Messages in this thread | | | From | KOSAKI Motohiro <> | Subject | Re: BUG: Bad page state [was: Strange oopses in 2.6.30] | Date | Tue, 23 Jun 2009 20:11:51 +0900 (JST) |
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> On Mon, Jun 22, 2009 at 12:02:33PM -0400, Lee Schermerhorn wrote: > > On Mon, 2009-06-22 at 10:16 +0100, Mel Gorman wrote: > > > On Mon, Jun 22, 2009 at 11:39:53AM +0900, KOSAKI Motohiro wrote: > > > > (cc to Mel and some reviewer) > > > > [added Rik so that he can get multiple copies, too. :)] > > > > > > > > > > > Flags are: > > > > > 0000000000400000 -- __PG_MLOCKED > > > > > 800000000050000c -- my page flags > > > > > 3650000c -- Maxim's page flags > > > > > 0000000000693ce1 -- my PAGE_FLAGS_CHECK_AT_FREE > > > > > > > > I guess commit da456f14d (page allocator: do not disable interrupts in > > > > free_page_mlock()) is a bit wrong. > > > > > > > > current code is: > > > > ------------------------------------------------------------- > > > > static void free_hot_cold_page(struct page *page, int cold) > > > > { > > > > (snip) > > > > int clearMlocked = PageMlocked(page); > > > > (snip) > > > > if (free_pages_check(page)) > > > > return; > > > > (snip) > > > > local_irq_save(flags); > > > > if (unlikely(clearMlocked)) > > > > free_page_mlock(page); > > > > ------------------------------------------------------------- > > > > > > > > Oh well, we remove PG_Mlocked *after* free_pages_check(). > > > > Then, it makes false-positive warning. > > > > > > > > Sorry, my review was also wrong. I think reverting this patch is better ;) > > > > > > > > > > I think a revert is way overkill. The intention of the patch is sound - > > > reducing the number of times interrupts are disabled. Having pages > > > with the PG_locked bit is now somewhat of an expected situation. I'd > > > prefer to go with either > > > > > > 1. Unconditionally clearing the bit with TestClearPageLocked as the > > > patch already posted does > > > 2. Removing PG_locked from the free_pages_check() > > > 3. Unlocking the pages as we go when an mlocked VMA is being torn town > > > > Mel, > > > > #3 SHOULD be happening in all cases. The free_page_mlocked() function > > counts when this is not happening. We tried to fix all cases that we > > encountered before this feature was submitted, but left the vm_stat > > there to report if more PG_mlocked leaks were introduced. > > That makes sense. I was surprised at the thought that the pages were > apparently not getting freed properly and upon investigation I could not > trivially reproduce the problem. Can someone with this problem post their > .config please in case I'm missing something in there? > > > We also, > > inadvertently, left PG_mlocked in the flags to check at free. We didn't > > hit this before your patch because free_page_mlock() did a test&clear on > > the PG_mlocked before checking the flags. Since you moved the call, and > > used PageMlocked() instead of TestClearPageMlocked(), any PG_locked page > > will cause the bug. > > > > So, we have another PG_mlocked flag leaking to free. I don't think this > > is terribly serious in itself, and probably not deserving of a BUG_ON. > > It probably doesn't deserve a vm_stat, either, I guess. However, it > > could indicate a more serious logic error and should be examined. So it > > would be nice to retain some indication that it's happening. > > > > > The patch that addresses 1 seemed ok to me. What do you think? > > > > > > > Your alternative #2 sounds less expensive that test&clear. > > > > How about the following? The intention is to warn once when PG_mlocked > is set but continue to count the number of times the event occured. > > ==== CUT HERE ==== > mm: Warn once when a page is freed with PG_mlocked set > > When a page is freed with the PG_mlocked set, it is considered an unexpected > but recoverable situation. A counter records how often this event happens > but due to commit da456f14d [page allocator: do not disable interrupts in > free_page_mlock()], the page state is being treated as a bad page which is > considered a severe bug. > > This bug drops the severity of the report in the event a page is freed > with PG_mlocked set. A warning is printed once and the subsequent events > counted. > > Signed-off-by: Mel Gorman <mel@csn.ul.ie> > --- > include/linux/page-flags.h | 10 +++++++++- > mm/page_alloc.c | 9 +++++++++ > 2 files changed, 18 insertions(+), 1 deletion(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index d6792f8..81731cf 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -389,7 +389,15 @@ static inline void __ClearPageTail(struct page *page) > 1 << PG_private | 1 << PG_private_2 | \ > 1 << PG_buddy | 1 << PG_writeback | 1 << PG_reserved | \ > 1 << PG_slab | 1 << PG_swapcache | 1 << PG_active | \ > - 1 << PG_unevictable | __PG_MLOCKED) > + 1 << PG_unevictable) > + > +/* > + * Flags checked when a page is freed. Pages being freed should not have > + * these set but the situation is easily resolved and should just be > + * reported as a once-off warning. > + */ > +#define PAGE_FLAGS_WARN_AT_FREE \ > + (__PG_MLOCKED) > > /* > * Flags checked when a page is prepped for return by the page allocator. > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index a5f3c27..c8c029e 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -497,6 +497,15 @@ static void free_page_mlock(struct page *page) { } > > static inline int free_pages_check(struct page *page) > { > + if (unlikely(page->flags & PAGE_FLAGS_WARN_AT_FREE)) {
this condition is always false. it's because caller clear PG_Mlocked flag before calling free_pages_check().
> + WARN_ONCE(1, KERN_WARNING > + "Sloppy page flags set process %s at pfn:%05lx\n" > + "page:%p flags:%p\n", > + current->comm, page_to_pfn(page), > + page, (void *)page->flags);
hmm, mystery (void*) casting is here.
> + page->flags &= ~PAGE_FLAGS_WARN_AT_FREE; > + } > + > if (unlikely(page_mapcount(page) | > (page->mapping != NULL) | > (atomic_read(&page->_count) != 0) |
Howerver, I like this patch concept. this warning is useful and meaningful IMHO.
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