linux-kernel - Re: [f2fs-dev] [PATCH v3] f2fs: fix race of pending

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Date:   Tue, 8 Dec 2020 08:51:45 +0900
From:   Daeho Jeong <daeho43@...il.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH v3] f2fs: fix race of pending_pages in decompression

Chao, Jaegeuk,

Thanks. I'll update it as your comments. :)

Eric,

Decompression and verity can be executed in different thread contexts
in different timing, so we need separate counts for each.

We already use STEP_VERITY for non-compression case, so I think using
this flag in here looks more making sense.

Thanks,

2020년 12월 8일 (화) 오전 5:31, Eric Biggers <ebiggers@...nel.org>님이 작성:
>
> On Sat, Dec 05, 2020 at 01:26:26PM +0900, Daeho Jeong wrote:
> > From: Daeho Jeong <daehojeong@...gle.com>
> >
> > I found out f2fs_free_dic() is invoked in a wrong timing, but
> > f2fs_verify_bio() still needed the dic info and it triggered the
> > below kernel panic. It has been caused by the race condition of
> > pending_pages value between decompression and verity logic, when
> > the same compression cluster had been split in different bios.
> > By split bios, f2fs_verify_bio() ended up with decreasing
> > pending_pages value before it is reset to nr_cpages by
> > f2fs_decompress_pages() and caused the kernel panic.
> >
> > [ 4416.564763] Unable to handle kernel NULL pointer dereference
> >                at virtual address 0000000000000000
> > ...
> > [ 4416.896016] Workqueue: fsverity_read_queue f2fs_verity_work
> > [ 4416.908515] pc : fsverity_verify_page+0x20/0x78
> > [ 4416.913721] lr : f2fs_verify_bio+0x11c/0x29c
> > [ 4416.913722] sp : ffffffc019533cd0
> > [ 4416.913723] x29: ffffffc019533cd0 x28: 0000000000000402
> > [ 4416.913724] x27: 0000000000000001 x26: 0000000000000100
> > [ 4416.913726] x25: 0000000000000001 x24: 0000000000000004
> > [ 4416.913727] x23: 0000000000001000 x22: 0000000000000000
> > [ 4416.913728] x21: 0000000000000000 x20: ffffffff2076f9c0
> > [ 4416.913729] x19: ffffffff2076f9c0 x18: ffffff8a32380c30
> > [ 4416.913731] x17: ffffffc01f966d97 x16: 0000000000000298
> > [ 4416.913732] x15: 0000000000000000 x14: 0000000000000000
> > [ 4416.913733] x13: f074faec89ffffff x12: 0000000000000000
> > [ 4416.913734] x11: 0000000000001000 x10: 0000000000001000
> > [ 4416.929176] x9 : ffffffff20d1f5c7 x8 : 0000000000000000
> > [ 4416.929178] x7 : 626d7464ff286b6b x6 : ffffffc019533ade
> > [ 4416.929179] x5 : 000000008049000e x4 : ffffffff2793e9e0
> > [ 4416.929180] x3 : 000000008049000e x2 : ffffff89ecfa74d0
> > [ 4416.929181] x1 : 0000000000000c40 x0 : ffffffff2076f9c0
> > [ 4416.929184] Call trace:
> > [ 4416.929187]  fsverity_verify_page+0x20/0x78
> > [ 4416.929189]  f2fs_verify_bio+0x11c/0x29c
> > [ 4416.929192]  f2fs_verity_work+0x58/0x84
> > [ 4417.050667]  process_one_work+0x270/0x47c
> > [ 4417.055354]  worker_thread+0x27c/0x4d8
> > [ 4417.059784]  kthread+0x13c/0x320
> > [ 4417.063693]  ret_from_fork+0x10/0x18
> >
> > Signed-off-by: Daeho Jeong <daehojeong@...gle.com>
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> > v3: back to v1 and enabled verity in a unit of cluster
> > v2: merged verity_pages with pending_pages, and increased the
> >     pending_pages count only if STEP_VERITY is set on bio
>
> I am trying to review this but it is very hard, as the f2fs compression code is
> very hard to understand.
>
> It looks like a 'struct decompress_io_ctx' represents the work to decompress a
> particular cluster.  Since the compressed data of the cluster can be read using
> multiple bios, there is a reference count of how many pages are remaining to be
> read before all the cluster's pages have been read and decompression can start.
>
> What I don't understand is why that reference counting needs to work differently
> depending on whether verity is enabled or not.  Shouldn't it be exactly the
> same?
>
> There also seems to be some confusion about the scope of STEP_VERITY.  Before
> f2fs compression was added, it was a per-bio thing.  But now in a compressed
> file, it's really a per-cluster thing, since all decompressed pages in a
> compressed cluster are verified (or not verified) at once.
>
> Wouldn't it make a lot more sense to, when a cluster needs both compression and
> verity, *not* set STEP_VERITY on the bios, but rather set a similar flag in the
> decompress_io_ctx?
>
> - Eric