linux-kernel - Re: [PATCH 0/3] aio: ctx->dead cleanups

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Message-ID: <20150617005012.GD17109@ZenIV.linux.org.uk>
Date:	Wed, 17 Jun 2015 01:50:12 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Jeff Moyer <jmoyer@...hat.com>, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] aio: ctx->dead cleanups

On Wed, Jun 17, 2015 at 01:39:06AM +0100, Al Viro wrote:

> Huh?  kill_ioctx() picks ctx->mmap_base and passes it to vm_munmap().
> Which tries to grab mmap_sem, blocks for mremap() from another thread
> and waits for it to drop mmap_sem.  By that time ctx->mmap_base has
> nothing whatsoever to the argument we'd passed to vm_munmap().  Sure,
> it had been recalculated by aio_ring_remap(), but it's too late for
> us - we'd already fetched the old value.

And yes, the leak you've spotted is real, but I would very much prefer
to avoid that goto - something like this instead:

diff --git a/mm/mremap.c b/mm/mremap.c
index 034e2d3..b36b530 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -291,7 +291,10 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 		if (err < 0) {
 			move_page_tables(new_vma, new_addr, vma, old_addr,
 					 moved_len, true);
-			return err;
+			vma = new_vma;
+			old_len = new_len;
+			old_addr = new_addr;
+			new_addr = err;
 		}
 	}
 
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