Re: [PATCH] parisc: Ensure page-aligned addresses in cache flush and copy functions

[Matthew Wilcox <willy@infradead.org>]

On Fri, Jan 27, 2023 at 10:39:41PM +0100, Helge Deller wrote:
> Matthew Wilcox noticed, that if ARCH_HAS_FLUSH_ON_KUNMAP is defined
> (which is the case for PA-RISC), __kunmap_local() calls
> kunmap_flush_on_unmap(), which may call the parisc flush functions with
> a non-page-aligned address and thus the page might not be fully flushed.
> 
> To prevent similiar cases, page-align any given address in the
> following parisc low-level calls:
> - clear_page_asm(),
> - copy_page_asm(),
> - copy_user_page_asm(),
> - clear_user_page_asm(),
> - flush_kernel_dcache_page_asm(),
> - purge_kernel_dcache_page_asm() and
> - flush_kernel_icache_page()

I don't think this is the right way to go.  Imagine that we enable
large folios on architectures that don't support TRANSPARENT_HUGEPAGE
(ie PA-RISC).  Then folio_zero_range() is going to call kmap_local()
and kunmap_local() once per folio instead of once per page, and so we'll
need to flush the entire folio, not just the page.