Re: [PATCH v9 03/10] mm/vmalloc: use pte_set_huge()/pte_clear_huge() for PTE-level block mappings

Le 23/09/2026 à 23:38, Barry Song a écrit :

On Wed, Sep 23, 2026 at 2:28 PM Wen Jiang [off-list ref] wrote:

quoted

vmap installs PTE-level block mappings by reusing set_huge_pte_at() and
huge_ptep_get_and_clear() under #ifdef CONFIG_HUGETLB_PAGE. This makes
the feature silently unavailable on CONFIG_HUGETLB_PAGE=n kernels and
couples mm/vmalloc.c to HugeTLB internals it does not otherwise need.

Now that arm64 and powerpc/8xx provide pte_set_huge() and
pte_clear_huge(), add the generic fallbacks next to the existing
pmd/pud_set_huge() family and convert vmap_pte_range() and
vunmap_pte_range() to the new helpers. The CONFIG_HUGETLB_PAGE guards
around the block-mapping paths are dropped, so PTE-level block mappings
now also work on CONFIG_HUGETLB_PAGE=n kernels, and mm/vmalloc.c no
longer includes <linux/hugetlb.h>.

The fallbacks exist only to keep the build working on architectures
without PTE-level block mapping support. They are unreachable there:

Since this is a `BUILD_BUG`, can the compiler simply eliminate the call?
I guess the build would still pass even without the fallbacks, so this is
more of a build-time check than something needed to keep the build passing?

Yes and that's what the compiler does, it eliminates the call.
The BUILD_BUG() is there to make sure it does eliminates the call.

The build won't pass without at least a forward declaration of the fallback.

In the old days when BUILD_BUG() didn't exist, the trick was the declare 
the function and never define it. But with that trick the problem is 
only detected at link time. With BUILD_BUG() the problem is detected at 
build time.

Am I missing something?

quoted

the callers only run when arch_vmap_pte_range_map_size() or
arch_vmap_pte_range_unmap_size() return a size other than PAGE_SIZE,
which requires an arch implementation. BUILD_BUG() makes that
explicit rather than silently doing nothing.

Signed-off-by: Wen Jiang <redacted>

Nice to see that `#ifdef CONFIG_HUGETLB_PAGE` and
`#include <linux/hugetlb.h>` are no longer needed in `vmalloc`.

Nothing concerns me except for the changelog issue mentioned above.
Please double-check whether the changelog needs to be corrected.

Reviewed-by: Barry Song <baohua@kernel.org>