Re: [PATCH 6/6] mm/zswap: zswap entry doesn't need refcount anymore

[Nhat Pham]

On Fri, Feb 2, 2024 at 2:37 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Fri, Feb 2, 2024 at 2:33 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> >
> > On Thu, Feb 1, 2024 at 7:50 AM Chengming Zhou
> > <zhouchengming@xxxxxxxxxxxxx> wrote:
> > >
> > > Since we don't need to leave zswap entry on the zswap tree anymore,
> > > we should remove it from tree once we find it from the tree.
> > >
> > > Then after using it, we can directly free it, no concurrent path
> > > can find it from tree. Only the shrinker can see it from lru list,
> > > which will also double check under tree lock, so no race problem.
> > >
> > > So we don't need refcount in zswap entry anymore and don't need to
> > > take the spinlock for the second time to invalidate it.
> > >
> > > The side effect is that zswap_entry_free() maybe not happen in tree
> > > spinlock, but it's ok since nothing need to be protected by the lock.
> > >
> > > Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> >
> > Oh this is sweet! Fewer things to keep in mind.
> > Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> >
> > > ---
> > >  mm/zswap.c | 63 +++++++++++---------------------------------------------------
> > >  1 file changed, 11 insertions(+), 52 deletions(-)
> > >
> > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > index cbf379abb6c7..cd67f7f6b302 100644
> > > --- a/mm/zswap.c
> > > +++ b/mm/zswap.c
> > > @@ -193,12 +193,6 @@ struct zswap_pool {
> > >   *
> > >   * rbnode - links the entry into red-black tree for the appropriate swap type
> > >   * swpentry - associated swap entry, the offset indexes into the red-black tree
> > > - * refcount - the number of outstanding reference to the entry. This is needed
> > > - *            to protect against premature freeing of the entry by code
> > > - *            concurrent calls to load, invalidate, and writeback.  The lock
> > > - *            for the zswap_tree structure that contains the entry must
> > > - *            be held while changing the refcount.  Since the lock must
> > > - *            be held, there is no reason to also make refcount atomic.
> > >   * length - the length in bytes of the compressed page data.  Needed during
> > >   *          decompression. For a same value filled page length is 0, and both
> > >   *          pool and lru are invalid and must be ignored.
> > > @@ -211,7 +205,6 @@ struct zswap_pool {
> > >  struct zswap_entry {
> > >         struct rb_node rbnode;
> > >         swp_entry_t swpentry;
> > > -       int refcount;
> >
> > Hah this should even make zswap a bit more space-efficient. IIRC Yosry
> > has some analysis regarding how much less efficient zswap will be
> > every time we add a new field to zswap entry - this should go in the
> > opposite direction :)
>
> Unfortunately in this specific case I think it won't change the size
> of the allocation for struct zswap_entry anyway, but it is a step
> nonetheless :)

Ah, is it because of the field alignment requirement? But yeah, one
day we will remove enough of them :)