Re: page-flags behavior on compound pages: a worry
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2015-08-12 21:16:47
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On Wed, 12 Aug 2015 17:35:09 +0300 "Kirill A. Shutemov" [off-list ref] wrote:
On Thu, Aug 06, 2015 at 12:24:22PM -0700, Hugh Dickins wrote:quoted
quoted
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:quoted
From e78eec7d7a8c4cba8b5952a997973f7741e704f4 Mon Sep 17 00:00:00 2001From: "Kirill A. Shutemov" <redacted> 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.
This is nuts :( page_count() should Just Work without us having to worry about bizarre races against splitting. Sigh.
quoted hunk ↗ jump to hunk
--- 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're going to do this sort of thing, can we please do it in a more transparent manner? Let's not sprinkle unexplained and incomprehensible direct accesses to ->_count all over the place. Create a formal function to do this, with an appropriate name and with documentation which fully explains what's going on. Then use that here, and in has_unmovable_pages() (at least). -- 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>