Messages in this thread Patch in this message | | | Date | Wed, 12 Aug 2015 17:35:09 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: page-flags behavior on compound pages: a worry |
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On Thu, Aug 06, 2015 at 12:24:22PM -0700, Hugh Dickins wrote: > > IIUC, the only potentially problematic callsites left are physical memory > > scanners. This code requires audit. I'll do that. > > Please.
I haven't finished the exercise yet. But here's an issue I believe present in current *Linus* tree:
From e78eec7d7a8c4cba8b5952a997973f7741e704f4 Mon Sep 17 00:00:00 2001 From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> Date: Wed, 12 Aug 2015 17:09:16 +0300 Subject: [PATCH] mm: fix potential race in isolate_migratepages_block()
Hugh has pointed that compound_head() call can be unsafe in some context. There's one example:
CPU0 CPU1
isolate_migratepages_block() page_count() compound_head() !!PageTail() == true put_page() tail->first_page = NULL head = tail->first_page alloc_pages(__GFP_COMP) prep_compound_page() tail->first_page = head __SetPageTail(p); !!PageTail() == true <head == NULL dereferencing>
The race is pure theoretical. I don't it's possible to trigger it in practice. But who knows.
This can be fixed by avoiding compound_head() in unsafe context.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Hugh Dickins <hughd@google.com> Cc: David Rientjes <rientjes@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> --- mm/compaction.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/compaction.c b/mm/compaction.c index 86f04e556f96..bec727b700d3 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -787,7 +787,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, * admittedly racy check. */ if (!page_mapping(page) && - page_count(page) > page_mapcount(page)) + atomic_read(&page->_count) > page_mapcount(page)) continue; /* If we already hold the lock, we can skip some rechecking */ -- Kirill A. Shutemov
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