Re: [PATCH RFC 07/10] fs/ext4: Fail truncate if pages are GUP pinned

[Ira Weiny]

On Thu, Jun 06, 2019 at 12:58:55PM +0200, Jan Kara wrote:
> On Wed 05-06-19 18:45:40, ira.weiny@xxxxxxxxx wrote:
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 
> > If pages are actively gup pinned fail the truncate operation.
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > ---
> >  fs/ext4/inode.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 75f543f384e4..1ded83ec08c0 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -4250,6 +4250,9 @@ int ext4_break_layouts(struct inode *inode, loff_t offset, loff_t len)
> >  		if (!page)
> >  			return 0;
> >  
> > +		if (page_gup_pinned(page))
> > +			return -ETXTBSY;
> > +
> >  		error = ___wait_var_event(&page->_refcount,
> >  				atomic_read(&page->_refcount) == 1,
> >  				TASK_INTERRUPTIBLE, 0, 0,
> 
> This caught my eye. Does this mean that now truncate for a file which has
> temporary gup users (such buffers for DIO) can fail with ETXTBUSY?

I thought about that before and I _thought_ I had accounted for it.  But I
think you are right...

>
> That
> doesn't look desirable.

No not desirable at all...  Ah it just dawned on my why I thought it was ok...
I was wrong.  :-/

> If we would mandate layout lease while pages are
> pinned as I suggested, this could be dealt with by checking for leases with
> pins (breaking such lease would return error and not break it) and if
> breaking leases succeeds (i.e., there are no long-term pinned pages), we'd
> just wait for the remaining references as we do now.

Agreed.

But I'm going to respond with some of the challenges of this (and ideas I had)
when replying to your other email.

Ira

> 
> 								Honza
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR