Re: [PATCH -mm -v6 3/9] mm, THP, swap: Add swap cluster allocate/free functions

[Tim Chen]

On Wed, 2017-03-15 at 09:19 +0800, Huang, Ying wrote:
> Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> writes:
> 
> > 
> > On Wed, 2017-03-08 at 15:26 +0800, Huang, Ying wrote:
> > > 
> > > From: Huang Ying <ying.huang@xxxxxxxxx>
> > > 
> > > The swap cluster allocation/free functions are added based on the
> > > existing swap cluster management mechanism for SSD.ÂÂThese functions
> > > don't work for the rotating hard disks because the existing swap cluster
> > > management mechanism doesn't work for them.ÂÂThe hard disks support may
> > > be added if someone really need it.ÂÂBut that needn't be included in
> > > this patchset.
> > > 
> > > This will be used for the THP (Transparent Huge Page) swap support.
> > > Where one swap cluster will hold the contents of each THP swapped out.
> > > 
> > > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > > Cc: Shaohua Li <shli@xxxxxxxxxx>
> > > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > > Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > > ---
> > > Âmm/swapfile.c | 217 +++++++++++++++++++++++++++++++++++++++++-----------------
> > > Â1 file changed, 156 insertions(+), 61 deletions(-)
> > > 
> > > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > > index a744604384ff..91876c33114b 100644
> > > --- a/mm/swapfile.c
> > > +++ b/mm/swapfile.c
> > > @@ -378,6 +378,14 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
> > > Â	schedule_work(&si->discard_work);
> > > Â}
> > > Â
> > > +static void __free_cluster(struct swap_info_struct *si, unsigned long idx)
> > > +{
> > > +	struct swap_cluster_info *ci = si->cluster_info;
> > > +
> > > +	cluster_set_flag(ci + idx, CLUSTER_FLAG_FREE);
> > > +	cluster_list_add_tail(&si->free_clusters, ci, idx);
> > > +}
> > > +
> > > Â/*
> > > Â * Doing discard actually. After a cluster discard is finished, the cluster
> > > Â * will be added to free cluster list. caller should hold si->lock.
> > > @@ -398,10 +406,7 @@ static void swap_do_scheduled_discard(struct swap_info_struct *si)
> > > Â
> > > Â		spin_lock(&si->lock);
> > > Â		ci = lock_cluster(si, idx * SWAPFILE_CLUSTER);
> > > -		cluster_set_flag(ci, CLUSTER_FLAG_FREE);
> > > -		unlock_cluster(ci);
> > > -		cluster_list_add_tail(&si->free_clusters, info, idx);
> > > -		ci = lock_cluster(si, idx * SWAPFILE_CLUSTER);
> > > +		__free_cluster(si, idx);
> > > Â		memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> > > Â				0, SWAPFILE_CLUSTER);
> > > Â		unlock_cluster(ci);
> > The __free_cluster definition and the above change to eliminate
> > the extra unlock_cluster and lock_cluster can perhaps be broken up
> > as a separate patch. ÂIt can be independent of THP changes.
> I think the change may have no value by itself without THP changes.
> There will be only 1 user of __free_cluster() and the lock change is
> trivial too.ÂÂSo I think it may be better just to keep it as that?
> 

Seems like the extra unlock and lock of cluster in existing code should be taken out
irrespective of the THP changes:
Â
		cluster_set_flag(ci, CLUSTER_FLAG_FREE);
-		unlock_cluster(ci);
		cluster_list_add_tail(&si->free_clusters, info, idx);
-		ci = lock_cluster(si, idx * SWAPFILE_CLUSTER);
		memset(si->swap_map + idx * SWAPFILE_CLUSTER,
ÂÂ				0, SWAPFILE_CLUSTER);

Tim