Date:   Wed, 21 Jun 2023 11:18:50 +0200
From:   Domenico Cerasuolo <cerasuolodomenico@...il.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Nhat Pham <nphamcs@...il.com>, Yu Zhao <yuzhao@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG mm-unstable] "kernel BUG at mm/swap.c:393!" on commit b9c91c43412f2e

On Wed, Jun 21, 2023 at 10:06 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Wed, Jun 21, 2023 at 12:01 AM Hyeonggon Yoo <42.hyeyoo@...il.com> wrote:
> >
> > On Wed, Jun 07, 2023 at 07:51:43PM +0000, Yosry Ahmed wrote:
> > > Commit 71024cb4a0bf ("frontswap: remove frontswap_tmem_exclusive_gets")
> > > removed support for exclusive loads from frontswap as it was not used.
> > > Bring back exclusive loads support to frontswap by adding an "exclusive"
> > > output parameter to frontswap_ops->load.
> > >
> > > On the zswap side, add a module parameter to enable/disable exclusive
> > > loads, and a config option to control the boot default value.
> > > Refactor zswap entry invalidation in zswap_frontswap_invalidate_page()
> > > into zswap_invalidate_entry() to reuse it in zswap_frontswap_load() if
> > > exclusive loads are enabled.
> > >
> > > With exclusive loads, we avoid having two copies of the same page in
> > > memory (compressed & uncompressed) after faulting it in from zswap. On
> > > the other hand, if the page is to be reclaimed again without being
> > > dirtied, it will be re-compressed. Compression is not usually slow, and
> > > a page that was just faulted in is less likely to be reclaimed again
> > > soon.
> > >
> > > Suggested-by: Yu Zhao <yuzhao@...gle.com>
> > > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> > > ---
> > >
> > > v1 -> v2:
> > > - Add a module parameter to control whether exclusive loads are enabled
> > >   or not, the config option now controls the default boot value instead.
> > >   Replaced frontswap_ops->exclusive_loads by an output parameter to
> > >   frontswap_ops->load() (Johannes Weiner).
> > > ---
> >
> > Hi Yosry, I was testing the latest mm-unstable and encountered a bug.
> > It was bisectable and this is the first bad commit.
> >
> >
> > Attached config file and bisect log.
> > The oops message is available at:
> >
> > https://social.kernel.org/media/eace06d71655b3cc76411366573e4a8ce240ad65b8fd20977d7c73eec9dc2253.jpg
> >
> > (the head commit is b9c91c43412f2e07 "mm: zswap: support exclusive loads")
> > (it's an image because I tested it on real machine)
> >
> >
> > This is what I have as swap space:
> >
> > $ cat /proc/swaps
> > Filename                                Type            Size            Used            Priority
> > /var/swap                               file            134217724       0               -2
> > /dev/zram0                              partition       8388604         0               100
>
>
> Hi Hyeonggon,
>
> Thanks for reporting this! I think I know what went wrong. Could you
> please verify if the below fix works if possible?
>
> Domenico, I believe the below fix would also fix a problem with the
> recent writeback series. If the entry is invalidated before we grab the
> lock to put the local ref in zswap_frontswap_load(), then the entry
> will be freed once we call zswap_entry_put(), and the movement to the
> beginning LRU will be operating on a freed entry. It also modifies
> your recently added commit 418fd29d9de5 ("mm: zswap: invaldiate entry
> after writeback"). I would appreciate it if you also take a look.

Hi Yosry,

Thanks, this makes sense indeed. I've been running a stress test too for
an hour now and it seems fine.

>
> If this works as intended, I can send a formal patch (applies on top
> of fd247f029cd0 ("mm/gup: do not return 0 from pin_user_pages_fast()
> for bad args")):
>
> From 4b7f949b3ffb42d969d525d5b576fad474f55276 Mon Sep 17 00:00:00 2001
> From: Yosry Ahmed <yosryahmed@...gle.com>
> Date: Wed, 21 Jun 2023 07:43:51 +0000
> Subject: [PATCH] mm: zswap: fix double invalidate with exclusive loads
>
> If exclusive loads are enabled for zswap, we invalidate the entry before
> returning from zswap_frontswap_load(), after dropping the local
> reference. However, the tree lock is dropped during decompression after
> the local reference is acquired, so the entry could be invalidated
> before we drop the local ref. If this happens, the entry is freed once
> we drop the local ref, and zswap_invalidate_entry() tries to invalidate
> an already freed entry.
>
> Fix this by:
> (a) Making sure zswap_invalidate_entry() is always called with a local
>     ref held, to avoid being called on a freed entry.
> (b) Making sure zswap_invalidate_entry() only drops the ref if the entry
>     was actually on the rbtree. Otherwise, another invalidation could
>     have already happened, and the initial ref is already dropped.
>
> With these changes, there is no need to check that there is no need to
> make sure the entry still exists in the tree in zswap_reclaim_entry()
> before invalidating it, as zswap_reclaim_entry() will make this check
> internally.
>
> Fixes: b9c91c43412f ("mm: zswap: support exclusive loads")
> Reported-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
>  mm/zswap.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 87b204233115..62195f72bf56 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -355,12 +355,14 @@ static int zswap_rb_insert(struct rb_root *root,
> struct zswap_entry *entry,
>         return 0;
>  }
>
> -static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
> +static bool zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
>  {
>         if (!RB_EMPTY_NODE(&entry->rbnode)) {
>                 rb_erase(&entry->rbnode, root);
>                 RB_CLEAR_NODE(&entry->rbnode);
> +               return true;
>         }
> +       return false;
>  }
>
>  /*
> @@ -599,14 +601,16 @@ static struct zswap_pool
> *zswap_pool_find_get(char *type, char *compressor)
>         return NULL;
>  }
>
> +/*
> + * If the entry is still valid in the tree, drop the initial ref and remove it
> + * from the tree. This function must be called with an additional ref held,
> + * otherwise it may race with another invalidation freeing the entry.
> + */
>  static void zswap_invalidate_entry(struct zswap_tree *tree,
>                                    struct zswap_entry *entry)
>  {
> -       /* remove from rbtree */
> -       zswap_rb_erase(&tree->rbroot, entry);
> -
> -       /* drop the initial reference from entry creation */
> -       zswap_entry_put(tree, entry);
> +       if (zswap_rb_erase(&tree->rbroot, entry))
> +               zswap_entry_put(tree, entry);
>  }
>
>  static int zswap_reclaim_entry(struct zswap_pool *pool)
> @@ -659,8 +663,7 @@ static int zswap_reclaim_entry(struct zswap_pool *pool)
>          * swapcache. Drop the entry from zswap - unless invalidate already
>          * took it out while we had the tree->lock released for IO.
>          */
> -       if (entry == zswap_rb_search(&tree->rbroot, swpoffset))
> -               zswap_invalidate_entry(tree, entry);
> +       zswap_invalidate_entry(tree, entry);
>
>  put_unlock:
>         /* Drop local reference */
> @@ -1466,7 +1469,6 @@ static int zswap_frontswap_load(unsigned type,
> pgoff_t offset,
>                 count_objcg_event(entry->objcg, ZSWPIN);
>  freeentry:
>         spin_lock(&tree->lock);
> -       zswap_entry_put(tree, entry);
>         if (!ret && zswap_exclusive_loads_enabled) {
>                 zswap_invalidate_entry(tree, entry);
>                 *exclusive = true;
> @@ -1475,6 +1477,7 @@ static int zswap_frontswap_load(unsigned type,
> pgoff_t offset,
>                 list_move(&entry->lru, &entry->pool->lru);
>                 spin_unlock(&entry->pool->lru_lock);
>         }
> +       zswap_entry_put(tree, entry);
>         spin_unlock(&tree->lock);
>
>         return ret;
> --
> 2.41.0.162.gfafddb0af9-goog

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