Date: Sat, 26 Sep 2009 12:09:21 +0100 (BST)
From: Hugh Dickins <hugh.dickins@tiscali.co.uk>
To: Wu Fengguang <fengguang.wu@intel.com>
cc: Andrew Morton <akpm@linux-foundation.org>, Nick Piggin <npiggin@suse.de>,
       Andi Kleen <andi@firstfloor.org>, linux-mm@kvack.org,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked()
In-Reply-To: <20090926031537.GA10176@localhost>
Message-ID: <Pine.LNX.4.64.0909261115530.12927@sister.anvils>
References: <20090926031537.GA10176@localhost>
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On Sat, 26 Sep 2009, Wu Fengguang wrote:

> The swap cache and page cache code assume that they 'own' the newly
> allocated page and therefore can disregard the locking rules. However
> now hwpoison can hit any time on any page.
> 
> So use the safer lock_page()/trylock_page(). The main intention is not
> to close such a small time window of memory corruption. But to avoid
> kernel oops that may result from such races, and also avoid raising
> false alerts in hwpoison stress tests.
> 
> This in theory will slightly increase page cache/swap cache overheads,
> however it seems to be too small to be measurable in benchmark.

No.

But I'd most certainly defer to Nick if he disagrees with me.

I don't think anyone would want to quarrel very long over the swap
and migration mods alone, but add_to_page_cache() is of a higher
order of magnitude.

I can't see any reason to surrender add_to_page_cache() optimizations
to the remote possibility of hwpoison (infinitely remote for most of
us); though I wouldn't myself want to run the benchmark to defend them.

You'd later be sending a patch to replace __SetPageUptodate()s by
SetPageUptodate()s etc, wouldn't you?  Because any non-atomic op
on page->flags might wipe your locked bit (or your hwpoison bit).

You could make #ifdef CONFIG_HWPOISON_INJECT select the slower
versions of these things; or #ifdef CONFIG_MEMORY_FAILURE? that
would pose distros with a harder choice.  But I'd much prefer
not to go that way.

Please accept that there will be quite a number of places where
the code "knows" it's the only user of the page, and hwpoison
handling and testing should work around those places (can shift
things around slightly to suit itself better, but not add cost).

Look into why you think you want the page lock: I can see it's
going to be useful if you're taking a page out of a file (but then
why bother if page->mapping not set?), or if you're using rmap-style
lookup (but then why bother if !page_mapped?).

I suspect if memory_failure() did something like:
	if (page->mapping)
		lock_page_nosync(p);
then you'd be okay, perhaps with a few additional _inexpensive_
tweaks here and there.  With the "necessary" memory barriers?
no, we probably wouldn't want to be adding any in hot paths.

But I definitely say "something like": remember that page_mapping()
does that weird thing with PageSwapCache (a mistake from day one in
my opinion), which might or might not be what you want.  There are
probably various reasons why it's not as simple as I suggest above.

It seems to me that the Intel hardware guys have done half a job
here: the sooner they get to remapping the bad pages, the better.

Hugh

> 
> CC: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> CC: Andi Kleen <andi@firstfloor.org> 
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  include/linux/pagemap.h |   13 ++++---------
>  mm/migrate.c            |    2 +-
>  mm/swap_state.c         |    4 ++--
>  3 files changed, 7 insertions(+), 12 deletions(-)
> 
> --- sound-2.6.orig/mm/swap_state.c	2009-09-14 10:50:19.000000000 +0800
> +++ sound-2.6/mm/swap_state.c	2009-09-25 18:42:23.000000000 +0800
> @@ -306,7 +306,7 @@ struct page *read_swap_cache_async(swp_e
>  		 * re-using the just freed swap entry for an existing page.
>  		 * May fail (-ENOMEM) if radix-tree node allocation failed.
>  		 */
> -		__set_page_locked(new_page);
> +		lock_page(new_page);
>  		SetPageSwapBacked(new_page);
>  		err = add_to_swap_cache(new_page, entry, gfp_mask & GFP_KERNEL);
>  		if (likely(!err)) {
> @@ -318,7 +318,7 @@ struct page *read_swap_cache_async(swp_e
>  			return new_page;
>  		}
>  		ClearPageSwapBacked(new_page);
> -		__clear_page_locked(new_page);
> +		unlock_page(new_page);
>  		swapcache_free(entry, NULL);
>  	} while (err != -ENOMEM);
>  
> --- sound-2.6.orig/include/linux/pagemap.h	2009-09-14 10:50:19.000000000 +0800
> +++ sound-2.6/include/linux/pagemap.h	2009-09-25 18:42:19.000000000 +0800
> @@ -292,11 +292,6 @@ extern int __lock_page_killable(struct p
>  extern void __lock_page_nosync(struct page *page);
>  extern void unlock_page(struct page *page);
>  
> -static inline void __set_page_locked(struct page *page)
> -{
> -	__set_bit(PG_locked, &page->flags);
> -}
> -
>  static inline void __clear_page_locked(struct page *page)
>  {
>  	__clear_bit(PG_locked, &page->flags);
> @@ -435,18 +430,18 @@ extern void remove_from_page_cache(struc
>  extern void __remove_from_page_cache(struct page *page);
>  
>  /*
> - * Like add_to_page_cache_locked, but used to add newly allocated pages:
> - * the page is new, so we can just run __set_page_locked() against it.
> + * Like add_to_page_cache_locked, but used to add newly allocated pages.
>   */
>  static inline int add_to_page_cache(struct page *page,
>  		struct address_space *mapping, pgoff_t offset, gfp_t gfp_mask)
>  {
>  	int error;
>  
> -	__set_page_locked(page);
> +	if (!trylock_page(page))
> +		return -EIO;	/* hwpoisoned */
>  	error = add_to_page_cache_locked(page, mapping, offset, gfp_mask);
>  	if (unlikely(error))
> -		__clear_page_locked(page);
> +		unlock_page(page);
>  	return error;
>  }
>  
> --- sound-2.6.orig/mm/migrate.c	2009-09-14 10:50:19.000000000 +0800
> +++ sound-2.6/mm/migrate.c	2009-09-25 18:42:19.000000000 +0800
> @@ -551,7 +551,7 @@ static int move_to_new_page(struct page 
>  	 * holding a reference to the new page at this point.
>  	 */
>  	if (!trylock_page(newpage))
> -		BUG();
> +		return -EAGAIN;		/* got by hwpoison */
>  
>  	/* Prepare mapping for the new page.*/
>  	newpage->index = page->index;
--
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