Re: [PATCH] percpu: clean up all mappings when pcpu_map_pages() fails

[Dennis Zhou]

Hi Yosry,

On Tue, Mar 19, 2024 at 01:08:26PM -0700, Yosry Ahmed wrote:
> On Mon, Mar 11, 2024 at 12:43 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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@xxxxxxxxxx>
> 
> Any thoughts about this change? Should I resend next week after the
> merge window?
> 

Sorry for the delay.

I'm looking at the code from mm/kmsan/hooks.c kmsan_ioremap_page_range().
It seems like __vunmap_range_noflush() is called on error for
successfully mapped pages similar to how it's being done in percpu-vm.c.

I haven't read in depth the expectations of vmap_pages_range_noflush()
but on first glance it doesn't seem like percpu is operating out of the
ordinary?

Thanks,
Dennis

> > ---
> >
> > 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
> >