Messages in this thread | | | Date | Fri, 2 Apr 2004 03:06:45 +0200 | From | Andrea Arcangeli <> | Subject | Re: [RFC][PATCH 1/3] radix priority search tree - objrmap complexity fix |
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On Thu, Apr 01, 2004 at 04:52:16PM -0800, Andrew Morton wrote: > 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.
that's because I must leave page->index free for the anon-vma tracking. Now an anonymous page while being swapped is just like a pagecache page, however the index on swap is different than the index in-address-space, because the swap is nonlinear. So I need to indexes, one for finding the page in the anon-vma in the task address space (page->index), the other (the swap-entry) for finding the page in the swap address-space (swapcache, or disk).
> (hugetlb pages are also added to pagecache, and they are compound, but the > code looks OK).
hugetlb is never swapped so yes, it cannot generate problems. The only thing swapping a compound page is swap suspend and that's why we didn't notice it yet.
> > 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;'?
I wouldn't like to reduce the hardness checks in the radix tree, I was very happy to find this robusteness checks trapping those bugs so reliably.
But I think the compound thing is overkill for 99% of usages, hugetlbfs is the only one really needing that sort of transparency in the refcounting I believe, so I'm now adding a __GFP_NO_COMP that swapsuspend will start to use to allocate the multipages, I'd better add it before somebody gets the idea of removing the order parameter to free_pages (something you could do just fine with page compound since the order is in page[1].index ;). This should fix it, I don't want to teach rw_swap_page_sync how to swapout a compound page, I'd rather make a compound page look like a regular page as far as rw_swap_page_sync is concerned. This will not slowdown the kernel at all since the additional check if to create a compound page or not will only trigger if the previous check of order > 0 is positive. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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