Re: [f2fs-dev] [PATCH] f2fs: speed up f2fs_empty_dir()

[Chao Yu]

On 2022/11/8 2:29, Eric Biggers wrote:
> On Sun, Nov 06, 2022 at 05:48:55PM +0800, Chao Yu wrote:
> > Wei Chen reports a kernel bug as blew:
> >
> > INFO: task syz-executor.0:29056 blocked for more than 143 seconds.
> >        Not tainted 5.15.0-rc5 #1
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > task:syz-executor.0  state:D stack:14632 pid:29056 ppid:  6574 flags:0x00000004
> > Call Trace:
> >   __schedule+0x4a1/0x1720
> >   schedule+0x36/0xe0
> >   rwsem_down_write_slowpath+0x322/0x7a0
> >   fscrypt_ioctl_set_policy+0x11f/0x2a0
> >   __f2fs_ioctl+0x1a9f/0x5780
> >   f2fs_ioctl+0x89/0x3a0
> >   __x64_sys_ioctl+0xe8/0x140
> >   do_syscall_64+0x34/0xb0
> >   entry_SYSCALL_64_after_hwframe+0x44/0xae
> >
> > Eric did some investigation on this issue, quoted from reply of Eric:
> >
> > "Well, the quality of this bug report has a lot to be desired (not on
> > upstream kernel, reproducer is full of totally irrelevant stuff, not
> > sent to the mailing list of the filesystem whose disk image is being
> > fuzzed, etc.).  But what is going on is that f2fs_empty_dir() doesn't
> > consider the case of a directory with an extremely large i_size on a
> > malicious disk image.
> >
> > Specifically, the reproducer mounts an f2fs image with a directory
> > that has an i_size of 14814520042850357248, then calls
> > FS_IOC_SET_ENCRYPTION_POLICY on it.
> >
> > That results in a call to f2fs_empty_dir() to check whether the
> > directory is empty.  f2fs_empty_dir() then iterates through all
> > 3616826182336513 blocks the directory allegedly contains to check
> > whether any contain anything.  i_rwsem is held during this, so
> > anything else that tries to take it will hang."
> >
> > In order to solve this issue, let's use f2fs_get_next_page_offset()
> > to speed up iteration by skipping holes for all below functions:
> > - f2fs_empty_dir
> > - f2fs_readdir
> > - find_in_level
> >
> > The way why we can speed up iteration was described in
> > 'commit 3cf4574705b4 ("f2fs: introduce get_next_page_offset to speed
> > up SEEK_DATA")'.
> >
> > Meanwhile, in f2fs_empty_dir(), let's use f2fs_find_data_page()
> > instead f2fs_get_lock_data_page(), due to i_rwsem was held in
> > caller of f2fs_empty_dir(), there shouldn't be any races, so it's
> > fine to not lock dentry page during lookuping dirents in the page.
> >
> > Link: https://lore.kernel.org/lkml/536944df-a0ae-1dd8-148f-510b476e1347@xxxxxxxxxx/T/
> > Reported-by: Wei Chen <harperchen1110@xxxxxxxxx>
> > Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> > Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
> > ---
> >   fs/f2fs/data.c | 17 ++++++++++++-----
> >   fs/f2fs/dir.c  | 34 ++++++++++++++++++++++++----------
> >   fs/f2fs/f2fs.h |  5 +++--
> >   fs/f2fs/gc.c   |  4 ++--
> >   4 files changed, 41 insertions(+), 19 deletions(-)
>
> Thanks.  I'm not an expert on all the details, but this patch looks good to me.
>
> Given that it optimizes lookups and readdirs too, a better title for the patch
> might be something like "f2fs: optimize iteration over sparse directories".

Yes, thanks for your suggestion, will update in v2.

Thanks,

> - Eric