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

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 23/09/2026 à 23:38, Barry Song a écrit :
> On Wed, Sep 23, 2026 at 2:28 PM Wen Jiang <jiangwenxiaomi@gmail.com> wrote:
>>
>> 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?
> 
>> 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 <jiangwen6@xiaomi.com>
> 
> 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>