Message-ID: <CAJD7tkYEZEihcQFVrb5KR18r6o5496uXSRJbDrs+woGHwv6zWg@mail.gmail.com>
Date:   Wed, 21 Jun 2023 01:05:56 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     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>,
        Domenico Cerasuolo <cerasuolodomenico@...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 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.

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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