Re: [PATCH][v6] filemap: drop the mmap_sem for all blocking operations

[Jan Kara <jack@suse.cz>]

On Wed 12-12-18 15:55:36, Andrew Morton wrote:
> On Wed, 12 Dec 2018 10:27:57 -0500 Josef Bacik <josef@toxicpanda.com> wrote:
> 
> > v5->v6:
> > - added more comments as per Andrew's suggestion.
> > - fixed the fpin leaks in the two error paths that were pointed out.
> > 
> 
> hm,
> 
> > --- a/mm/filemap.c~filemap-drop-the-mmap_sem-for-all-blocking-operations-v6
> > +++ a/mm/filemap.c
> > @@ -2461,7 +2476,8 @@ static struct file *do_sync_mmap_readahe
> >  
> >  /*
> >   * Asynchronous readahead happens when we find the page and PG_readahead,
> > - * so we want to possibly extend the readahead further..
> > + * so we want to possibly extend the readahead further.  We return the file that
> > + * was pinned if we have to drop the mmap_sem in order to do IO.
> >   */
> >  static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
> >  					    struct page *page)
> > @@ -2545,14 +2561,15 @@ retry_find:
> >  		page = pagecache_get_page(mapping, offset,
> >  					  FGP_CREAT|FGP_FOR_MMAP,
> >  					  vmf->gfp_mask);
> > -		if (!page)
> > +		if (!page) {
> > +			if (fpin)
> > +				goto out_retry;
> 
> Is this right?  If pagecache_get_page() returns NULL we can now return
> VM_FAULT_MAJOR|VM_FAULT_RETRY whereas we used to return ENOMEM.

Yes, but once we've dropped mmap_sem, there's no way safely return -ENOMEM.
So VM_FAULT_RETRY is really the only option to tell the caller that
mmap_sem is not held anymore...

So the patch looks good to me now. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>


								Honza

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