Re: Kernel bug: Bad page state: related to generic symlink code andmmap

[Linus Torvalds]

On Fri, 19 Aug 2005, Anton Altaparmakov wrote:
>
> 		struct page *page;
> 		page = find_get_page(dentry->d_inode->i_mapping, 0);
> 		if (!page)
> ---->			BUG();

Something has truncated the mapping. 

My guess is that you had a cache invalidate event that removed the page
from the mapping while it was being used. That might also explain why this
only happens for ncpfs. I bet that in the other cases, the mapping was 
also invalidated, but re-filled immediately, and your strace slowed the 
other process down enough that it didn't get to re-fill the cache it 
invalidated or something like that.

The fact that it happens only for cross-volume things might also be 
explained that way - is there something ncpfs does when switching volumes 
that might trigger a cache invalidate for another volume (either on the 
client side or the server side - maybe the server tends to try to break 
the connection for the old volume when you start traversing a new one?)

The generic "page cache for symlinks" code does _not_ support invalidating 
the cache while it's being used. A local filesystem will obviously never 
invalidate the cache at all. 

Hmm.. NFS _does_ use the page cache for symlinks, but uses it slightly 
differently: instead of relying on the page cache entry being the same 
when freeing the page, it just caches the page it looked up in the page 
cache (ie "nfs_follow_link()" does look up the page cache, but then hides 
the page pointer inside the page data itself (uglee), and thus does not 
depend on the mapping staying the same (nfs_put_link() just takes the page 
from the symlink data).

		Linus
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