed the code and fixed bugs. I'll
have a closer look at what you are suggesting (it seems reasonable
just from a quick scan) and see how much it cleans up.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com