Re: [PATCH v2 3/8] vringh: replace kmap_atomic() with kmap_local_page()

Jason Wang
2023-03-16T02:54:23Z

On Thu, Mar 16, 2023 at 5:12 AM Fabio M. De Francesco
<fmdefrancesco@gmail.com> wrote:
>
> On martedì 14 marzo 2023 04:56:08 CET Jason Wang wrote:
> > On Thu, Mar 2, 2023 at 7:34 PM Stefano Garzarella <sgarzare@redhat.com>
> wrote:
> > > kmap_atomic() is deprecated in favor of kmap_local_page().
> >
> > It's better to mention the commit or code that introduces this.
> >
> > > With kmap_local_page() the mappings are per thread, CPU local, can take
> > > page-faults, and can be called from any context (including interrupts).
> > > Furthermore, the tasks can be preempted and, when they are scheduled to
> > > run again, the kernel virtual addresses are restored and still valid.
> > >
> > > kmap_atomic() is implemented like a kmap_local_page() which also disables
> > > page-faults and preemption (the latter only for !PREEMPT_RT kernels,
> > > otherwise it only disables migration).
> > >
> > > The code within the mappings/un-mappings in getu16_iotlb() and
> > > putu16_iotlb() don't depend on the above-mentioned side effects of
> > > kmap_atomic(),
> >
> > Note we used to use spinlock to protect simulators (at least until
> > patch 7, so we probably need to re-order the patches at least) so I
> > think this is only valid when:
> >
> > The vringh IOTLB helpers are not used in atomic context (e.g spinlock,
> > interrupts).
>
> I'm probably missing some context but it looks that you are saying that
> kmap_local_page() is not suited for any use in atomic context (you are
> mentioning spinlocks).
>
> The commit message (that I know pretty well since it's the exact copy, word by
> word, of my boiler plate commits) explains that kmap_local_page() is perfectly
> usable in atomic context (including interrupts).

Thanks for the confirmation, I misread the change log and thought it
said it can't be used in interrupts.

>
> I don't know this code, however I am not able to see why these vringh IOTLB
> helpers cannot work if used under spinlocks. Can you please elaborate a little
> more?

My fault, see above.

>
> > If yes, should we document this? (Or should we introduce a boolean to
> > say whether an IOTLB variant can be used in an atomic context)?
>
> Again, you'll have no problems from the use of kmap_local_page() and so you
> don't need any boolean to tell whether or not the code is running in atomic
> context.
>
> Please take a look at the Highmem documentation which has been recently
> reworked and extended by me: https://docs.kernel.org/mm/highmem.html

This is really helpful.

>
> Anyway, I have been ATK 12 or 13 hours in a row. So I'm probably missing the
> whole picture.

It's me that misses the movitiation of kmap_local().

Thanks

>
> Thanks,
>
> Fabio
>
> > Thanks
> >
> > > so that mere replacements of the old API with the new one
> > > is all that is required (i.e., there is no need to explicitly add calls
> > > to pagefault_disable() and/or preempt_disable()).
> > >
> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > ---
> > >
> > > Notes:
> > > v2:
> > > - added this patch since checkpatch.pl complained about deprecation
> > >
> > > of kmap_atomic() touched by next patch
> > >
> > > drivers/vhost/vringh.c | 8 ++++----
> > > 1 file changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > > index a1e27da54481..0ba3ef809e48 100644
> > > --- a/drivers/vhost/vringh.c
> > > +++ b/drivers/vhost/vringh.c
> > > @@ -1220,10 +1220,10 @@ static inline int getu16_iotlb(const struct vringh
> > > *vrh,>
> > > if (ret < 0)
> > >
> > > return ret;
> > >
> > > - kaddr = kmap_atomic(iov.bv_page);
> > > + kaddr = kmap_local_page(iov.bv_page);
> > >
> > > from = kaddr + iov.bv_offset;
> > > *val = vringh16_to_cpu(vrh, READ_ONCE(*(__virtio16 *)from));
> > >
> > > - kunmap_atomic(kaddr);
> > > + kunmap_local(kaddr);
> > >
> > > return 0;
> > >
> > > }
> > >
> > > @@ -1241,10 +1241,10 @@ static inline int putu16_iotlb(const struct vringh
> > > *vrh,>
> > > if (ret < 0)
> > >
> > > return ret;
> > >
> > > - kaddr = kmap_atomic(iov.bv_page);
> > > + kaddr = kmap_local_page(iov.bv_page);
> > >
> > > to = kaddr + iov.bv_offset;
> > > WRITE_ONCE(*(__virtio16 *)to, cpu_to_vringh16(vrh, val));
> > >
> > > - kunmap_atomic(kaddr);
> > > + kunmap_local(kaddr);
> > >
> > > return 0;
> > >
> > > }
> > >
> > > --
> > > 2.39.2
>
>
>
>