Re: [PATCH 1/4] compcache: xvmalloc memory allocator

[Nitin Gupta]

On 08/25/2009 03:16 AM, Hugh Dickins wrote:
> On Tue, 25 Aug 2009, Nitin Gupta wrote:
> > On 08/25/2009 02:09 AM, Hugh Dickins wrote:
> > > On Tue, 25 Aug 2009, Nitin Gupta wrote:
> > > > On 08/24/2009 11:03 PM, Pekka Enberg wrote:
> > > > > What's the purpose of passing PFNs around? There's quite a lot of PFN
> > > > > to struct page conversion going on because of it. Wouldn't it make
> > > > > more sense to return (and pass) a pointer to struct page instead?
> > > >
> > > > PFNs are 32-bit on all archs
> > >
> > > Are you sure?  If it happens to be so for all machines built today,
> > > I think it can easily change tomorrow.  We consistently use unsigned long
> > > for pfn (there, now I've said that, I bet you'll find somewhere we don't!)
> > >
> > > x86_64 says MAX_PHYSMEM_BITS 46 and ia64 says MAX_PHYSMEM_BITS 50 and
> > > mm/sparse.c says
> > > unsigned long max_sparsemem_pfn = 1UL<<   (MAX_PHYSMEM_BITS-PAGE_SHIFT);
> >
> > For PFN to exceed 32-bit we need to have physical memory>  16TB (2^32 * 4KB).
> > So, maybe I can simply add a check in ramzswap module load to make sure that
> > RAM is indeed<  16TB and then safely use 32-bit for PFN?
>
> Others know much more about it, but I believe that with sparsemem you
> may be handling vast holes in physical memory: so a relatively small
> amount of physical memory might in part be mapped with gigantic pfns.
>
> So if you go that route, I think you'd rather have to refuse pages
> with oversized pfns (or refuse configurations with any oversized pfns),
> than base it upon the quantity of physical memory in the machine.
>
> Seems ugly to me, as it did to Pekka; but I can understand that you're
> very much in the business of saving memory, so doubling the size of some
> of your tables (I may be oversimplifying) would be repugnant to you.
>
> You could add a CONFIG option, rather like CONFIG_LBDAF, to switch on
> u64-sized pfns; but you'd still have to handle what happens when the
> pfn is too big to fit in u32 without that option; and if distros always
> switch the option on, to accomodate the larger machines, then there may
> have been no point to adding it.

Thanks for these details.

Now I understand that use of 32-bit PFN on 64-bit archs is unsafe. So,
there is no option but to include extra bits for PFNs or use struct page.

* Solution of ramzswap block device:

Use 48 bit PFNs (32 + 8) and have a compile time error to make sure that
that MAX_PHYSMEM_BITS is < 48 + PAGE_SHIFT. The ramzswap table can accommodate
48-bits without any increase in table size.

--- ramzswap_new.h	2009-08-25 20:10:38.054033804 +0530
+++ ramzswap.h	2009-08-25 20:09:28.386069100 +0530
@@ -110,9 +110,9 @@

 /* Indexed by page no. */
 struct table {
-	u32 pagenum_1;
+	u32 pagenum;
 	u16 offset;
-	u8 pagenum_2;
+	u8 count;	/* object ref count (not yet used) */
 	u8 flags;
 };


(removal for 'count' field will hurt later when we implement
memory defragmentation support).


* Solution for allocator:

Use struct page instead of PFN. This is better than always using 64-bit PFNs
since we get rid of all casts. Use of 48-bit PFNs as above will create too
much mess. However, use of struct page increases per-pool overhead by 4K on
64-bit systems. This should be okay.


Please let me know if you have any comments. I will make these changes in next
revision.

There is still some problem with memory allocator naming. Its no longer a
separate module, the symbols are not exported and its now compiled with ramzswap
block driver itself. So, I am hoping xv_malloc() does not causes any confusion
with any existing name now. It really should not cause any confustion. I would
love to retain this name for allocator.

Thanks,
Nitin




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