Re: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 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:" ;)

Agghh, thanks.


>> @@ -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.



>>       /*
>> +      * 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 ?

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