linux-kernel - Re: [PATCH] aoe: adjust ref of head for compound page tails

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Date:	Wed, 7 Aug 2013 19:54:45 -0400
From:	Ed Cashin <ecashin@...aid.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] aoe: adjust ref of head for compound page tails

On Aug 7, 2013, at 7:35 PM, Andrew Morton wrote:

> On Wed, 7 Aug 2013 19:27:40 -0400 Ed Cashin <ecashin@...aid.com> wrote:
> 
>> On Aug 7, 2013, at 5:26 PM, Andrew Morton wrote:
>> 
>>> On Wed, 7 Aug 2013 17:21:32 -0400 Ed Cashin <ecashin@...aid.com> wrote:
>> ...
>>>> Sorry.  I tried to cover that but maybe should have separated it more clearly from the running text.
>>>> 
>>>> The patch changes the current behavior encountered with direct I/O and aoe:
>>>> 
>>>> aoe can BUG when direct I/O used
>>>> 
>>>> ... to a new behavior:
>>>> 
>>>> aoe does not BUG when direct I/O is used
>>>> 
>>>> And the user will never know that instead aoe is manipulating the _count of the head page, even though that's what's happening with this patch applied.
>>>> 
>>>> (The user-visible effects that motivated the older patch that added the BUG to aoe when page tails are used for block I/O was a VM_BUG_ON that occurred when the aoe driver manipulated the _count of a compound page tail.)
>>> 
>>> OK, I added "Fix a BUG which can trigger when direct-IO is used with
>>> AOE".  Which kernel versions are affected?
>> 
>> 
>> That BUG was added to the aoe driver in a commit that first appears in v3.7.  It persists until this patch adds the code that handles the situation that the BUG was intended to detect.
>> 
>> [ecashin@...ino linux]$ git describe --contains --match 'v*' 69cf2d85de773d998798e47e3335b85e5645d157
>> v3.7-rc1~110^2~22
>> [ecashin@...ino linux]$ 
>> 
> 
> ah, is it this?
> 
> +bio_pageinc(struct bio *bio)
> +{
> +       struct bio_vec *bv;
> +       struct page *page;
> +       int i;
> +
> +       bio_for_each_segment(bv, bio, i) {
> +               page = bv->bv_page;
> +               /* Non-zero page count for non-head members of
> +                * compound pages is no longer allowed by the kernel,
> +                * but this has never been seen here.
> +                */
> +               if (unlikely(PageCompound(page)))
> +                       if (compound_trans_head(page) != page) {
> +                               pr_crit("page tail used for block I/O\n");
> +                               BUG();
> +                       }
> 
> But get_page/put_page against a tail page of a compound page should
> Just Work.  The core MM will hunt down the head page and manipulate its
> refcount.  
> 
> Perhaps the problem here is that AOE is diving under the covers and is
> using low-level page refcount alterations:
> 
> +               atomic_inc(&page->_count);
> 
> Why does AOE do this?  It would be better if it were to use the
> official published MM interfaces.  If those interfaces need
> alteration/augmentation then we can do that.

The get_page cannot be used when the _count is 0 (which is exactly when we need to increment it), because it will VM_BUG_ON.

	/*
	 * Getting a normal page or the head of a compound page
	 * requires to already have an elevated page->_count.
	 */
	VM_BUG_ON(atomic_read(&page->_count) <= 0);

(And seeing the code reminds me that I forgot to include atomic_read in my last email's proposal.  LKML + dinner isn't a good mix. ;)

-- 
  Ed Cashin
  ecashin@...aid.com


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