Messages in this thread Patch in this message | | | From | Borislav Petkov <> | Subject | [PATCH 1/3] mm: make page freeing path RCU-safe | Date | Sun, 11 Apr 2010 15:19:56 +0200 |
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From: Linus Torvalds <torvalds@linux-foundation.org>
On Sat, 10 Apr 2010, Linus Torvalds wrote: > On Sat, 10 Apr 2010, Borislav Petkov wrote: > > > > And I got an oops again, this time the #GP from couple of days ago. > > Oh damn. So the list corruption really does happen still.
Ho humm.
Maybe I'm crazy, but something started bothering me. And I started wondering: when is the 'page->mapping' of an anonymous page actually cleared?
The thing is, the mapping of an anonymous page is actually cleared only when the page is _freed_, in "free_hot_cold_page()".
Now, let's think about that. And in particular, let's think about how that relates to the freeing of the 'anon_vma' that the page->mapping points to.
The way the anon_vma is freed is when the mapping is torn down, and we do roughly:
tlb = tlb_gather_mmu(mm,..) .. unmap_vmas(&tlb, vma .. .. free_pgtables() .. tlb_finish_mmu(tlb, start, end);
and we actually unmap all the pages in "unmap_vmas()", and then _after_ unmapping all the pages we do the "unlink_anon_vmas(vma);" in "free_pgtables()". Fine so far - the anon_vma stay around until after the page has been happily unmapped.
But "unmapped all the pages" is _not_ actually the same as "free'd all the pages". The actual _freeing_ of the page happens generally in tlb_finish_mmu(), because we can free the page only after we've flushed any TLB entries.
So what we have in that tlb_gather structure is a list of _pending_ pages to be freed, while we already actually free'd the anon_vmas earlier!
Now, the thing is, tlb_gather_mmu() begins a preempt-safe region (because we use a per-cpu variable), but as far as I can tell it is _not_ an RCU-safe region.
So I think we might actually get a real RCU freeing event while this all happens. So now the 'anon_vma' that 'page->mapping' points to has not just been released back to the SLUB caches, the page itself might have been released too.
I dunno. Does the above sound at all sane? Or am I just raving?
Something hacky like the above might fix it if I'm not just raving. I really might be missing something here.
Linus --- include/asm-generic/tlb.h | 3 +++ 1 files changed, 3 insertions(+), 0 deletions(-)
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h index e43f976..2678118 100644 --- a/include/asm-generic/tlb.h +++ b/include/asm-generic/tlb.h @@ -14,6 +14,7 @@ #define _ASM_GENERIC__TLB_H #include <linux/swap.h> +#include <linux/rcupdate.h> #include <asm/pgalloc.h> #include <asm/tlbflush.h> @@ -62,6 +63,7 @@ tlb_gather_mmu(struct mm_struct *mm, unsigned int full_mm_flush) tlb->fullmm = full_mm_flush; + rcu_read_lock(); return tlb; } @@ -90,6 +92,7 @@ tlb_finish_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end) /* keep the page table cache within bounds */ check_pgt_cache(); + rcu_read_unlock(); put_cpu_var(mmu_gathers); } -- 1.7.0.3
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