linux-kernel - Re: [RFC][PATCH 2/2] vmalloc: vread vwrite avoid memory hole.

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Date:	Fri, 31 Jul 2009 17:38:30 +0800
From:	Amerigo Wang <xiyou.wangcong@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Amerigo Wang <xiyou.wangcong@...il.com>,
	Mike Smith <scgtrp@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/2] vmalloc: vread vwrite avoid memory hole.

On Wed, Jul 29, 2009 at 08:51:23PM +0900, KAMEZAWA Hiroyuki wrote:
>On Wed, 29 Jul 2009 17:59:32 +0800
>Amerigo Wang <xiyou.wangcong@...il.com> wrote:
>
>> On Wed, Jul 29, 2009 at 05:48:10PM +0900, KAMEZAWA Hiroyuki wrote:
>> >Considering recent changes, I think this an answer to this bug.
>> >
>> >All get_vm_area() calls specified VM_IOREMAP before 2.6.30.
>> >Now, percpu calls get_vm_area() without IOREMAP (it means kcore doesn't skip this)
>> >And it seems pcpu does delayed map to pre-allocated vmalloc-area.
>> >Then, now, vmalloc() area has memory holes. (I'm sorry if I misunderstand.)
>> 
>> 
>> You mean the guard holes in vmalloc area?
>> 
>no, not guard hole.  memory hole in the middle of valid vmalloc area, like this
>
>start             end
>      XXXXX0XXXXXG   XXXXXXXXG  XXXXXXXXG
>X= pte is present
>O= pte is none
>G= pte is none as guard page.
>


Hmm, I get your meaning now... thanks


>> >+
>> >+static int aligned_vwrite(char *buf, char *addr, unsigned long count)
>> >+{
>> >+	struct page *p;
>> >+	int copied;
>> >+
>> >+	while (count) {
>> >+		unsigned long offset, length;
>> >+
>> >+		offset = (unsinged long)addr & PAGE_MASK;
>> >+		length = PAGE_SIZE - offset;
>> >+		if (length > count)
>> >+			length = count;
>> >+		/* confirm the page is present */
>> >+		p = vmalloc_to_page(addr);
>> >+		if (p)
>> >+			memcpy(addr, buf, length);
>> 
>> 
>> So when !p, you copy nothing, but still increase the length,
>> *silently*?
>Ah, thanks, it's careless bug...will fix.
>
>
>
>> 
>> This looks a bit weird for me... I am thinking if we need
>> to return the number of bytes that is *actually* copied?
>> 
>I think it makes the caller complex.
>
>Then, I have 2 options.
> a) skip memory hole
> b) if we find memory hole, return immediately.
>
>Do you like b) ? 
>But in old behavior, vwrite()/vread() incremetns its "addr"
>(at read zero-fill).
>
>Then, I used a).


Ok, just want to make sure it's safe to do a).



>
>
>> 
>> >+		addr += length;
>> >+		buf += length;
>> >+		copiled += length;
>> >+		count -= length;
>> >+	}
>> >+	return copied;
>> >+}
>> >+
>> >+long vread(char *buf, char *addr, unsigned long count, bool skip_ioremap)
>> > {
>> > 	struct vm_struct *tmp;
>> > 	char *vaddr, *buf_start = buf;
>> >@@ -1640,10 +1694,11 @@ long vread(char *buf, char *addr, unsign
>> > 		count = -(unsigned long) addr;
>> > 
>> > 	read_lock(&vmlist_lock);
>> >-	for (tmp = vmlist; tmp; tmp = tmp->next) {
>> >+	for (tmp = vmlist; count && tmp; tmp = tmp->next) {
>> > 		vaddr = (char *) tmp->addr;
>> > 		if (addr >= vaddr + tmp->size - PAGE_SIZE)
>> > 			continue;
>> >+
>> > 		while (addr < vaddr) {
>> > 			if (count == 0)
>> > 				goto finished;
>> >@@ -1653,14 +1708,15 @@ long vread(char *buf, char *addr, unsign
>> > 			count--;
>> > 		}
>> > 		n = vaddr + tmp->size - PAGE_SIZE - addr;
>> >-		do {
>> >-			if (count == 0)
>> >-				goto finished;
>> >-			*buf = *addr;
>> >-			buf++;
>> >-			addr++;
>> >-			count--;
>> >-		} while (--n > 0);
>> >+		if (n > count)
>> >+			n = count;
>> >+		if (!(tmp->flags & VM_IOREMAP) || !skip_ioremap)
>> >+			aligned_vread(buf, addr, n);
>> >+		else
>> >+			memset(buf, 0, n);
>> >+		buf += n;
>> >+		addr += n;
>> >+		count -= n;
>> 
>> 
>> If I set 'skip_ioremap' true, I want the return value can be
>> what it _actually_ copies, i.e. kick out the bytes counted
>> for VM_IOREMAP. What do you think?
>> 
>
>One one od the reasons is The same reason to above. 
>(adjusted to old behavior)
>
>IIUC, In following kind of codes,
>
>	len = vread(buf,addr,xxx);
>	bud += len;
>	addr += len
>
>len should be the length that the caller should increment buffer address.
>So, I just skipped hole.


Well... old code doesn't have the parameter 'skip_ioremap'. :-)
For new code, we can do:

  len = vread(buf, addr, count, true);
  buf += len;
  addr += count;

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