Re: Using the upcoming fsinfo()

[Bruce Dubbs <bruce.dubbs@gmail.com>]

On 5/13/19 4:08 AM, Karel Zak wrote:
> On Mon, May 13, 2019 at 01:33:22PM +0800, Ian Kent wrote:
>> Some of you may know that David Howells is working on getting
>> a new system call fsinfo() merged into the Linux kernel.
>>
>> This system call will provide access to information about mounted
>> mounts without having to read and parse file based mount tables
>> such as /proc/self/mountinfo, etc.
>>
>> Essentially all mounts have an id and one can get the id of a
>> mount by it's path and then use that to obtain a large range
>> of information about it.
>>
>> The information can include a list of mounts within the mount
>> which can be used to traverse a tree of mounts or the id used
>> to lookup information on an individual mount without the need
>> to traverse a file based mount table.
>>
>> I'd like to update libmount to use the fsinfo() system call
>> because I believe using file based methods to get mount
>> information introduces significant overhead that can be
>> avoided.
>>
>> Because the fsinfo() system call provides a very different way
>> to get information
>> about mounts, and having looked at the current
>> code, I'm wondering what will be
>> the best way to go about it.
>>
>> Any suggestions about the way this could best be done, given
>> that the existing methods must still work, will be very much
>> appreciated.
> 
> It would be nice to start with some low-level things to read info
> about a target (mountpoint) into libmnt_fs, something like:
> 
>      int mnt_fsinfo_fill_fs(chat char *tgt, struct libmnt_fs *fs)
> 
> and fill create a complete mount table by fsinfo():
> 
>      int mnt_fsinfo_fill_table(struct libmnt_table *tab)
> 
> ... probably add fsinfo.c to code to keep it all together.
> 
> So, after then we can use these functions in our code.
> 
> The nice place where is ugly overhead with the current mountinfo is
> context_umount.c code, see lookup_umount_fs() and
> mnt_context_find_umount_fs(). In this code we have mountpoint and we
> need more information about it (due to redirection to umount.<type>
> helpers, userspace mount options, etc.). It sounds like ideal to use
> mnt_fsinfo_fill_fs() if possible.
> 
> The most visible change will be to use mnt_fsinfo_fill_table() with in
> mnt_table_parse_file() if the file name is "/proc/self/mountinfo".
> This will be huge improvement as we use this function in systemd on
> each mount table change...
> 
> The question is how easily will be to replace mountinfo with fsinfo().

I may be stating the obvious, but this proposal does not appear to 
simplify anything because it is kernel version dependent.  From what I 
understand, the new and old methods will both need to be supported for 
quite some time.

I'm not suggesting that the changes not be made, but I suggest going slow.

   -- Bruce