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

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Message-ID: <20150617012245.GA22709@redhat.com>
Date:	Wed, 17 Jun 2015 03:22:45 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
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 06/17, Al Viro wrote:
>
> 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;

Personally, I'd really prefer to factor out at least this
move_page_tables() with six args. Although I agree, "goto previous_if"
doesn't look nice too, this needs cleanup.

But this is minor. I am already sleeping, most probably I misread
this code. But it seems that there is another bug with VM_ACCOUNT.

I'll recheck tomorrow and write another email.

Oleg.

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