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

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(a)intel.com&gt; > > 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);