> -----Original Message-----
> From: LeantionX <[email protected]>
> Sent: Tuesday, June 2, 2026 11:52 AM
> To: [email protected]
> Cc: [email protected]; KY Srinivasan <[email protected]>;
> Haiyang Zhang <[email protected]>; [email protected]; Dexuan Cui
> <[email protected]>; Long Li <[email protected]>;
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected]; linux-
> [email protected]; Anton Leontev <[email protected]>
> Subject: [EXTERNAL] [PATCH net] hv_netvsc: use kmap_local_page in
> netvsc_copy_to_send_buf
>
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>
> From: Anton Leontev <[email protected]>
>
> 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
