linux-kernel - Re: [PATCH -next] mm,vmacache: also flush cache for VM

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Date:	Thu, 13 Mar 2014 16:32:18 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Davidlohr Bueso <davi@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] mm,vmacache: also flush cache for VM_CLONE

On 03/13, Oleg Nesterov wrote:
>
> Yes. But it seems that use_mm() and unuse_mm() should invalidate vmacache too.
>
> Suppose that a kernel thread T does, say,
>
> 	use_mm(foreign_mm);
> 	get_user(...);
> 	unuse_mm();
>
> This can trigger a fault and populate T->vmacache[]. If this code is called
> again vmacache_find() can use the stale entries.
>
> Or, assuming that only a kernel thread can do use_mm(), we can change
> vmacache_valid() to also check !PF_KTHREAD.

Yes, I think we should check PF_KTHREAD, because

> Hmm. Another problem is that use_mm() doesn't take ->mmap_sem and thus
> it can race with vmacache_flush_all()...

this also closes this race. use_mm() users should not use vmacache at all.

> Finally. Shouldn't vmacache_update() check current->mm == mm as well?
> What if access_remote_vm/get_user_pages trigger find_vma() ??? Unless
> I missed something this is not theoretical at all and can lead to the
> corrupted vmacache, no?

Looks like a real problem or I am totally confused. I think we need
something like below (uncompiled).

Oleg.

--- x/mm/vmacache.c
+++ x/mm/vmacache.c
@@ -30,20 +30,24 @@ void vmacache_flush_all(struct mm_struct
 	rcu_read_unlock();
 }
 
+static bool vmacache_valid_mm(mm)
+{
+	return current->mm == mm && !(current->flags & PF_KTHREAD);
+}
+
 void vmacache_update(unsigned long addr, struct vm_area_struct *newvma)
 {
-	int idx = VMACACHE_HASH(addr);
-	current->vmacache[idx] = newvma;
+	if (vmacache_valid_mm(newvma->vm_mm))
+		current->vmacache[VMACACHE_HASH(addr)] = newvma;
 }
 
 static bool vmacache_valid(struct mm_struct *mm)
 {
-	struct task_struct *curr = current;
-
-	if (mm != curr->mm)
+	if (!vmacache_valid_mm(mm))
 		return false;
 
 	if (mm->vmacache_seqnum != curr->vmacache_seqnum) {
+		struct task_struct *curr = current;
 		/*
 		 * First attempt will always be invalid, initialize
 		 * the new cache for this task here.

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