Re: [PATCH 02/24] shrinkers: use will_defer for GFP_NOFS sensitive shrinkers

Brian Foster
2019-08-02T15:27:40Z

On Thu, Aug 01, 2019 at 12:17:30PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> For shrinkers that currently avoid scanning when called under
> GFP_NOFS contexts, conver them to use the new ->will_defer flag
> rather than checking and returning errors during scans.
>
> This makes it very clear that these shrinkers are not doing any work
> because of the context limitations, not because there is no work
> that can be done.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> drivers/staging/android/ashmem.c | 8 ++++----
> fs/gfs2/glock.c | 5 +++--
> fs/gfs2/quota.c | 6 +++---
> fs/nfs/dir.c | 6 +++---
> fs/super.c | 6 +++---
> fs/xfs/xfs_buf.c | 4 ++++
> fs/xfs/xfs_qm.c | 11 ++++++++---
> net/sunrpc/auth.c | 5 ++---
> 8 files changed, 30 insertions(+), 21 deletions(-)
>
...
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index ca0849043f54..6e0f76532535 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -1680,6 +1680,10 @@ xfs_buftarg_shrink_count(
> {
> struct xfs_buftarg *btp = container_of(shrink,
> struct xfs_buftarg, bt_shrinker);
> +
> + if (!(sc->gfp_mask & __GFP_FS))
> + sc->will_defer = true;
> +
> return list_lru_shrink_count(&btp->bt_lru, sc);
> }

This hunk looks like a behavior change / bug fix..? The rest of the
patch converts existing logic to bail out of scans to use the new count
time defer mechanism. The change is probably fine, but I think we should
have a separate patch to introduce this behavior in the first place
(which BTW could be sent as a standalone patch and just picked up by
this on eventual rebase).

Brian

>
> diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
> index 5e7a37f0cf84..13c842e8f13b 100644
> --- a/fs/xfs/xfs_qm.c
> +++ b/fs/xfs/xfs_qm.c
> @@ -502,9 +502,6 @@ xfs_qm_shrink_scan(
> unsigned long freed;
> int error;
>
> - if ((sc->gfp_mask & (__GFP_FS|__GFP_DIRECT_RECLAIM)) != (__GFP_FS|__GFP_DIRECT_RECLAIM))
> - return 0;
> -
> INIT_LIST_HEAD(&isol.buffers);
> INIT_LIST_HEAD(&isol.dispose);
>
> @@ -534,6 +531,14 @@ xfs_qm_shrink_count(
> struct xfs_quotainfo *qi = container_of(shrink,
> struct xfs_quotainfo, qi_shrinker);
>
> + /*
> + * __GFP_DIRECT_RECLAIM is used here to avoid blocking kswapd
> + */
> + if ((sc->gfp_mask & (__GFP_FS|__GFP_DIRECT_RECLAIM)) !=
> + (__GFP_FS|__GFP_DIRECT_RECLAIM)) {
> + sc->will_defer = true;
> + }
> +
> return list_lru_shrink_count(&qi->qi_lru, sc);
> }
>
> diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
> index cdb05b48de44..6babcbac4a00 100644
> --- a/net/sunrpc/auth.c
> +++ b/net/sunrpc/auth.c
> @@ -527,9 +527,6 @@ static unsigned long
> rpcauth_cache_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>
> {
> - if ((sc->gfp_mask & GFP_KERNEL) != GFP_KERNEL)
> - return SHRINK_STOP;
> -
> /* nothing left, don't come back */
> if (list_empty(&cred_unused))
> return SHRINK_STOP;
> @@ -541,6 +538,8 @@ static unsigned long
> rpcauth_cache_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
>
> {
> + if ((sc->gfp_mask & GFP_KERNEL) != GFP_KERNEL)
> + sc->will_defer = true;
> return number_cred_unused * sysctl_vfs_cache_pressure / 100;
> }
>
> --
> 2.22.0
>