Re: [PATCH 5/5] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()

[Hugh Dickins <hughd@google.com>]

On Tue, 4 Dec 2012, Hugh Dickins wrote:
> 
> Yes, your patch fixes it Mel, but I prefer it as below, with a couple
> of mods: removing the no longer true comment, and leaving shmem_swapin()
> alone with just a comment.  It appears to be the job of the rather weird
> mpol_cond_copy() to drop the reference on the original mempolicy, and
> clear MPOL_F_SHARED so the copy won't need one (it's trying to cope with
> the fact that swapin_readahead will make an unknown number of calls to
> alloc_page_vma).  So I'd rather not add another mpol_cond_put there,
> whose cond will never be met.

Hold on, ignore that patch for now, I think I had my priorities
upside down: it would be better for shmem_swapin() to behave as
you proposed, and we delete the mpol_cond_copy() weirdness instead.

Your 00442ad04a5e changed alloc_pages_vma() to keep its refcounting
in balance, so it now does not matter that swapin_readahead() makes
an unknown number of calls to it: we should simply take a reference
before and drop it after, just as you do in shmem_alloc_page().

I'd still like to revisit alloc_page_vma(), and its refcount
manipulations do now appear redundant; but changing that is not
something I want to get into in a last minute rush.  But getting rid
of mpol_cond_copy() should be safe and clear, I'll test that out now
and reply with an updated patch (or else admit I got confused).

Hugh

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