Re: [PATCH 7/7] btrfs: drop mmap_sem in mkwrite for btrfs

[Jan Kara]

On Thu 25-10-18 09:58:51, Josef Bacik wrote:
> On Thu, Oct 25, 2018 at 03:22:30PM +0200, Jan Kara wrote:
> > On Thu 18-10-18 16:23:18, Josef Bacik wrote:
> > > ->page_mkwrite is extremely expensive in btrfs.  We have to reserve
> > > space, which can take 6 lifetimes, and we could possibly have to wait on
> > > writeback on the page, another several lifetimes.  To avoid this simply
> > > drop the mmap_sem if we didn't have the cached page and do all of our
> > > work and return the appropriate retry error.  If we have the cached page
> > > we know we did all the right things to set this page up and we can just
> > > carry on.
> > > 
> > > Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> > ...
> > > @@ -8828,6 +8830,29 @@ vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf)
> > >  
> > >  	reserved_space = PAGE_SIZE;
> > >  
> > > +	/*
> > > +	 * We have our cached page from a previous mkwrite, check it to make
> > > +	 * sure it's still dirty and our file size matches when we ran mkwrite
> > > +	 * the last time.  If everything is OK then return VM_FAULT_LOCKED,
> > > +	 * otherwise do the mkwrite again.
> > > +	 */
> > > +	if (vmf->flags & FAULT_FLAG_USED_CACHED) {
> > > +		lock_page(page);
> > > +		if (vmf->cached_size == i_size_read(inode) &&
> > > +		    PageDirty(page))
> > > +			return VM_FAULT_LOCKED;
> > > +		unlock_page(page);
> > > +	}
> > 
> > I guess this is similar to Dave's comment: Why is i_size so special? What
> > makes sure that file didn't get modified between time you've prepared
> > cached_page and now such that you need to do the preparation again?
> > And if indeed metadata prepared for a page cannot change, what's so special
> > about it being that particular cached_page?
> > 
> > Maybe to phrase my objections differently: Your preparations in
> > btrfs_page_mkwrite() are obviously related to your filesystem metadata. So
> > why cannot you infer from that metadata (extent tree, whatever - I'd use
> > extent status tree in ext4) whether that particular file+offset is already
> > prepared for writing and just bail out with success in that case?
> > 
> 
> I was just being overly paranoid, I was afraid of the case where we would
> truncate and then extend in between, but now that I actually think about it that
> would end up with the page not being on the mapping anymore so we would catch
> that case.  I've dropped this part from my current version.  I'm getting some
> testing on these patches in production and I'll post them sometime next week
> once I'm happy with them.  Thanks,

OK, but do you still need the vmf->cached_page stuff? Because I don't see
why even that is necessary...

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