Subject: Re: [PATCH v1 08/16] kmsan: convert kmsan_handle_dma to use physical addresses

From: Leon Romanovsky <[email-masked]>
Date: Thu, 14 Aug 2025 16:31:06 +0300

On Thu, Aug 14, 2025 at 09:44:48AM -0300, Jason Gunthorpe wrote:
> On Thu, Aug 14, 2025 at 03:35:06PM +0300, Leon Romanovsky wrote:
> > > Then check attrs here, not pfn_valid.
> > 
> > attrs are not available in kmsan_handle_dma(). I can add it if you prefer.
> 
> That makes more sense to the overall design. The comments I gave
> before were driving at a promise to never try to touch a struct page
> for ATTR_MMIO and think this should be comphrensive to never touching
> a struct page even if pfnvalid.
> 
> > > > So let's keep this patch as is.
> > > 
> > > Still need to fix the remarks you clipped, do not check PageHighMem
> > > just call kmap_local_pfn(). All thie PageHighMem stuff is new to this
> > > patch and should not be here, it is the wrong way to use highmem.
> > 
> > Sure, thanks
> 
> I am wondering if there is some reason it was written like this in the
> first place. Maybe we can't even do kmap here.. So perhaps if there is
> not a strong reason to change it just continue to check pagehighmem
> and fail.
> 
> if (!(attrs & ATTR_MMIO) && PageHighMem(phys_to_page(phys)))
>    return;

Does this version good enough? There is no need to call to
kmap_local_pfn() if we prevent PageHighMem pages.

diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
index eab7912a3bf0..d9cf70f4159c 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -337,13 +337,13 @@ static void kmsan_handle_dma_page(const void *addr, size_t size,

 /* Helper function to handle DMA data transfers. */
 void kmsan_handle_dma(phys_addr_t phys, size_t size,
-                     enum dma_data_direction dir)
+                     enum dma_data_direction dir, unsigned long attrs)
 {
        u64 page_offset, to_go, addr;
        struct page *page;
        void *kaddr;

-       if (!pfn_valid(PHYS_PFN(phys)))
+       if ((attrs & ATTR_MMIO) || PageHighMem(phys_to_page(phys)))
                return;

        page = phys_to_page(phys);
@@ -357,19 +357,12 @@ void kmsan_handle_dma(phys_addr_t phys, size_t size,
        while (size > 0) {
                to_go = min(PAGE_SIZE - page_offset, (u64)size);

-               if (PageHighMem(page))
-                       /* Handle highmem pages using kmap */
-                       kaddr = kmap_local_page(page);
-               else
-                       /* Lowmem pages can be accessed directly */
-                       kaddr = page_address(page);
+               /* Lowmem pages can be accessed directly */
+               kaddr = page_address(page);

                addr = (u64)kaddr + page_offset;
                kmsan_handle_dma_page((void *)addr, to_go, dir);

-               if (PageHighMem(page))
-                       kunmap_local(page);
-
                phys += to_go;
                size -= to_go;

(END)

> Jason