Re: [PATCH 15/24] xfs: eagerly free shadow buffers to reduce CIL footprint

Brian Foster
2019-08-05T18:03:06Z

On Thu, Aug 01, 2019 at 12:17:43PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> The CIL can pin a lot of memory and effectively defines the lower
> free memory boundary of operation for XFS. The way we hang onto
> log item shadow buffers "just in case" effectively doubles the
> memory footprint of the CIL for dubious reasons.
>
> That is, we hang onto the old shadow buffer in case the next time
> we log the item it will fit into the shadow buffer and we won't have
> to allocate a new one. However, we only ever tend to grow dirty
> objects in the CIL through relogging, so once we've allocated a
> larger buffer the old buffer we set as a shadow buffer will never
> get reused as the amount we log never decreases until the item is
> clean. And then for buffer items we free the log item and the shadow
> buffers, anyway. Inode items will hold onto their shadow buffer
> until they are reclaimed - this could double the inode's memory
> footprint for it's lifetime...
>
> Hence we should just free the old log item buffer when we replace it
> with a new shadow buffer rather than storing it for later use. It's
> not useful, get rid of it as early as possible.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_log_cil.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index fa5602d0fd7f..1863a9bdf4a9 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -238,9 +238,7 @@ xfs_cil_prepare_item(
> /*
> * If there is no old LV, this is the first time we've seen the item in
> * this CIL context and so we need to pin it. If we are replacing the
> - * old_lv, then remove the space it accounts for and make it the shadow
> - * buffer for later freeing. In both cases we are now switching to the
> - * shadow buffer, so update the the pointer to it appropriately.
> + * old_lv, then remove the space it accounts for and free it.
> */

The comment above xlog_cil_alloc_shadow_bufs() needs a similar update
around how we handle the old buffer when the shadow buffer is used.

> if (!old_lv) {
> if (lv->lv_item->li_ops->iop_pin)
> @@ -251,7 +249,8 @@ xfs_cil_prepare_item(
>
> *diff_len -= old_lv->lv_bytes;
> *diff_iovecs -= old_lv->lv_niovecs;
> - lv->lv_item->li_lv_shadow = old_lv;
> + kmem_free(old_lv);
> + lv->lv_item->li_lv_shadow = NULL;
> }

So IIUC this is the case where we allocated a shadow buffer, the item
was already pinned (so old_lv is still around) but we ended up using the
shadow buffer for this relog. Instead of keeping the old buffer around
as a new shadow, we toss it. That makes sense, but if the objective is
to not leave dangling shadow buffers around as such, what about the case
where we allocated a shadow buffer but didn't end up using it because
old_lv was reusable? It looks like we still keep the shadow buffer
around in that scenario with a similar lifetime as the swapout scenario
this patch removes. Hm?

Brian

>
> /* attach new log vector to log item */
> --
> 2.22.0
>