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

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Message-ID: <CACQD4-4bbwk_LOUVamTyB6V+Fg_F+Q4q2g8DxroTM7YiA=eJzQ@mail.gmail.com>
Date:	Fri, 28 Feb 2014 22:36:29 -0800
From:	Ning Qu <quning@...gle.com>
To:	Hugh Dickins <hughd@...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>, 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

Btw, should we first check if page returned by radix_tree_deref_slot is NULL?

diff --git a/mm/filemap.c b/mm/filemap.c
index 1bc12a9..c129ee5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1745,6 +1745,8 @@ void filemap_map_pages(struct vm_area_struct
*vma, struct vm_fault *vmf)
                        break;
 repeat:
                page = radix_tree_deref_slot(slot);
+               if (unlikely(!page))
+                       continue;
                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page))


Best wishes,
-- 
Ning Qu (曲宁) | Software Engineer | quning@...gle.com | +1-408-418-6066


On Fri, Feb 28, 2014 at 5:20 PM, Hugh Dickins <hughd@...gle.com> wrote:
> 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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