Re: [BUG mm-unstable] "kernel BUG at mm/swap.c:393!" on commit b9c91c43412f2e

[Yosry Ahmed]

On Wed, Jun 21, 2023 at 2:06 AM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
>
> On Wed, Jun 21, 2023 at 01:05:56AM -0700, Yosry Ahmed wrote:
> > On Wed, Jun 21, 2023 at 12:01 AM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
> > > 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?
> >
>
> Works fine and I was not able to reproduce the bug with the patch
> applied.
>
> Not sure Andrew would prefer squashing it into original one or applying it
> as separate patch, though (I'm totally fine with both way).

I think it already landed in mm-stable so it cannot be squashed at this point.

>
> Anyway:
>
> Tested-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>

Thanks a lot for reporting and testing this!

I will wait for Domenico to also respond then send the fix to Andrew.
Hopefully it's not too late for this rc.

>
> > 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@xxxxxxxxxx>
> > 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@xxxxxxxxx>
> > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
>
> <...snip...>
>
> --
> Hyeonggon Yoo
>
> Undergraduate | Chungnam National University