linux-kernel - Re: [PATCH 4/4] add ksm kernel shared memory driver.

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Date:	Wed, 15 Apr 2009 17:43:03 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, avi@...hat.com,
	aarcange@...hat.com, chrisw@...hat.com, mtosatti@...hat.com,
	hugh@...itas.com, kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 4/4] add ksm kernel shared memory driver.

Andrew Morton wrote:
>> +static pte_t *get_pte(struct mm_struct *mm, unsigned long addr)
>> +{
>> +	pgd_t *pgd;
>> +	pud_t *pud;
>> +	pmd_t *pmd;
>> +	pte_t *ptep = NULL;
>> +
>> +	pgd = pgd_offset(mm, addr);
>> +	if (!pgd_present(*pgd))
>> +		goto out;
>> +
>> +	pud = pud_offset(pgd, addr);
>> +	if (!pud_present(*pud))
>> +		goto out;
>> +
>> +	pmd = pmd_offset(pud, addr);
>> +	if (!pmd_present(*pmd))
>> +		goto out;
>> +
>> +	ptep = pte_offset_map(pmd, addr);
>> +out:
>> +	return ptep;
>> +}
>>     
>
> hm, this looks very generic.  Does it duplicate anything which core
> kernel already provides?  If not, perhaps core kernel should provide
> this (perhaps after some reorganisation).
>   

It is lookup_address() which works on user addresses, and as such is 
very useful.  But it would need to deal with returning a level so it can 
deal with large pages in usermode, and have some well-defined semantics 
on whether the caller is responsible for unmapping the returned thing 
(ie, only if its a pte).

I implemented this myself a couple of months ago, but I can't find it 
anywhere...

>> +static int memcmp_pages(struct page *page1, struct page *page2)
>> +{
>> +	char *addr1, *addr2;
>> +	int r;
>> +
>> +	addr1 = kmap_atomic(page1, KM_USER0);
>> +	addr2 = kmap_atomic(page2, KM_USER1);
>> +	r = memcmp(addr1, addr2, PAGE_SIZE);
>> +	kunmap_atomic(addr1, KM_USER0);
>> +	kunmap_atomic(addr2, KM_USER1);
>> +	return r;
>> +}
>>     
>
> I wonder if this code all does enough cpu cache flushing to be able to
> guarantee that it's looking at valid data.  Not my area, and presumably
> not an issue on x86.
>   

Shouldn't that be kmap_atomic's job anyway?  Otherwise it would be hard 
to use on any virtual-tag/indexed cache machine.

    J
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