linux-kernel - Re: [PATCH 5/8] kmsan: convert kmsan_handle

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Message-ID: <20250626184504.GK17401@unreal>
Date: Thu, 26 Jun 2025 21:45:04 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Christoph Hellwig <hch@....de>, Jonathan Corbet <corbet@....net>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Marco Elver <elver@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, iommu@...ts.linux.dev,
	virtualization@...ts.linux.dev, kasan-dev@...glegroups.com,
	linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 5/8] kmsan: convert kmsan_handle_dma to use physical
 addresses

On Thu, Jun 26, 2025 at 07:43:06PM +0200, Alexander Potapenko wrote:
> On Wed, Jun 25, 2025 at 3:19 PM Leon Romanovsky <leon@...nel.org> wrote:
> >
> > From: Leon Romanovsky <leonro@...dia.com>
> 
> 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