On Sun, 9 May 2010 12:56:49 -0700 (PDT)
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>
> On Sun, 9 May 2010, Mel Gorman wrote:
> >
> > It turns out not to be easy to the preallocating of PUDs, PMDs and PTEs
> > move_page_tables() needs. To avoid overallocating, it has to follow the same
> > logic as move_page_tables duplicating some code in the process. The ugliest
> > aspect of all is passing those pre-allocated pages back into move_page_tables
> > where they need to be passed down to such functions as __pte_alloc. It turns
> > extremely messy.
>
> Umm. What?
>
> That's crazy talk. I'm not talking about preallocating stuff in order to
> pass it in to move_page_tables(). I'm talking about just _creating_ the
> dang page tables early - preallocating them IN THE PROCESS VM SPACE.
>
> IOW, a patch like this (this is a pseudo-patch, totally untested, won't
> compile, yadda yadda - you need to actually make the people who call
> "move_page_tables()" call that prepare function first etc etc)
>
> Yeah, if we care about holes in the page tables, we can certainly copy
> more of the move_page_tables() logic, but it certainly doesn't matter for
> execve(). This just makes sure that the destination page tables exist
> first.
>
IMHO, I think move_page_tables() itself should be implemented as your patch.
But, move_page_tables()'s failure is not a big problem. At failure,
exec will abort and no page fault will occur later. What we have to do in
this migration-patch-series is avoding inconsistent update of sets of
[page, vma->vm_start, vma->pg_off, ptes] or "dont' migrate pages in exec's
statk".
Considering cost, as Mel shows, "don't migrate pages in exec's stack" seems
reasonable. But, I still doubt this check.
+static bool is_vma_temporary_stack(struct vm_area_struct *vma)
+{
+ int maybe_stack = vma->vm_flags & (VM_GROWSDOWN | VM_GROWSUP);
+
+ if (!maybe_stack)
+ return false;
+
+ /* If only the stack is mapped, assume exec is in progress */
+ if (vma->vm_mm->map_count == 1) -------------------(*)
+ return true;
+
+ return false;
+}
+
Mel, can (*) be safe even on a.out format (format other than ELFs) ?
Thanks,
-Kame
> Linus
>
> ---
> mm/mremap.c | 22 +++++++++++++++++++++-
> 1 files changed, 21 insertions(+), 1 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index cde56ee..c14505c 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -128,6 +128,26 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
>
> #define LATENCY_LIMIT (64 * PAGE_SIZE)
>
> +/*
> + * Preallocate the page tables, so that we can do the actual move
> + * without any allocations, and thus no error handling etc.
> + */
> +int prepare_move_page_tables(struct vm_area_struct *vma,
> + unsigned long old_addr, struct vm_area_struct *new_vma,
> + unsigned long new_addr, unsigned long len)
> +{
> + unsigned long end_addr = new_addr + len;
> +
> + while (new_addr < end_addr) {
> + pmd_t *new_pmd;
> + new_pmd = alloc_new_pmd(vma->vm_mm, new_addr);
> + if (!new_pmd)
> + return -ENOMEM;
> + new_addr = (new_addr + PMD_SIZE) & PMD_MASK;
> + }
> + return 0;
> +}
> +
> unsigned long move_page_tables(struct vm_area_struct *vma,
> unsigned long old_addr, struct vm_area_struct *new_vma,
> unsigned long new_addr, unsigned long len)
> @@ -147,7 +167,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> old_pmd = get_old_pmd(vma->vm_mm, old_addr);
> if (!old_pmd)
> continue;
> - new_pmd = alloc_new_pmd(vma->vm_mm, new_addr);
> + new_pmd = get_old_pmd(vma->vm_mm, new_addr);
> if (!new_pmd)
> break;
> next = (new_addr + PMD_SIZE) & PMD_MASK;
>
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