Re: [RFC][PATCH 1/3] radix priority search tree - objrmap complexity fix

From: Andrew Morton (akpm_at_osdl.org)
Date: 04/02/04

  • Next message: Tomas Vinar: "bug: hid module crashed" 
    Date:	Thu, 1 Apr 2004 16:52:16 -0800
To: Andrea Arcangeli <andrea@suse.de>

    Andrea Arcangeli <andrea@suse.de> wrote:
    >
    > > @@ -149,8 +149,14 @@ int rw_swap_page_sync(int rw, swp_entry_
    > > };
    > >
    > > lock_page(page);
    > > -
    > > + /*
    > > + * This library call can be only used to do I/O
    > > + * on _private_ pages just allocated with alloc_pages().
    > > + */
    > > BUG_ON(page->mapping);
    > > + BUG_ON(PageSwapCache(page));
    > > + BUG_ON(PageAnon(page));
    > > + BUG_ON(PageLRU(page));
    > > ret = add_to_page_cache(page, &swapper_space, entry.val, GFP_KERNEL);
    > > if (unlikely(ret)) {
    > > unlock_page(page);
    >
    >
    > the good thing is that I believe this fix will make it work with the -mm
    > writeback changes. However this fix now collides with anon-vma since
    > swapsuspend passes compound pages to rw_swap_page_sync and
    > add_to_page_cache overwrites page->private and the kernel crashes at the
    > next page_cache_get() since page->private is now the swap entry and not
    > a page_t pointer.

    Why do swapcache pages have their ->index in ->private? That should have
    been commented.

    (hugetlb pages are also added to pagecache, and they are compound, but the
    code looks OK).

    > So I guess I've a good reason now to giveup trying to
    > add the page to the swapcache, and to just fake the radix tree like I
    > did in my original fix. That way the page won't be swapcache either so I
    > don't even need to use get_page to avoid remove_exclusive_swap_page to
    > mess with it.

    The BUG_ON in radix_tree_tag_set() is a fairly arbitrary sanity check:
    "hey, why are you tagging a non-existent item?".

    We could simply replace it with a `return NULL;'?
    -
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