Re: [PATCHv4 02/10] mm: convert mm->nr_ptes to atomic_t

From: Dave Hansen
Date: Fri Sep 27 2013 - 17:02:03 EST

On 09/27/2013 01:46 PM, Cody P Schafer wrote:
> On 09/27/2013 06:16 AM, Kirill A. Shutemov wrote:
>> @@ -339,6 +339,7 @@ struct mm_struct {
>> pgd_t * pgd;
>> atomic_t mm_users; /* How many users with user space? */
>> atomic_t mm_count; /* How many references to "struct
>> mm_struct" (users count as 1) */
>> + atomic_t nr_ptes; /* Page table pages */
>> int map_count; /* number of VMAs */
> Will 32bits always be enough here? Should atomic_long_t be used instead?

There are 48 bits of virtual address space on x86 today. 12 bits of
that is the address inside the page, so we've at *most* 2^36 pages. 2^9
(512) pages are mapped by a pte page, so that means the page tables only
hold 2^27 pte pages in a single process.

We've got 31 bits of usable space in the atomic_t, so that definitely
works _today_. If the virtual address space ever gets bigger, we might
have problems, though.

In practice, though, we steal a big chunk of that virtual address space
for the kernel, and that doesn't get accounted in mm->nr_ptes, so we've
got a _bit_ more wiggle room than just 4 bits. Also, anybody that's
mapping >4 petabytes of memory with 4k ptes is just off their rocker.

I'm also not sure what the virtual address limits are for the more
obscure architectures, so I guess it's also possible they'll hit this.
I guess it wouldn't hurt to stick an overflow check in there for VM
debugging purposes.
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