Re: [PATCH 3/5] mm: Rework return value for copy_one_pte()

John Hubbard
2020-09-22T07:11:32Z

On 9/21/20 2:17 PM, Peter Xu wrote:
> There's one special path for copy_one_pte() with swap entries, in which
> add_swap_count_continuation(GFP_ATOMIC) might fail. In that case we'll return

I might be looking at the wrong place, but the existing code seems to call
add_swap_count_continuation(GFP_KERNEL), not with GFP_ATOMIC?

> the swp_entry_t so that the caller will release the locks and redo the same
> thing with GFP_KERNEL.
>
> It's confusing when copy_one_pte() must return a swp_entry_t (even if all the
> ptes are non-swap entries). More importantly, we face other requirement to
> extend this "we need to do something else, but without the locks held" case.
>
> Rework the return value into something easier to understand, as defined in enum
> copy_mm_ret. We'll pass the swp_entry_t back using the newly introduced union

I like the documentation here, but it doesn't match what you did in the patch.
Actually, the documentation had the right idea (enum, rather than #define, for
COPY_MM_* items). Below...

> copy_mm_data parameter.
>
> Another trivial change is to move the reset of the "progress" counter into the
> retry path, so that we'll reset it for other reasons too.
>
> This should prepare us with adding new return codes, very soon.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> mm/memory.c | 42 +++++++++++++++++++++++++++++-------------
> 1 file changed, 29 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 7525147908c4..1530bb1070f4 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -689,16 +689,24 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
> }
> #endif
>
> +#define COPY_MM_DONE 0
> +#define COPY_MM_SWAP_CONT 1

Those should be enums, so as to get a little type safety and other goodness from
using non-macro items.

...
> @@ -866,13 +877,18 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> pte_unmap_unlock(orig_dst_pte, dst_ptl);
> cond_resched();
>
> - if (entry.val) {
> - if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
> + switch (copy_ret) {
> + case COPY_MM_SWAP_CONT:
> + if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0)
> return -ENOMEM;
> - progress = 0;

Yes. Definitely a little cleaner to reset this above, instead of here.

> + break;
> + default:
> + break;

I assume this no-op noise is to placate the compiler and/or static checkers. :)

I'm unable to find any actual problems with the diffs, aside from the nit about
using an enum.

thanks,
--
John Hubbard
NVIDIA