Re: Should activate_page()/__set_page_dirty_buffers() use _irqsave locking?

[Andrew Morton <akpm@osdl.org>]

Richard Purdie <rpurdie@rpsys.net> wrote:
>
> I've been experimenting with oprofile on an arm system without a PMU.
> Whenever I enable callgraphing I see a BUG from run_posix_cpu_timers()
> due to irqs being enabled when they should be disabled.
> 
> Tracing this back shows interrupts are enabled after the arm backtrace
> code completes. Further tracing reveals its the call to
> check_user_page_readable() (within an interrupt) that is causing the
> problem.
> 
> check_user_page_readable() can potentially result in calls to
> activate_page() (mm/swap.c) and __set_page_dirty_buffers()
> (fs/buffer.c). Both functions use *_lock_irq()/*_unlock_irq rather than
> the *_lock_irqsave/*_unlock_irqrestore counterparts.
> 
> Switching them to use the save/restore locks makes everything work. Is
> there a reason for not using these here? Would such a patch be accepted?
> 
> Both the arm and i386 backtrace code would seem to be vulnerable to this
> problem.

ow, yes, ug.

check_page_readable() won't actually call set_page_dirty() because it
passes in `write = 0'.  So it should be sufficient to use
spin_lock_irqsave() in mark_page_accessed().

But then again, that's fragile and obscure and it isn't even correct: if
someone calls check_page_readable(), that doesn't imply an actual read of
the page's contents.

So how about we add a new flag to __follow_page() telling it whether to
consider this as an access to the page contents?

diff -puN mm/memory.c~check_user_page_readable-deadlock-fix mm/memory.c
--- devel/mm/memory.c~check_user_page_readable-deadlock-fix	2005-07-26 11:34:38.000000000 -0700
+++ devel-akpm/mm/memory.c	2005-07-26 11:37:21.000000000 -0700
@@ -776,8 +776,8 @@ unsigned long zap_page_range(struct vm_a
  * Do a quick page-table lookup for a single page.
  * mm->page_table_lock must be held.
  */
-static struct page *
-__follow_page(struct mm_struct *mm, unsigned long address, int read, int write)
+static struct page *__follow_page(struct mm_struct *mm, unsigned long address,
+			int read, int write, int accessed)
 {
 	pgd_t *pgd;
 	pud_t *pud;
@@ -818,9 +818,11 @@ __follow_page(struct mm_struct *mm, unsi
 		pfn = pte_pfn(pte);
 		if (pfn_valid(pfn)) {
 			page = pfn_to_page(pfn);
-			if (write && !pte_dirty(pte) && !PageDirty(page))
-				set_page_dirty(page);
-			mark_page_accessed(page);
+			if (accessed) {
+				if (write && !pte_dirty(pte) &&!PageDirty(page))
+					set_page_dirty(page);
+				mark_page_accessed(page);
+			}
 			return page;
 		}
 	}
@@ -829,16 +831,14 @@ out:
 	return NULL;
 }
 
-struct page *
-follow_page(struct mm_struct *mm, unsigned long address, int write)
+struct page *follow_page(struct mm_struct *mm, unsigned long address, int write)
 {
-	return __follow_page(mm, address, /*read*/0, write);
+	return __follow_page(mm, address, 0, write, 1);
 }
 
-int
-check_user_page_readable(struct mm_struct *mm, unsigned long address)
+int check_user_page_readable(struct mm_struct *mm, unsigned long address)
 {
-	return __follow_page(mm, address, /*read*/1, /*write*/0) != NULL;
+	return __follow_page(mm, address, 1, 0, 0) != NULL;
 }
 EXPORT_SYMBOL(check_user_page_readable);
 
_

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