Re: [PATCH]: ufs: ufs_get_locked_patch race fix

[Nick Piggin]

Andrew Morton wrote:

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