Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

[Mike Rapoport <rppt@kernel.org>]

On Tue, Jul 05, 2022 at 04:34:09PM +0100, Catalin Marinas wrote:
> On Tue, Jul 05, 2022 at 06:02:02PM +0300, Mike Rapoport wrote:
> > +void __init remap_crashkernel(void)
> > +{
> > +#ifdef CONFIG_KEXEC_CORE
> > +	phys_addr_t start, end, size;
> > +	phys_addr_t aligned_start, aligned_end;
> > +
> > +	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> > +	    return;
> > +
> > +	if (!crashk_res.end)
> > +	    return;
> > +
> > +	start = crashk_res.start & PAGE_MASK;
> > +	end = PAGE_ALIGN(crashk_res.end);
> > +
> > +	aligned_start = ALIGN_DOWN(crashk_res.start, PUD_SIZE);
> > +	aligned_end = ALIGN(end, PUD_SIZE);
> > +
> > +	/* Clear PUDs containing crash kernel memory */
> > +	unmap_hotplug_range(__phys_to_virt(aligned_start),
> > +			    __phys_to_virt(aligned_end), false, NULL);
> 
> What I don't understand is what happens if there's valid kernel data
> between aligned_start and crashk_res.start (or the other end of the
> range).

Data shouldn't go anywhere :)

There is 

+	/* map area from PUD start to start of crash kernel with large pages */
+	size = start - aligned_start;
+	__create_pgd_mapping(swapper_pg_dir, aligned_start,
+			     __phys_to_virt(aligned_start),
+			     size, PAGE_KERNEL, early_pgtable_alloc, 0);

and 

+	/* map area from end of crash kernel to PUD end with large pages */
+	size = aligned_end - end;
+	__create_pgd_mapping(swapper_pg_dir, end, __phys_to_virt(end),
+			     size, PAGE_KERNEL, early_pgtable_alloc, 0);

after the unmap, so after we tear down a part of a linear map we
immediately recreate it, just with a different page size.

This all happens before SMP, so there is no concurrency at that point.

> -- 
> Catalin

-- 
Sincerely yours,
Mike.