Messages in this thread | | | Date | Mon, 16 Apr 2012 17:50:40 +0200 | From | Martin Schwidefsky <> | Subject | Re: [RFC PATCH] s390: mm: rmap: Transfer storage key to struct page under the page lock |
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On Mon, 16 Apr 2012 15:14:23 +0100 Mel Gorman <mgorman@suse.de> wrote:
> This patch is horribly ugly and there has to be a better way of doing > it. I'm looking for suggestions on what s390 can do here that is not > painful or broken. > > However, s390 needs a better way of guarding against > PageSwapCache pages being removed from the radix tree while set_page_dirty() > is being called. The patch would be marginally better if in the PageSwapCache > case we simply tried to lock once and in the contended case just fail to > propogate the storage key. I lack familiarity with the s390 architecture > to be certain if this is safe or not. Suggestions on a better fix?
One though that crossed my mind is that maybe a better approach would be to move the page_test_and_clear_dirty check out of page_remove_rmap. What we need to look out for are code sequences of the form:
if (pte_dirty(pte)) set_page_dirty(page); ... page_remove_rmap(page);
There are four of those as far as I can see: in try_to_unmap_one, try_to_unmap_cluster, zap_pte, and zap_pte_range.
A valid implementation for s390 would be to test and clear the changed bit in the storage key for every of those pte_dirty() calls.
if (pte_dirty(pte) || page_test_and_clear_dirty(page)) set_page_dirty(page); ... page_remove_rmap(page); /* w/o page_test_clear_dirty */
Trouble is that the ISKE and SSKE instructions are very expensive, that is why we currently have the operation in page_remove_rmap after the map counter dropped to zero (which is wrong as we now have learned the hard way). The additional check for (!PageAnon || PageSwapCache) is just another variation of avoiding ISKE/SSKE.
Thinking about a function like this:
static inline int page_test_dirty_eco(struct page *page) { if (page_mapcount(page) > 1) return 0; if (PageAnon(page) && !PageSwapCache(page)) return 0; return page_test_and_clear_dirty(page); }
and use it alongside the pte_dirty() check. The worry I have is the map counter. What guarantees us that the map counter is not decremented concurrently? Which is probably a problem with the current patch as well, checking atomic_add_negative(-1, &page->_mapcount) against zero works, doing (page_mapcount(page) == 1) followed by the decrement can race. And we better not forget a dirty bit ..
-- blue skies, Martin.
"Reality continues to ruin my life." - Calvin.
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