Re: [PATCH 5/6] dax: Call ->iomap_begin without entry lock during dax fault

[Jan Kara <jack@suse.cz>]

On Thu 01-12-16 16:27:04, Ross Zwisler wrote:
> On Thu, Dec 01, 2016 at 03:24:47PM -0700, Ross Zwisler wrote:
> > On Thu, Nov 24, 2016 at 10:46:35AM +0100, Jan Kara wrote:
> > > Currently ->iomap_begin() handler is called with entry lock held. If the
> > > filesystem held any locks between ->iomap_begin() and ->iomap_end()
> > > (such as ext4 which will want to hold transaction open), this would cause
> > > lock inversion with the iomap_apply() from standard IO path which first
> > > calls ->iomap_begin() and only then calls ->actor() callback which grabs
> > > entry locks for DAX.
> > 
> > I don't see the dax_iomap_actor() grabbing any entry locks for DAX?  Is this
> > an issue currently, or are you just trying to make the code consistent so we
> > don't run into issues in the future?
> 
> Ah, I see that you use this new ordering in patch 6/6 so that you can change
> your interaction with the ext4 journal.  I'm still curious if we have a lock
> ordering inversion within DAX, but if this ordering helps you with ext4, good
> enough.
> 
> One quick comment:
> 
> > @@ -1337,19 +1353,10 @@ int dax_iomap_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> >        */                                                                     
> >       entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD);              
> >       if (IS_ERR(entry))                                                      
> > -             goto fallback;                                                  
> > +             goto finish_iomap;                                              
> >                                                                               
> > -     /*                                                                      
> > -      * Note that we don't use iomap_apply here.  We aren't doing I/O, only  
> > -      * setting up a mapping, so really we're using iomap_begin() as a way   
> > -      * to look up our filesystem block.                                     
> > -      */                                                                     
> > -     pos = (loff_t)pgoff << PAGE_SHIFT;                                      
> > -     error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap);    
> > -     if (error)                                                              
> > -             goto unlock_entry;                                              
> >       if (iomap.offset + iomap.length < pos + PMD_SIZE)                       
> > -             goto finish_iomap;                                              
> > +             goto unlock_entry;       
> 
> I think this offset+length bounds check could be moved along with the
> iomap_begin() call up above the grab_mapping_entry().  You would then goto
> 'finish_iomap' if you hit this error condition, allowing you to avoid grabbing
> and releasing of the mapping entry.

Yes, that is nicer. Changed.

> Other than that one small nit, this looks fine to me:
> Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>

Thanks.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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