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

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Date: Fri, 2 Feb 2024 14:36:43 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Chengming Zhou <zhouchengming@...edance.com>, Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH 6/6] mm/zswap: zswap entry doesn't need refcount anymore

On Fri, Feb 2, 2024 at 2:33 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Thu, Feb 1, 2024 at 7:50 AM Chengming Zhou
> <zhouchengming@...edance.com> 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@...edance.com>
>
> Oh this is sweet! Fewer things to keep in mind.
> Reviewed-by: Nhat Pham <nphamcs@...il.com>
>
> > ---
> >  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 :)