linux-kernel - Re: [PATCH 1/1] mm: implement ->map

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Date:	Fri, 28 Feb 2014 17:20:23 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Ning Qu <quning@...gle.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>, Ning Qu <quning@...il.com>,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm: implement ->map_pages for shmem/tmpfs

On Fri, 28 Feb 2014, Ning Qu wrote:

> In shmem/tmpfs, we also use the generic filemap_map_pages,
> seems the additional checking is not worth a separate version
> of map_pages for it.
> 
> Signed-off-by: Ning Qu <quning@...gle.com>
> ---
>  mm/shmem.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 1f18c9d..2ea4e89 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2783,6 +2783,7 @@ static const struct super_operations shmem_ops = {
>  
>  static const struct vm_operations_struct shmem_vm_ops = {
>  	.fault		= shmem_fault,
> +	.map_pages	= filemap_map_pages,
>  #ifdef CONFIG_NUMA
>  	.set_policy     = shmem_set_policy,
>  	.get_policy     = shmem_get_policy,
> -- 

(There's no need for a 0/1, all the info should go into the one patch.)

I expect this will prove to be a very sensible and adequate patch,
thank you: it probably wouldn't be worth more effort to give shmem
anything special of its own, and filemap_map_pages() is already
(almost) coping with exceptional entries.

But I can't Ack it until I've tested it some more, won't be able to
do so until Sunday; and even then some doubt, since this and Kirill's
are built upon mmotm/next, which after a while gives me spinlock
lockups under load these days, yet to be investigated.

"almost" above because, Kirill, even without Ning's extension to
shmem, your filemap_map_page() soon crashes on an exceptional entry:

Don't try to dereference an exceptional entry.

Signed-off-by: Hugh Dickins <hughd@...gle.com>

--- mmotm+kirill/mm/filemap.c	2014-02-28 15:17:50.984019060 -0800
+++ linux/mm/filemap.c	2014-02-28 16:38:04.976633308 -0800
@@ -2084,7 +2084,7 @@ repeat:
 			if (radix_tree_deref_retry(page))
 				break;
 			else
-				goto next;
+				continue;
 		}
 
 		if (!page_cache_get_speculative(page))
--
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