Re: [PATCH v3 3/5] dax: use common 4k zero page for dax mmap reads

[Jan Kara]

On Wed 28-06-17 16:01:50, Ross Zwisler wrote:
> Another major change is that we remove dax_pfn_mkwrite() from our fault
> flow, and instead rely on the page fault itself to make the PTE dirty and
> writeable.  The following description from the patch adding the
> vm_insert_mixed_mkwrite() call explains this a little more:
> 
> ***
>   To be able to use the common 4k zero page in DAX we need to have our PTE
>   fault path look more like our PMD fault path where a PTE entry can be
>   marked as dirty and writeable as it is first inserted, rather than
>   waiting for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.
> 
>   Right now we can rely on having a dax_pfn_mkwrite() call because we can
>   distinguish between these two cases in do_wp_page():
> 
>   	case 1: 4k zero page => writable DAX storage
>   	case 2: read-only DAX storage => writeable DAX storage
> 
>   This distinction is made by via vm_normal_page().  vm_normal_page()
>   returns false for the common 4k zero page, though, just as it does for
>   DAX ptes.  Instead of special casing the DAX + 4k zero page case, we will
>   simplify our DAX PTE page fault sequence so that it matches our DAX PMD
>   sequence, and get rid of dax_pfn_mkwrite() completely.
> 
>   This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
>   and allow us to pass in a 'mkwrite' flag.  If 'mkwrite' is set
>   insert_pfn() will do the work that was previously done by wp_page_reuse()
>   as part of the dax_pfn_mkwrite() call path.
> ***

Hum, thinking about this in context of this patch... So what if we have
allocated storage, a process faults it read-only, we map it to page tables
writeprotected. Then the process writes through mmap to the area - the code
in handle_pte_fault() ends up in do_wp_page() if I'm reading it right.
Then, since we are missing ->pfn_mkwrite() handlers, the PTE will be marked
writeable but radix tree entry stays clean - bug. Am I missing something?

								Honza

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