Re: [TOPIC] Last iput() from flusher thread, last fput() from munmap()...

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Mar 27, 2012 at 11:08:58PM +0200, Jan Kara wrote:
>   Hello,
> 
>   maybe the name of this topic could be "How hard should be life of
> filesystems?" but that's kind of broad topic and suggests too much of
> bikeshedding. I'd like to concentrate on concrete possible pain points
> between filesystems & VFS (possibly writeback or even generally MM).
> Lately, I've myself came across the two issues in $SUBJECT:
> 1) dropping of last file reference can happen from munmap() and in that
>    case mmap_sem will be held when ->release() is called. Even more it
>    could be held when ->evict_inode() is called to delete inode because
>    inode was unlinked.

Yes, it can.

> 2) since flusher thread takes inode reference when writing inode out, the
>    last inode reference can be dropped from flusher thread. Thus inode may
>    get deleted in the flusher thread context. This does not seem that
>    problematic on its own but if we realize progress of memory reclaim
>    depends (at least from a longterm perspective) on flusher thread making
>    progress, things start looking a bit uncertain. Even more so when we
>    would like avoid ->writepage() calls from reclaim and let flusher thread
>    do the work instead. That would then require filesystems to carefully
>    design their ->evict_inode() routines so that things are not
>    deadlockable.

You mean "use GFP_NOIO for allocations when holding fs-internal locks"?

>   Both these issues should be avoidable (we can postpone fput() after we
> drop mmap_sem; we can tweak inode refcounting to avoid last iput() from
> flusher thread) but obviously there's some cost in the complexity of generic
> layer. So the question is, is it worth it?

I don't thing it is.  ->i_mutex in ->release() is never needed; existing
cases are racy and dropping preallocation that way is simply wrong.  And
->evict_inode() is a non-issue, since it has no reason whatsoever to take
*any* locks in mutex - the damn thing is called when nobody has references
to struct inode anymore.  Deadlocks with flusher... that's what NOIO and
NOFS are for.

As for the IMA issues...  We probably ought to use a separate mutex for
xattr and relying on ->i_mutex for its internal locking is unconvincing,
to put it mildly...