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    SubjectRe: [PATCH -mm -v6 3/9] mm, THP, swap: Add swap cluster allocate/free functions
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    On Wed, 2017-03-15 at 09:19 +0800, Huang, Ying wrote:
    > Tim Chen <tim.c.chen@linux.intel.com> writes:
    >
    > >
    > > On Wed, 2017-03-08 at 15:26 +0800, Huang, Ying wrote:
    > > >
    > > > From: Huang Ying <ying.huang@intel.com>
    > > >
    > > > The swap cluster allocation/free functions are added based on the
    > > > existing swap cluster management mechanism for SSD.  These functions
    > > > don't work for the rotating hard disks because the existing swap cluster
    > > > management mechanism doesn't work for them.  The hard disks support may
    > > > be added if someone really need it.  But 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@redhat.com>
    > > > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    > > > Cc: Hugh Dickins <hughd@google.com>
    > > > Cc: Shaohua Li <shli@kernel.org>
    > > > Cc: Minchan Kim <minchan@kernel.org>
    > > > Cc: Rik van Riel <riel@redhat.com>
    > > > Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
    > > > ---
    > > >  mm/swapfile.c | 217 +++++++++++++++++++++++++++++++++++++++++-----------------
    > > >  1 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,
    > > >   schedule_work(&si->discard_work);
    > > >  }
    > > >  
    > > > +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);
    > > > +}
    > > > +
    > > >  /*
    > > >   * Doing discard actually. After a cluster discard is finished, the cluster
    > > >   * 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)
    > > >  
    > > >   spin_lock(&si->lock);
    > > >   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);
    > > >   memset(si->swap_map + idx * SWAPFILE_CLUSTER,
    > > >   0, SWAPFILE_CLUSTER);
    > > >   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.  It 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.  So 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:
     
    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,
       0, SWAPFILE_CLUSTER);

    Tim

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