linux-kernel - Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion

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Date:   Sat, 16 Sep 2017 10:11:26 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>, Jaegeuk Kim <jaegeuk@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion

On Sat, Sep 16, 2017 at 1:12 AM, Theodore Ts'o <tytso@....edu> wrote:
> On Fri, Sep 15, 2017 at 11:44:33AM -0700, Jaegeuk Kim wrote:
>>
>> So, I digged it in more detail, and found, in drivers/android/binder.c [1],
>> - binder_ioctl()
>>  - create a kernel thread
>>  - zombie_cleanup_check()
>>   - binder_defer_work()
>>     - queue_work(..., &binder_deferred_work);
>>
>> - binder_deferred_func()
>>  - binder_clear_zombies()
>>   - binder_proc_clear_zombies()
>>    - put_files_struct()
>>     - close_files()
>>      - filp_close()
>>       - fput()
>>
>> It seems binder holds some proc files.
>
> If binder was holding some files open, then umount should have failed
> with EBUSY, no?
>
> Does Android use mount namespaces at all?
>

Extensively.
Every user (i.e. from multi user) has its own mount ns with private /data
Every app has its own mount ns, with /sdcard mounted to one of 3 FUSE
sdcard mounts depending of app storage permission (none, rdonly, rdwr).

Amir.