linux-kernel - Re: [PATCH] xfs: make xfs_log_iovec independent from xfs_log

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Date: Tue, 25 Jun 2024 04:33:00 -0700
From: Christoph Hellwig <hch@...radead.org>
To: alexjlzheng@...il.com
Cc: chandan.babu@...cle.com, djwong@...nel.org, linux-xfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, alexjlzheng@...cent.com
Subject: Re: [PATCH] xfs: make xfs_log_iovec independent from xfs_log_vec and
 release it early

> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -2526,6 +2526,8 @@ xlog_write(
>  			xlog_write_full(lv, ticket, iclog, &log_offset,
>  					 &len, &record_cnt, &data_cnt);
>  		}
> +		if (lv->lv_flags & XFS_LOG_VEC_DYNAMIC)
> +			kvfree(lv->lv_iovecp);

This should porbably be a function paramter to xlog_write, with
xlog_cil_write_chain asking for the iovecs to be freed because they
are dynamically allocated, and the other two not becaue the iovecs
are on-stack.  With that we don't need to grow a new field in
struct xfs_log_vec.

>  	list_for_each_entry(lip, &tp->t_items, li_trans) {
>  		struct xfs_log_vec *lv;
> +		struct xfs_log_iovec *lvec;
>  		int	niovecs = 0;
>  		int	nbytes = 0;
>  		int	buf_size;
> @@ -339,18 +339,23 @@ xlog_cil_alloc_shadow_bufs(
>  			 * the buffer, only the log vector header and the iovec
>  			 * storage.
>  			 */
> -			kvfree(lip->li_lv_shadow);
> -			lv = xlog_kvmalloc(buf_size);
> -
> -			memset(lv, 0, xlog_cil_iovec_space(niovecs));
> +			if (lip->li_lv_shadow) {
> +				kvfree(lip->li_lv_shadow->lv_iovecp);
> +				kvfree(lip->li_lv_shadow);
> +			}
> +			lv = xlog_kvmalloc(sizeof(struct xfs_log_vec));
> +			memset(lv, 0, sizeof(struct xfs_log_vec));
> +			lvec = xlog_kvmalloc(buf_size);
> +			memset(lvec, 0, xlog_cil_iovec_space(niovecs));

This area can use quite a bit of a redo.  The xfs_log_vec is tiny,
so it doesn't really need a vmalloc fallback but can simply use
kmalloc.

But more importantly there is no need to really it, you just
need to allocate it.  So this should probably become:

	lv = lip->li_lv_shadow;
	if (!lv) {
		/* kmalloc and initialize, set lv_size to zero */
	}

	if (buf_size > lv->lv_size) {
		/* grow case that rallocates ->lv_iovecp */
	} else {
		/* same or smaller, optimise common overwrite case */
		..
	}