Re: [PATCH v4 10/12] filesystem-dax: Introduce dax_lock_page()

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Mon, Jun 11, 2018 at 05:41:46PM +0200, Jan Kara wrote:
> On Fri 08-06-18 16:51:14, Dan Williams wrote:
> > In preparation for implementing support for memory poison (media error)
> > handling via dax mappings, implement a lock_page() equivalent. Poison
> > error handling requires rmap and needs guarantees that the page->mapping
> > association is maintained / valid (inode not freed) for the duration of
> > the lookup.
> > 
> > In the device-dax case it is sufficient to simply hold a dev_pagemap
> > reference. In the filesystem-dax case we need to use the entry lock.
> > 
> > Export the entry lock via dax_lock_page() that uses rcu_read_lock() to
> > protect against the inode being freed, and revalidates the page->mapping
> > association under xa_lock().
> > 
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> 
> Some comments below...
> 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index cccf6cad1a7a..b7e71b108fcf 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -361,6 +361,82 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
> >  	}
> >  }
> >  
> > +struct page *dax_lock_page(unsigned long pfn)
> > +{
> 
> Why do you return struct page here? Any reason behind that? Because struct
> page exists and can be accessed through pfn_to_page() regardless of result
> of this function so it looks a bit confusing. Also dax_lock_page() name
> seems a bit confusing. Maybe dax_lock_pfn_mapping_entry()?

It's also a bit awkward that the functions are asymmetric in their arguments:
dax_lock_page(pfn) vs dax_unlock_page(struct page)

Looking at dax_lock_page(), we only use 'pfn' to get 'page', so maybe it would
be cleaner to just always deal with struct page, i.e.:

void dax_lock_page(struct page *page);
void dax_unlock_page(struct page *page);