linux-kernel - Re: [PATCH v3 1/7] mm: zswap: add pool shrinking mechanism

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Message-ID: <CAJD7tkbcf+MEMJdGeruB7XL=J8tJNgkB7_1UQ2S4VEKjob-NQw@mail.gmail.com>
Date:   Sat, 17 Jun 2023 22:28:48 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        vitaly.wool@...sulko.com, minchan@...nel.org, linux-mm@...ck.org,
        ddstreet@...e.org, sjenning@...hat.com, nphamcs@...il.com,
        hannes@...xchg.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v3 1/7] mm: zswap: add pool shrinking mechanism

On Sat, Jun 17, 2023 at 10:25 PM Sergey Senozhatsky
<senozhatsky@...omium.org> wrote:
>
> On (23/06/17 21:48), Yosry Ahmed wrote:
> > > On (23/06/12 11:38), Domenico Cerasuolo wrote:
> > > > +static int zswap_reclaim_entry(struct zswap_pool *pool)
> > > > +{
> > > > +     struct zswap_header *zhdr;
> > > > +     struct zswap_entry *entry;
> > > > +     struct zswap_tree *tree;
> > > > +     pgoff_t swpoffset;
> > > > +     int ret;
> > > > +
> > > > +     /* Get an entry off the LRU */
> > > > +     spin_lock(&pool->lru_lock);
> > > > +     if (list_empty(&pool->lru)) {
> > > > +             spin_unlock(&pool->lru_lock);
> > > > +             return -EINVAL;
> > > > +     }
> > > > +     entry = list_last_entry(&pool->lru, struct zswap_entry, lru);
> > > > +     list_del_init(&entry->lru);
> > >
> > > A quick question: should we zswap_entry_get() here?
> >
> > We need to hold the tree lock for that, and the lock ordering is tree
> > lock -> lru lock. If we try to grab the tree lock here we may
> > deadlock.
>
> We can deadlock doing this?
>
>         lock tree_lock
>         lock lru_lock
>         list_del_init
>         unlock lru_lock
>         entry_get
>         unlock tree_lock
>         writeback

We don't know which tree the zswap entry belongs to until we get from
the LRU -- so we can't hold the tree lock before getting the entry
from the lru (and to get the entry from the LRU we need the lru_lock).