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SubjectRe: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page
Date
On Tuesday 14 April 2009 16:16:52 KOSAKI Motohiro wrote:

> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Sugessted-by: Linus Torvalds <torvalds@osdl.org>

"Suggested-by:" ;)

> Cc: Hugh Dickins <hugh@veritas.com>
> Cc: Andrew Morton <akpm@osdl.org>
> Cc: Nick Piggin <nickpiggin@yahoo.com.au>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Jeff Moyer <jmoyer@redhat.com>
> Cc: linux-mm@kvack.org
> ---
> mm/rmap.c | 21 +++++++++++++++++++++
> mm/swapfile.c | 10 +++++++++-
> 2 files changed, 30 insertions(+), 1 deletion(-)
>
> Index: b/mm/swapfile.c
> ===================================================================
> --- a/mm/swapfile.c 2009-04-11 21:38:33.000000000 +0900
> +++ b/mm/swapfile.c 2009-04-11 21:38:45.000000000 +0900
> @@ -533,6 +533,8 @@ static inline int page_swapcount(struct
> * to it. And as a side-effect, free up its swap: because the old content
> * on disk will never be read, and seeking back there to write new content
> * later would only waste time away from clustering.
> + * Caller must hold pte_lock. try_to_unmap() decrement page::_mapcount
> + * and get_user_pages() increment page::_count under pte_lock.
> */
> int reuse_swap_page(struct page *page)
> {
> @@ -547,7 +549,13 @@ int reuse_swap_page(struct page *page)
> SetPageDirty(page);
> }
> }
> - return count == 1;
> +
> + /*
> + * If we can re-use the swap page _and_ the end
> + * result has only one user (the mapping), then
> + * we reuse the whole page
> + */
> + return count + page_count(page) == 2;
> }

I guess this patch does work to close the read-side race, but I slightly don't
like using page_count for things like this. page_count can be temporarily
raised for reasons other than access through their user mapping. Swapcache,
page reclaim, LRU pagevecs, concurrent do_wp_page, etc.


> /*
> Index: b/mm/rmap.c
> ===================================================================
> --- a/mm/rmap.c 2009-04-11 21:38:33.000000000 +0900
> +++ b/mm/rmap.c 2009-04-12 00:58:58.000000000 +0900
> @@ -773,6 +773,27 @@ static int try_to_unmap_one(struct page
> goto out;
>
> /*
> + * Don't pull an anonymous page out from under get_user_pages.
> + * GUP carefully breaks COW and raises page count (while holding
> + * pte_lock, as we have here) to make sure that the page
> + * cannot be freed. If we unmap that page here, a user write
> + * access to the virtual address will bring back the page, but
> + * its raised count will (ironically) be taken to mean it's not
> + * an exclusive swap page, do_wp_page will replace it by a copy
> + * page, and the user never get to see the data GUP was holding
> + * the original page for.
> + *
> + * This test is also useful for when swapoff (unuse_process) has
> + * to drop page lock: its reference to the page stops existing
> + * ptes from being unmapped, so swapoff can make progress.
> + */
> + if (PageSwapCache(page) &&
> + page_count(page) != page_mapcount(page) + 2) {
> + ret = SWAP_FAIL;
> + goto out_unmap;
> + }

I guess it does add another constraint to the VM, ie. not allowed to
unmap an anonymous page with elevated refcount. Maybe not a big deal
now, but I think it is enough that it should be noted. If you squint,
this could actually be more complex/intrusive to the wider VM than my
copy on fork (which is basically exactly like a manual do_wp_page at
fork time).

And.... I don't think this is safe against a concurrent gup_fast()
(which helps my point).


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