linux-kernel - Re: Flushing whole page instead of work for ptrace

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Message-Id: <20101130233250.35603401C8@magilla.sf.frob.com>
Date:	Tue, 30 Nov 2010 15:32:50 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	michal.simek@...alogix.com
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	John Williams <john.williams@...alogix.com>,
	"Edgar E. Iglesias" <edgar.iglesias@...il.com>
Subject: Re: Flushing whole page instead of work for ptrace

This is a VM question more than a ptrace question.  
I can't give you any authoritative answers about the VM issues.

Documentation/cachetlb.txt says:

	Any time the kernel writes to a page cache page, _OR_
	the kernel is about to read from a page cache page and
	user space shared/writable mappings of this page potentially
	exist, this routine is called.

In your case, the kernel is only reading (write=0 passed to
access_process_vm and get_user_pages).  In normal situations,
the page in question will have only a private and read-only
mapping in user space.  So the call should not be required in
these cases--if the code can tell that's so.

Perhaps something like the following would be safe.
But you really need some VM folks to tell you for sure.

diff --git a/mm/memory.c b/mm/memory.c
index 02e48aa..2864ee7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1484,7 +1484,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 				pages[i] = page;
 
 				flush_anon_page(vma, page, start);
-				flush_dcache_page(page);
+				if ((vm_flags & VM_WRITE) || (vma->vm_flags & VM_SHARED)
+					flush_dcache_page(page);
 			}
 			if (vmas)
 				vmas[i] = vma;


Thanks,
Roland
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