Messages in this thread | | | Subject | Re: BUG: Bad page state [was: Strange oopses in 2.6.30] | From | Lee Schermerhorn <> | Date | Mon, 22 Jun 2009 12:02:33 -0400 |
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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. 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.
Lee
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