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    SubjectRe: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page
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    On Tuesday 14 April 2009 22:02:47 KOSAKI Motohiro wrote:
    > > On Tuesday 14 April 2009 16:16:52 KOSAKI Motohiro wrote:
    > >> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

    > >> @@ -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.
    >
    > Yes, that's trade-off.
    > your early decow also can misjudge and make unnecessary copy.

    Yes indeed it can. Although it would only ever do so in case of pages
    that have had get_user_pages run against them previously, and not from
    random interactions from any other parts of the kernel.

    I would be interested, using an anon vma field as you say for keeping
    a gup count... it could potentially be used to avoid the extra copy.
    But hmm, I don't have much time to go down that path so long as the
    basic concept of my proposal is in question.


    > >> /*
    > >> + * 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).
    >
    > I agree this code effect widely kernel activity.
    > but actually, in past days, the kernel did the same behavior. then
    > almost core code is
    > page_count checking safe.
    >
    > but Yes, we need to afraid newer code don't works with this code...
    >
    >
    > > And.... I don't think this is safe against a concurrent gup_fast()
    > > (which helps my point).
    >
    > Could you please explain more detail ?
    >

    + if (PageSwapCache(page) &&
    + page_count(page) != page_mapcount(page) + 2) {
    + ret = SWAP_FAIL;
    + goto out_unmap;
    + }

    Now if another thread does a get_user_pages_fast after it passes this
    check, it can take a gup reference to the page which is now about to
    be unmapped. Then after it is unmapped, if a wp fault is caused on the
    page, then it will not be reused and thus you lose data as explained
    in your big comment.


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