Re: [PATCH v4] kexec: Enable CMA based contiguous allocation

[Alexander Graf]

On 21.05.25 17:29, Alexander Graf wrote:
> When booting a new kernel with kexec_file, the kernel picks a target
> location that the kernel should live at, then allocates random pages,
> checks whether any of those patches magically happens to coincide with
> a target address range and if so, uses them for that range.
>
> For every page allocated this way, it then creates a page list that the
> relocation code - code that executes while all CPUs are off and we are
> just about to jump into the new kernel - copies to their final memory
> location. We can not put them there before, because chances are pretty
> good that at least some page in the target range is already in use by
> the currently running Linux environment. Copying is happening from a
> single CPU at RAM rate, which takes around 4-50 ms per 100 MiB.
>
> All of this is inefficient and error prone.
>
> To successfully kexec, we need to quiesce all devices of the outgoing
> kernel so they don't scribble over the new kernel's memory. We have seen
> cases where that does not happen properly (*cough* GIC *cough*) and hence
> the new kernel was corrupted. This started a month long journey to root
> cause failing kexecs to eventually see memory corruption, because the new
> kernel was corrupted severely enough that it could not emit output to
> tell us about the fact that it was corrupted. By allocating memory for the
> next kernel from a memory range that is guaranteed scribbling free, we can
> boot the next kernel up to a point where it is at least able to detect
> corruption and maybe even stop it before it becomes severe. This increases
> the chance for successful kexecs.
>
> Since kexec got introduced, Linux has gained the CMA framework which
> can perform physically contiguous memory mappings, while keeping that
> memory available for movable memory when it is not needed for contiguous
> allocations. The default CMA allocator is for DMA allocations.
>
> This patch adds logic to the kexec file loader to attempt to place the
> target payload at a location allocated from CMA. If successful, it uses
> that memory range directly instead of creating copy instructions during
> the hot phase. To ensure that there is a safety net in case anything goes
> wrong with the CMA allocation, it also adds a flag for user space to force
> disable CMA allocations.
>
> Using CMA allocations has two advantages:
>
>    1) Faster by 4-50 ms per 100 MiB. There is no more need to copy in the
>       hot phase.
>    2) More robust. Even if by accident some page is still in use for DMA,
>       the new kernel image will be safe from that access because it resides
>       in a memory region that is considered allocated in the old kernel and
>       has a chance to reinitialize that component.
>
> Signed-off-by: Alexander Graf <graf@xxxxxxxxxx>


Ping? Not seeing this patch in Linus' master tree :)

Alex





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