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Message-ID: <485493B3.8080001@linux.vnet.ibm.com>
Date: Sun, 15 Jun 2008 09:29:47 +0530
From: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To: Lee Schermerhorn <Lee.Schermerhorn@...com>
CC: Andrew Morton <akpm@...ux-foundation.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
linux-mm@...ck.org, Nick Piggin <npiggin@...e.de>,
Andy Whitcroft <apw@...dowen.org>,
"riel@...hat.com" <riel@...hat.com>
Subject: Re: [PATCH] fix double unlock_page() in 2.6.26-rc5-mm3 kernel BUG
at mm/filemap.c:575!
Lee Schermerhorn wrote:
> On Thu, 2008-06-12 at 19:13 -0700, Andrew Morton wrote:
>> On Fri, 13 Jun 2008 10:44:44 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
>>
>>> This is reproducer of panic. "quick fix" is attached.
>> Thanks - I put that in
>> ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.26-rc5/2.6.26-rc5-mm3/hot-fixes/
>>
>>> But I think putback_lru_page() should be re-designed.
>> Yes, it sounds that way.
>
> Here's a proposed replacement patch that reworks putback_lru_page()
> slightly and cleans up the call sites. I still want to balance the
> get_page() in isolate_lru_page() with a put_page() in putback_lru_page()
> for the primary users--vmscan and page migration. So, I need to drop
> the lock before the put_page() when handed a page with null mapping and
> a single reference count as the page will be freed on put_page() and a
> locked page would bug out in free_pages_check()/bad_page().
>
> Lee
>
> PATCH fix page unlocking protocol for putback_lru_page()
>
> Against: 2.6.26-rc5-mm3
>
> Replaces Kame-san's hotfix:
> fix-double-unlock_page-in-2626-rc5-mm3-kernel-bug-at-mm-filemapc-575.patch
>
> Applies at end of vmscan/unevictable/mlock series to avoid patch conflicts.
>
> 1) modified putback_lru_page() to drop page lock only if both page_mapping()
> NULL and page_count() == 1 [rather than VM_BUG_ON(page_count(page) != 1].
> I want to balance the put_page() from isolate_lru_page() here for vmscan
> and, e.g., page migration rather than requiring explicit checks of the
> page_mapping() and explicit put_page() in these areas. However, the page
> could be truncated while one of these subsystems holds it isolated from
> the LRU. So, need to handle this case. Callers of putback_lru_page()
> need to be aware of this and only call it with a page with NULL
> page_mapping() when they will no longer reference the page afterwards.
> This is the case for vmscan and page migration.
>
> 2) m[un]lock_vma_page() already will not be called for page with NULL
> mapping. Added VM_BUG_ON() to assert this.
>
> 3) modified clear_page_lock() to skip the isolate/putback shuffle for
> pages with NULL mapping, as they are being truncated/freed. Thus,
> any future callers of clear_page_lock() need not be concerned about
> the putback_lru_page() semantics for truncated pages.
>
Hi Lee,
Thanks, After applying the patch, the kernel does not panic's while
bootup.
Tested-by: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
> Signed-off-by: Lee Schermerhorn <lee.schermerhorn@...com>
>
> mm/mlock.c | 29 +++++++++++++++++++----------
> mm/vmscan.c | 12 +++++++-----
> 2 files changed, 26 insertions(+), 15 deletions(-)
>
> Index: linux-2.6.26-rc5-mm3/mm/mlock.c
> ===================================================================
> --- linux-2.6.26-rc5-mm3.orig/mm/mlock.c 2008-06-12 11:42:59.000000000 -0400
> +++ linux-2.6.26-rc5-mm3/mm/mlock.c 2008-06-13 09:47:14.000000000 -0400
> @@ -59,27 +59,33 @@ void __clear_page_mlock(struct page *pag
>
> dec_zone_page_state(page, NR_MLOCK);
> count_vm_event(NORECL_PGCLEARED);
> - if (!isolate_lru_page(page)) {
> - putback_lru_page(page);
> - } else {
> - /*
> - * Page not on the LRU yet. Flush all pagevecs and retry.
> - */
> - lru_add_drain_all();
> - if (!isolate_lru_page(page))
> + if (page->mapping) { /* truncated ? */
> + if (!isolate_lru_page(page)) {
> putback_lru_page(page);
> - else if (PageUnevictable(page))
> - count_vm_event(NORECL_PGSTRANDED);
> + } else {
> + /*
> + * Page not on the LRU yet.
> + * Flush all pagevecs and retry.
> + */
> + lru_add_drain_all();
> + if (!isolate_lru_page(page))
> + putback_lru_page(page);
> + else if (PageUnevictable(page))
> + count_vm_event(NORECL_PGSTRANDED);
> + }
> }
> }
>
> /*
> * Mark page as mlocked if not already.
> * If page on LRU, isolate and putback to move to unevictable list.
> + *
> + * Called with page locked and page_mapping() != NULL.
> */
> void mlock_vma_page(struct page *page)
> {
> BUG_ON(!PageLocked(page));
> + VM_BUG_ON(!page_mapping(page));
>
> if (!TestSetPageMlocked(page)) {
> inc_zone_page_state(page, NR_MLOCK);
> @@ -92,6 +98,8 @@ void mlock_vma_page(struct page *page)
> /*
> * called from munlock()/munmap() path with page supposedly on the LRU.
> *
> + * Called with page locked and page_mapping() != NULL.
> + *
> * Note: unlike mlock_vma_page(), we can't just clear the PageMlocked
> * [in try_to_munlock()] and then attempt to isolate the page. We must
> * isolate the page to keep others from messing with its unevictable
> @@ -110,6 +118,7 @@ void mlock_vma_page(struct page *page)
> static void munlock_vma_page(struct page *page)
> {
> BUG_ON(!PageLocked(page));
> + VM_BUG_ON(!page_mapping(page));
>
> if (TestClearPageMlocked(page)) {
> dec_zone_page_state(page, NR_MLOCK);
> Index: linux-2.6.26-rc5-mm3/mm/vmscan.c
> ===================================================================
> --- linux-2.6.26-rc5-mm3.orig/mm/vmscan.c 2008-06-12 11:39:09.000000000 -0400
> +++ linux-2.6.26-rc5-mm3/mm/vmscan.c 2008-06-13 09:44:44.000000000 -0400
> @@ -1,4 +1,4 @@
> -/*
> + /*
> * linux/mm/vmscan.c
> *
> * Copyright (C) 1991, 1992, 1993, 1994 Linus Torvalds
> @@ -488,6 +488,9 @@ int remove_mapping(struct address_space
> * lru_lock must not be held, interrupts must be enabled.
> * Must be called with page locked.
> *
> + * If page truncated [page_mapping() == NULL] and we hold the last reference,
> + * the page will be freed here. For vmscan and page migration.
> + *
> * return 1 if page still locked [not truncated], else 0
> */
> int putback_lru_page(struct page *page)
> @@ -502,12 +505,11 @@ int putback_lru_page(struct page *page)
> lru = !!TestClearPageActive(page);
> was_unevictable = TestClearPageUnevictable(page); /* for page_evictable() */
>
> - if (unlikely(!page->mapping)) {
> + if (unlikely(!page->mapping && page_count(page) == 1)) {
> /*
> - * page truncated. drop lock as put_page() will
> - * free the page.
> + * page truncated and we hold last reference.
> + * drop lock as put_page() will free the page.
> */
> - VM_BUG_ON(page_count(page) != 1);
> unlock_page(page);
> ret = 0;
> } else if (page_evictable(page, NULL)) {
>
>
--
Thanks & Regards,
Kamalesh Babulal,
Linux Technology Center,
IBM, ISTL.
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