Re: [PATCH]: ufs: ufs_get_locked_patch race fix

Nick Piggin
2006-08-01T07:18:19Z

Andrew Morton wrote:

>On Mon, 31 Jul 2006 16:57:02 +0400
>Evgeniy Dushistov <dushistov@mail.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@mail.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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