Re: [EXTERNAL] [PATCH net] hv_netvsc: use kmap_local_page in netvsc_copy_to_send_buf

[Anton Leontev]

>
>
>
> > -----Original Message-----
> > From: LeantionX <leontyevantony@xxxxxxxxx>
> > Sent: Tuesday, June 2, 2026 11:52 AM
> > To: netdev@xxxxxxxxxxxxxxx
> > Cc: linux-hyperv@xxxxxxxxxxxxxxx; KY Srinivasan <kys@xxxxxxxxxxxxx>;
> > Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; wei.liu@xxxxxxxxxx; Dexuan Cui
> > <DECUI@xxxxxxxxxxxxx>; Long Li <longli@xxxxxxxxxxxxx>;
> > andrew+netdev@xxxxxxx; kuba@xxxxxxxxxx; pabeni@xxxxxxxxxx;
> > edumazet@xxxxxxxxxx; davem@xxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; Anton Leontev <leontyevantony@xxxxxxxxx>
> > Subject: [EXTERNAL] [PATCH net] hv_netvsc: use kmap_local_page in
> > netvsc_copy_to_send_buf
> >
> > [You don't often get email from leontyevantony@xxxxxxxxx. Learn why this
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> >
> > From: Anton Leontev <leontyevantony@xxxxxxxxx>
> >
> > netvsc_copy_to_send_buf() copies skb fragment pages into the shared
> > VMBus send buffer using phys_to_virt() on the fragment PFN. On 32-bit
> > x86 with CONFIG_HIGHMEM=y, phys_to_virt() (i.e. __va()) is only valid
> > for LOWMEM addresses below 896 MiB. For a HIGHMEM page it returns an
> > address that has no kernel page table entry and lies outside the
> > kernel direct map, so the subsequent memcpy() faults. As this happens
> > on the transmit softirq path, the fault is fatal.
> Please include the stack trace in patch description.
>
> > A HIGHMEM fragment reaches this path whenever the page backing an skb
> > fragment lives above the LOWMEM boundary, which is common on a 32-bit
> > guest with several GiB of RAM (for example when the in-kernel NFS
> > server splices page cache pages directly into the reply skb).
> >
> > Map the fragment page on demand with kmap_local_page()/kunmap_local()
> > instead. Using pfn_to_page() on pb[i].pfn maps exactly the page
> > described by the page buffer entry. On configurations without HIGHMEM
> > (amd64, i386 without CONFIG_HIGHMEM) kmap_local_page() reduces to
> > page_address(), so this is a no-op there.
>
> So, on 64bit kernel, it has no performance impact?
>
> Thanks,
> - Haiyang
>

Correct. On 64-bit (and any !CONFIG_HIGHMEM config) all pages are
permanently present in the kernel direct map, so kmap_local_page()
folds to page_address() and kunmap_local() is a no-op. The generated
code is therefore equivalent to the previous direct-map access, with
no extra mapping cost on the tx path.

The kmap is only meaningful on 32-bit CONFIG_HIGHMEM, where the
fragment page may live above the LOWMEM boundary and the old
phys_to_virt() result is invalid.

Thanks,
Anton