linux-kernel - Re: [PATCH] percpu: clean up all mappings when pcpu_map

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Date: Tue, 19 Mar 2024 13:08:26 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>, 
	Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] percpu: clean up all mappings when pcpu_map_pages() fails

On Mon, Mar 11, 2024 at 12:43 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> In pcpu_map_pages(), if __pcpu_map_pages() fails on a CPU, we call
> __pcpu_unmap_pages() to clean up mappings on all CPUs where mappings
> were created, but not on the CPU where __pcpu_map_pages() fails.
>
> __pcpu_map_pages() and __pcpu_unmap_pages() are wrappers around
> vmap_pages_range_noflush() and vunmap_range_noflush(). All other callers
> of vmap_pages_range_noflush() call vunmap_range_noflush() when mapping
> fails, except pcpu_map_pages(). The reason could be that partial
> mappings may be left behind from a failed mapping attempt.
>
> Call __pcpu_unmap_pages() for the failed CPU as well in
> pcpu_map_pages().
>
> This was found by code inspection, no failures or bugs were observed.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Any thoughts about this change? Should I resend next week after the
merge window?

> ---
>
> Perhaps the reason __pcpu_unmap_pages() is not currently being called
> for the failed CPU is that the size and alignment requirements make sure
> we never leave any partial mappings behind? I have no idea. Nonetheless,
> I think we want this change as that could be fragile, and is
> inconsistent with other callers.
>
> ---
>  mm/percpu-vm.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
> index 2054c9213c433..cd69caf6aa8d8 100644
> --- a/mm/percpu-vm.c
> +++ b/mm/percpu-vm.c
> @@ -231,10 +231,10 @@ static int pcpu_map_pages(struct pcpu_chunk *chunk,
>         return 0;
>  err:
>         for_each_possible_cpu(tcpu) {
> -               if (tcpu == cpu)
> -                       break;
>                 __pcpu_unmap_pages(pcpu_chunk_addr(chunk, tcpu, page_start),
>                                    page_end - page_start);
> +               if (tcpu == cpu)
> +                       break;
>         }
>         pcpu_post_unmap_tlb_flush(chunk, page_start, page_end);
>         return err;
> --
> 2.44.0.278.ge034bb2e1d-goog
>