Re: [PATCH 1/3] When migrate_pages returns 0, all pages must have been released

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, Jan 21, 2011 at 06:36:18PM +0100, Andrea Arcangeli wrote:
> On Fri, Jan 21, 2011 at 10:11:03AM -0600, Christoph Lameter wrote:
> > On Thu, 20 Jan 2011, Andrea Arcangeli wrote:
> > 
> > > Which following putback_lru_page()?  You mean
> > > putback_lru_page(newpage)? That is for the newly allocated page
> > > (allocated at the very top, so always needed), it's not relevant to
> > > the page_count(page) = 1. The page_count 1 is hold by the caller, so
> > > it's leaking memory right now (for everything but compaction).
> > 
> > Ahh yes we removed the putback_lru_pages call from migrate_pages()
> > and broke the existing release logic. The caller has to call
> > putback_release_pages() as per commit
> 
> putback_lru_paeges
> 
> > cf608ac19c95804dc2df43b1f4f9e068aa9034ab
> 
> That is the very commit that introduced the two bugs that I've fixed
> by code review.
> 
> > 
> > If that is still the case then we still have the double free.
> 
> The caller only calls putback_lru_pages if ret != 0 (the two cases you
> refer to happen with ret = 0).
> 
> Even if caller unconditionally calls putback_lru_pages (kind of what
> compaction did), it can't double free because migrate_pages already
> unlinked the pages before calling putback_lru_page(page), so there's
> no way to do a double free (however if the caller unconditionally
> called putback_lru_pages there would be no memleak to fix, but it
> doesn't).
> 
> > Could we please document the calling conventions exactly in the source?
> > Right now it says that the caller should call putback_lru_pages().
> 
> The caller should call putback_lru_pages only if ret != 0. Minchan
> this is your commit we're discussing can you check the commentary?

No problem.
I will send the patch.

Thanks Adnrea, Christoph.


> 
> Thanks!
> Andrea
> 

-- 
Kind regards,
Minchan Kim