linux-kernel - Re: [PATCH v3] mm/zswap: invalidate old entry when store fail or !zswap

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Date: Tue, 6 Feb 2024 21:52:54 -0800
From: Chris Li <chrisl@...nel.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: chengming.zhou@...ux.dev, hannes@...xchg.org, nphamcs@...il.com, 
	akpm@...ux-foundation.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Chengming Zhou <zhouchengming@...edance.com>, stable@...r.kernel.org
Subject: Re: [PATCH v3] mm/zswap: invalidate old entry when store fail or !zswap_enabled

On Tue, Feb 6, 2024 at 9:46 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> > > @@ -1608,14 +1598,12 @@ bool zswap_store(struct folio *folio)
> > >         /* map */
> > >         spin_lock(&tree->lock);
> > >         /*
> > > -        * A duplicate entry should have been removed at the beginning of this
> > > -        * function. Since the swap entry should be pinned, if a duplicate is
> > > -        * found again here it means that something went wrong in the swap
> > > -        * cache.
> > > +        * The folio may have been dirtied again, invalidate the
> > > +        * possibly stale entry before inserting the new entry.
> > >          */
> > > -       while (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
> > > -               WARN_ON(1);
> > > +       if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
> > >                 zswap_invalidate_entry(tree, dupentry);
> > > +               VM_WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry));
> >
> > It seems there is only one path called zswap_rb_insert() and there is
> > no loop to repeat the insert any more. Can we have the
> > zswap_rb_insert() install the entry and return the dupentry? We can
> > still just call zswap_invalidate_entry() on the duplicate. The mapping
> > of the dupentry has been removed when  zswap_rb_insert() returns. That
> > will save a repeat lookup on the duplicate case.
> > After this change, the zswap_rb_insert() will map to the xarray
> > xa_store() pretty nicely.
>
> I brought this up in v1 [1]. We agreed to leave it as-is for now since
> we expect the xarray conversion soon-ish. No need to update
> zswap_rb_insert() only to replace it with xa_store() later anyway.
>
> [1] https://lore.kernel.org/lkml/ZcFne336KJdbrvvS@google.com/
>
Ah, thanks for the pointer. I miss your earlier reply.

Acked-by: Chris Li <chrisl@...nel.org>

Chris