On Sat, 15 Oct 2005, Nick Piggin wrote:
>
> Is there anything that prevents PageDirty from theoretically being
> speculatively loaded before page_count here? (see patch)
>
> It would result in pagecache corruption in the following situation:
>
> 1 2
> find_get_page();
> write to page write_lock(tree_lock);
> SetPageDirty(); if (page_count != 2
> put_page(); || PageDirty())
>
> Now I'm worried that 2 might see PageDirty *before* SetPageDirty in
page->flags
> 1, and page_count *after* put_page in 1.
I think you're right. But I'm the last person to ask
barrier/ordering questions of. CC'ed Ben and Andrea.
Hugh
> --- linux-2.6.orig/mm/vmscan.c
> +++ linux-2.6/mm/vmscan.c
> @@ -511,7 +511,12 @@ static int shrink_list(struct list_head
> * PageDirty _after_ making sure that the page is freeable and
> * not in use by anybody. (pagecache + us == 2)
> */
> - if (page_count(page) != 2 || PageDirty(page)) {
> + if (page_count(page) != 2) {
> + write_unlock_irq(&mapping->tree_lock);
> + goto keep_locked;
> + }
> + smp_rmb();
> + if (PageDirty(page)) {
> write_unlock_irq(&mapping->tree_lock);
> goto keep_locked;
> }
-
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