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    SubjectRe: [RFC][PATCH 1/3] radix priority search tree - objrmap complexity fix
    On Thu, Apr 01, 2004 at 05:15:34PM +0200, Andrea Arcangeli wrote:
    > > diff -urNp --exclude CVS --exclude BitKeeper --exclude {arch} --exclude .arch-ids x-ref/mm/page_io.c x/mm/page_io.c
    > > --- x-ref/mm/page_io.c 2004-04-01 17:07:10.231289760 +0200
    > > +++ x/mm/page_io.c 2004-04-01 17:07:33.182800600 +0200
    > > @@ -139,7 +139,7 @@ struct address_space_operations swap_aop
    > >
    > > /*
    > > * A scruffy utility function to read or write an arbitrary swap page
    > > - * and wait on the I/O.
    > > + * and wait on the I/O. The caller must have a ref on the page.
    > > */
    > > int rw_swap_page_sync(int rw, swp_entry_t entry, struct page *page)
    > > {
    > > @@ -151,8 +151,16 @@ int rw_swap_page_sync(int rw, swp_entry_
    > > lock_page(page);
    > >
    > > BUG_ON(page->mapping);
    > > - page->mapping = &swapper_space;
    > > - page->index = entry.val;
    > > + ret = add_to_page_cache(page, &swapper_space, entry.val, GFP_KERNEL);
    > > + if (unlikely(ret)) {
    > > + unlock_page(page);
    > > + return ret;
    > > + }
    > > + /*
    > > + * get one more reference to make page non-exclusive so
    > > + * remove_exclusive_swap_page won't mess with it.
    > > + */
    > > + page_cache_get(page);
    > >
    > > if (rw == READ) {
    > > ret = swap_readpage(NULL, page);
    > > @@ -161,7 +169,13 @@ int rw_swap_page_sync(int rw, swp_entry_
    > > ret = swap_writepage(page, &swap_wbc);
    > > wait_on_page_writeback(page);
    > > }
    > > - page->mapping = NULL;
    > > +
    > > + lock_page(page);
    > > + remove_from_page_cache(page);
    > > + unlock_page(page);
    > > + page_cache_release(page);
    > > + page_cache_release(page); /* For add_to_page_cache() */
    > > +
    > > if (ret == 0 && (!PageUptodate(page) || PageError(page)))
    > > ret = -EIO;
    > > return ret;
    >
    > incrementally to the above I applied this hardness checks in the
    > anon-vma patch, so we're safe against the problem Andrew outlined
    > (somebody attempting to do swapin/swapouts of anonymous pages through
    > that interface, something that shouldn't happen since we want only the
    > VM to deal with userspace mapped pages).
    >
    > @@ -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. 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.
    -
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