Messages in this thread | | | Date | Tue, 13 Apr 2010 00:17:56 +0900 | Subject | Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas of a mergeable VMA | From | Minchan Kim <> |
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On Mon, Apr 12, 2010 at 11:40 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Sat, 2010-04-10 at 11:21 -0700, Linus Torvalds wrote: >> > >> 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. > > Right, so unless you have CONFIG_TREE_PREEMPT_RCU=y, the preempt-disable > == RCU read lock assumption does hold.
Indeed.
> > But even with your patch it doesn't close all holes because while > zap_pte_range() can remove the last mapcount of the page, the > page_remove_tlb() et al. don't need to be the last use count of the > page. > > Concurrent reclaim/gup/whatever could still have a count out on the page > delaying the actual free beyond the tlb gather RCU section.
anon_vma lock is just valid in case of page_mapped. if reclaim/gup/whatever want to use anon_vma, it should check with page_mapped. And last put_page doesn't touch anon_vma for freeing the page so I think it's not a problem. Do I miss something?
> > This also complicates the whole page_lock_anon_vma() thing, so it would > be nice to be able to remove this race and clear page->mapping in > page_remove_rmap(). >
BTW, I totally agree with you. Now anon_vma is very complicated. SLAB_DESTROY_BY_RCU, vma merge, when page->mapping is cleared, anon_vma_chain and so on.. :(
-- 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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