[PATCH] mm: get_user_pages vs. try_to_unmap
Andrea Arcangeli's fix to an ironic weakness with get_user_pages. try_to_unmap_one must check page_count against page->mapcount before unmapping a swapcache page: because the raised pagecount by which get_user_pages ensures the page cannot be freed, will cause any write fault to see that page as not exclusively owned, and therefore a copy page will be substituted for it - the reverse of what's intended. rmap.c was entirely free of such page_count heuristics before, I tried hard to avoid putting this in. But Andrea's fix rarely gives a false positive; and although it might be nicer to change exclusive_swap_page etc. to rely on page->mapcount instead, it seems likely that we'll want to get rid of page->mapcount later, so better not to entrench its use. Signed-off-by: Hugh Dickins <hugh@veritas.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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