On 08/03/22 at 06:00pm, Fabio M. De Francesco wrote:
Thanks for the log update. And Cc Andrew.
> kmap() is being deprecated in favor of kmap_local_page().
>
> There are two main problems with kmap(): (1) It comes with an overhead as
> mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
>
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and are still valid.
>
> Since its use in kexec_core.c is safe everywhere, it should be preferred.
>
> Therefore, replace kmap() with kmap_local_page() in kexec_core.c.
>
> Tested on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
> HIGHMEM64GB enabled.
>
> Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> Acked-by: Baoquan He <bhe@xxxxxxxxxx>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
> ---
>
> v2->v3: Add more information in the commit message as requested by Baoquan
> He and forward two tags, respectively from Ira Weiny and Baoquan He (many
> thanks to them two!).
>
> v1->v2: A sentence of the commit message contained an error due to a
> mistake in copy-pasting from a previous patch. Replace "aio.c" with
> "kexec_core.c".
>
> kernel/kexec_core.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 4d34c78334ce..6f98274765d4 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -814,7 +814,7 @@ static int kimage_load_normal_segment(struct kimage *image,
> if (result < 0)
> goto out;
>
> - ptr = kmap(page);
> + ptr = kmap_local_page(page);
> /* Start with a clear page */
> clear_page(ptr);
> ptr += maddr & ~PAGE_MASK;
> @@ -827,7 +827,7 @@ static int kimage_load_normal_segment(struct kimage *image,
> memcpy(ptr, kbuf, uchunk);
> else
> result = copy_from_user(ptr, buf, uchunk);
> - kunmap(page);
> + kunmap_local(ptr);
> if (result) {
> result = -EFAULT;
> goto out;
> @@ -878,7 +878,7 @@ static int kimage_load_crash_segment(struct kimage *image,
> goto out;
> }
> arch_kexec_post_alloc_pages(page_address(page), 1, 0);
> - ptr = kmap(page);
> + ptr = kmap_local_page(page);
> ptr += maddr & ~PAGE_MASK;
> mchunk = min_t(size_t, mbytes,
> PAGE_SIZE - (maddr & ~PAGE_MASK));
> @@ -894,7 +894,7 @@ static int kimage_load_crash_segment(struct kimage *image,
> else
> result = copy_from_user(ptr, buf, uchunk);
> kexec_flush_icache_page(page);
> - kunmap(page);
> + kunmap_local(ptr);
> arch_kexec_pre_free_pages(page_address(page), 1);
> if (result) {
> result = -EFAULT;
> --
> 2.37.1
>
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