On Wed, Mar 28, 2012 at 03:38:52AM +0100, Al Viro wrote: > 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. The alternative to using ->release is ....? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
