Messages in this thread | | | Date | Wed, 17 Mar 2010 12:00:15 +0900 | Subject | Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages | From | Minchan Kim <> |
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On Wed, Mar 17, 2010 at 11:12 AM, KAMEZAWA Hiroyuki > BTW, I doubt freeing anon_vma can happen even when we check mapcount. > > "unmap" is 2-stage operation. > 1. unmap_vmas() => modify ptes, free pages, etc. > 2. free_pgtables() => free pgtables, unlink vma and free it. > > Then, if migration is enough slow. > > Migration(): Exit(): > check mapcount > rcu_read_lock > pte_lock > replace pte with migration pte > pte_unlock > pte_lock > copy page etc... zap pte (clear pte) > pte_unlock > free_pgtables > ->free vma > ->free anon_vma > pte_lock > remap pte with new pfn(fail) > pte_unlock > > lock anon_vma->lock # modification after free. > check list is empty
check list is empty? Do you mean anon_vma->head?
If it is, is it possible that that list isn't empty since anon_vma is used by others due to SLAB_DESTROY_BY_RCU?
but such case is handled by page_check_address, vma_address, I think.
> unlock anon_vma->lock > free anon_vma > rcu_read_unlock > > > Hmm. IIUC, anon_vma is allocated as SLAB_DESTROY_BY_RCU. Then, while > rcu_read_lock() is taken, anon_vma is anon_vma even if freed. But it > may reused as anon_vma for someone else. > (IOW, it may be reused but never pushed back to general purpose memory > until RCU grace period.) > Then, touching anon_vma->lock never cause any corruption. > > Does use-after-free check for SLAB_DESTROY_BY_RCU correct behavior ?
Could you elaborate your point?
> Above case is not use-after-free. It's safe and expected sequence. > > Thanks, > -Kame > > > >> > --- >> > mm/migrate.c | 13 +++++++++++++ >> > 1 files changed, 13 insertions(+), 0 deletions(-) >> > >> > diff --git a/mm/migrate.c b/mm/migrate.c >> > index 98eaaf2..6eb1efe 100644 >> > --- a/mm/migrate.c >> > +++ b/mm/migrate.c >> > @@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, >> > */ >> > if (PageAnon(page)) { >> > rcu_read_lock(); >> > + >> > + /* >> > + * If the page has no mappings any more, just bail. An >> > + * unmapped anon page is likely to be freed soon but worse, >> > + * it's possible its anon_vma disappeared between when >> > + * the page was isolated and when we reached here while >> > + * the RCU lock was not held >> > + */ >> > + if (!page_mapcount(page)) { >> > + rcu_read_unlock(); >> > + goto uncharge; >> > + } >> > + >> > rcu_locked = 1; >> > anon_vma = page_anon_vma(page); >> > atomic_inc(&anon_vma->migrate_refcount); >> > >> >> -- >> To unsubscribe, send a message with 'unsubscribe linux-mm' in >> the body to majordomo@kvack.org. For more info on Linux MM, >> see: http://www.linux-mm.org/ . >> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a> >> > >
-- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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