Re: [patch 3/3] mm: fault handler to replace nopage and populate

[Thomas Hellstrom <thomas@tungstengraphics.com>]

Nick Piggin wrote:
> On Mon, Oct 09, 2006 at 10:07:50PM +1000, Benjamin Herrenschmidt wrote:
> 
>>On Mon, 2006-10-09 at 13:58 +0200, Nick Piggin wrote:
>>
>>>The VM won't see that you have struct pages backing the ptes, and won't
>>>do the right refcounting or rmap stuff... But for file backed mappings,
>>>all the critical rmap stuff should be set up at mmap time, so you might
>>>have another option to simply always do the nopfn thing, as far as the
>>>VM is concerned (ie. even when you do have a struct page)
>>
>>Any reason why it wouldn't work to flip that bit on the first no_page()
>>after a migration ? A migration always involves destroying all PTEs and
>>is done with a per-object mutex held that no_page() takes too, so we can
>>be pretty sure that the first nopage can set that bit before any PTE is
>>actually inserted in the mapping after all the previous ones have been
>>invalidated... That would avoid having to walk the vma's.
> 
> 
> Ok I guess that would work. I was kind of thinking that one needs to
> hold the mmap_sem for writing when changing the flags, but so long
> as everyone *else* does, then I guess you can get exclusion from just
> the read lock. And your per-object mutex would prevent concurrent
> nopages from modifying it.

Wouldn't that confuse concurrent readers?

Could it be an option to make it safe for the fault handler to 
temporarily drop the mmap_sem read lock given that some conditions TBD 
are met?
In that case it can retake the mmap_sem write lock, do the VMA flags 
modifications, downgrade and do the pte modifications using a helper, or 
even use remap_pfn_range() during the time the write lock is held?

/Thomas