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

[Jeremy Fitzhardinge]

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/