Cody P Schafer wrote:
> On 09/27/2013 06:16 AM, Kirill A. Shutemov wrote:
> > With split page table lock for PMD level we can't hold
> > mm->page_table_lock while updating nr_ptes.
> >
> > Let's convert it to atomic_t to avoid races.
> >
>
> > ---
>
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 84e0c56e1e..99f19e850d 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -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 */
> >
> > spinlock_t page_table_lock; /* Protects page tables and some counters */
> > @@ -360,7 +361,6 @@ struct mm_struct {
> > unsigned long exec_vm; /* VM_EXEC & ~VM_WRITE */
> > unsigned long stack_vm; /* VM_GROWSUP/DOWN */
> > unsigned long def_flags;
> > - unsigned long nr_ptes; /* Page table pages */
> > unsigned long start_code, end_code, start_data, end_data;
> > unsigned long start_brk, brk, start_stack;
> > unsigned long arg_start, arg_end, env_start, env_end;
>
> Will 32bits always be enough here? Should atomic_long_t be used instead?
Good question!
On x86_64 we need one table to cover 2M (512 entries by 4k, 21 bits) of
virtual address space. Total size of virtual memory which can be covered
by 31-bit (32 - sign) nr_ptes is 52 bits (31 + 21).
Currently, on x86_64 with 4-level page tables we can use at most 48 bit of
virtual address space (only half of it available for userspace), so we
pretty safe here.
Although, it can be a potential problem, if (when) x86_64 will implement
5-level page tables -- 57-bits of virtual address space.
Any thoughts?
--
Kirill A. Shutemov
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