linux-kernel - Re: [PATCH]: ufs: ufs_get_locked

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Date:	Tue, 01 Aug 2006 17:18:14 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...l.org>
CC:	Evgeniy Dushistov <dushistov@...l.ru>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH]: ufs: ufs_get_locked_patch race fix

Andrew Morton wrote:

>On Mon, 31 Jul 2006 16:57:02 +0400
>Evgeniy Dushistov <dushistov@...l.ru> wrote:
>
>
>>As discussed earlier:
>>http://lkml.org/lkml/2006/6/28/136
>>this patch fixes such issue:
>>`ufs_get_locked_page' takes page from cache
>>after that `vmtruncate' takes page and deletes it from cache
>>`ufs_get_locked_page' locks page, and reports about EIO error.
>>
>>Also because of find_lock_page always return valid page or NULL,
>>we have no need check it if page not NULL.
>>
>>Signed-off-by: Evgeniy Dushistov <dushistov@...l.ru>
>>
>>
>>---
>>
>>
>>Index: linux-2.6.18-rc2-mm1/fs/ufs/util.c
>>===================================================================
>>--- linux-2.6.18-rc2-mm1.orig/fs/ufs/util.c
>>+++ linux-2.6.18-rc2-mm1/fs/ufs/util.c
>>@@ -257,6 +257,7 @@ try_again:
>> 		page = read_cache_page(mapping, index,
>> 				       (filler_t*)mapping->a_ops->readpage,
>> 				       NULL);
>>+
>> 		if (IS_ERR(page)) {
>> 			printk(KERN_ERR "ufs_change_blocknr: "
>> 			       "read_cache_page error: ino %lu, index: %lu\n",
>>@@ -266,6 +267,13 @@ try_again:
>> 
>> 		lock_page(page);
>> 
>>+		if (unlikely(page->mapping != mapping ||
>>+			     page->index != index)) {
>>+			unlock_page(page);
>>+			page_cache_release(page);
>>+			goto try_again;
>>+		}
>>+
>> 		if (!PageUptodate(page) || PageError(page)) {
>> 			unlock_page(page);
>> 			page_cache_release(page);
>>
>
>Looks good to me.
>
>Is there any need to be checking ->index?  Normally we simply use the
>sequence:
>
>	lock_page(page);
>	if (page->mapping == NULL)
>		/* truncate got there first */
>
>to handle this case.
>

If you already have a reference on the page, I believe no. We have to
be a bit careful about it in the core vm now that splice can move
pages between different parts of the same mapping, but any code outside
mm/ shouldn't need to worry (exactly because they should have a ref
before doing the lock_page).
--

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