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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 09/12] xfs: Introduce delayed logging core code
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Mon, 10 May 2010 07:44:35 -0400
Cc: xfs@xxxxxxxxxxx
In-reply-to: <1273210860-23414-10-git-send-email-david@xxxxxxxxxxxxx>
References: <1273210860-23414-1-git-send-email-david@xxxxxxxxxxxxx> <1273210860-23414-10-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.19 (2009-01-05)
Looks good to me,


Reviewed-by: Christoph Hellwig <hch@xxxxxx>

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.

> +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?

> +     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;

> +/*
> + * 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.

> +     /* 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.

> +             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.

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