Re: [RFC] memory defragmentation to satisfy high order allocations

[IWAMOTO Toshihiro <iwamoto@valinux.co.jp>]

At Sun, 3 Oct 2004 11:07:23 -0300,
Marcelo Tosatti wrote:
> 
> On Sun, Oct 03, 2004 at 01:13:38PM +0900, Hirokazu Takahashi wrote:
> > > 2) 
> > > At migrate_onepage you add anonymous pages which aren't swap allocated
> > > to the swap cache
> > > +       /*
> > > +        * Put the page in a radix tree if it isn't in the tree yet.
> > > +        */
> > > +#ifdef CONFIG_SWAP
> > > +       if (PageAnon(page) && !PageSwapCache(page))
> > > +               if (!add_to_swap(page, GFP_KERNEL)) {
> > > +                       unlock_page(page);
> > > +                       return ERR_PTR(-ENOSPC);
> > > +               }
> > > +#endif /* CONFIG_SWAP */
> > > 
> > > Why's that? You can copy anonymous pages without adding them to swap (thats
> > > what the patch I posted does).
> > 
> > The reason is to guarantee that any anonymous page can be migrated anytime.
> > I want to block newly occurred accesses to the page during the migration
> > because it can't be migrated if there remain some references on it by
> > system calls, direct I/O and page faults.
> 
> It would be nice if we could block pte faults in a way such to not need
> adding each anonymous page to swap. It can be too costly if you have a lot memory
> and it makes the whole operation dependable on swap size (if you dont have enough
> swap, you're dead).
> 
> Maybe hold mm->page_table_lock (might be too costly in terms of CPU time, but since
> migration is not a common operation anyway), or create a semaphore? 

I chose the swap cache based implementation in order to minimize
slowdown of the normal code path.  (I thought there's zero code
addition on the normal pagefault path when I designed this, but it's
no longer true...)

If we can agree on adding a new lock, there might be a better
implementation.

--
IWAMOTO Toshihiro
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