Subject: Re: [PATCH 5/8] kmsan: convert kmsan_handle_dma to use physical addresses

From: Leon Romanovsky <[email-masked]>
Date: Thu, 26 Jun 2025 21:45:04 +0300

On Thu, Jun 26, 2025 at 07:43:06PM +0200, Alexander Potapenko wrote:
> On Wed, Jun 25, 2025 at 3:19 PM Leon Romanovsky <[email-masked]> wrote:
> >
> > From: Leon Romanovsky <[email-masked]>
> 
> Hi Leon,
> 
> >
> > Convert the KMSAN DMA handling function from page-based to physical
> > address-based interface.
> >
> > The refactoring renames kmsan_handle_dma() parameters from accepting
> > (struct page *page, size_t offset, size_t size) to (phys_addr_t phys,
> > size_t size).
> 
> Could you please elaborate a bit why this is needed? Are you fixing
> some particular issue?

It is soft of the fix and improvement at the same time.
Improvement: 
It allows direct call to kmsan_handle_dma() without need
to convert from phys_addr_t to struct page for newly introduced
dma_map_phys() routine.

Fix:
It prevents us from executing kmsan for addresses that don't have struct page
(for example PCI_P2PDMA_MAP_THRU_HOST_BRIDGE pages), which we are doing
with original code.

dma_map_sg_attrs()
 -> __dma_map_sg_attrs()
  -> dma_direct_map_sg()
   -> PCI_P2PDMA_MAP_THRU_HOST_BRIDGE and nents > 0
    -> kmsan_handle_dma_sg();
     -> kmsan_handle_dma(g_page(item) <---- this is "fake" page.

We are trying to build DMA API that doesn't require struct pages.

> 
> > A PFN_VALID check is added to prevent KMSAN operations
> > on non-page memory, preventing from non struct page backed address,
> >
> > As part of this change, support for highmem addresses is implemented
> > using kmap_local_page() to handle both lowmem and highmem regions
> > properly. All callers throughout the codebase are updated to use the
> > new phys_addr_t based interface.
> 
> KMSAN only works on 64-bit systems, do we actually have highmem on any of these?

I don't know, but the original code had this check:
  344         if (PageHighMem(page)) 
  345                 return;

Thanks