On 26/11/2024 12:36, Vlastimil Babka wrote: > On 11/26/24 13:18, Ryan Roberts wrote: >> On 14/11/2024 10:09, Vlastimil Babka wrote: >>> On 11/1/24 21:16, Dave Kleikamp wrote: >>>> When boot-time page size is enabled, the test against KMALLOC_MAX_CACHE_SIZE >>>> is no longer optimized out with a constant size, so a build bug may >>>> occur on a path that won't be reached. >>> >>> That's rather unfortunate, the __builtin_constant_p(size) part of >>> kmalloc_noprof() really expects things to resolve at compile time and it >>> would be better to keep it that way. >>> >>> I think it would be better if we based KMALLOC_MAX_CACHE_SIZE itself on >>> PAGE_SHIFT_MAX and kept it constant, instead of introducing >>> KMALLOC_SHIFT_HIGH_MAX only for some sanity checks. >>> >>> So if the kernel was built to support 4k to 64k, but booted as 4k, it would >>> still create and use kmalloc caches up to 128k. SLUB should handle that fine >>> (if not, please report it :) >> >> So when PAGE_SIZE_MAX=64K and PAGE_SIZE=4K, kmalloc will support up to 128K >> whereas before it only supported up to 8K. I was trying to avoid that since I >> assumed that would be costly in terms of extra memory allocated for those higher >> order buckets that will never be used. But I have no idea how SLUB works in >> practice. Perhaps memory for the cache is only lazily allocated so we won't see >> an issue in practice? > > Yes the e.g. 128k slabs themselves will be lazily allocated. There will be > some overhead with the management structures (struct kmem_cache etc) but > much smaller. > To be completely honest, some extra overhead might come to be when the slabs > are allocated ans later the user frees those allocations. kmalloc_large() > wwould return them immediately, while a regular kmem_cache will keep one or > more per cpu for reuse. But if that becomes a visible problem we can tune > those caches to discard slabs more aggressively. > >> I'm happy to make this change if you're certain it's the right approach; please >> confirm. > > Yes it's much better option than breaking the build-time-constant part of > kmalloc_noprof(). OK, I'll take this approach as you suggest. Thanks, Ryan > >>> >>> Maybe we could also stop adding + 1 to PAGE_SHIFT_MAX if it's >=64k, so the >>> cache size is max 64k and not 128k but that should be probably evaluated >>> separately from this series. >> >> I'm inferring from this that perhaps there is a memory cost with having the >> higher orders defined but unused. > > Yeah as per above, should not be too large and we could tune it down if > necessary. > >> Thanks, >> Ryan >> >>> >>> Vlastimil >>> >>>> Found compiling drivers/net/ethernet/qlogic/qed/qed_sriov.c >>>> >>>> Signed-off-by: Dave Kleikamp <dave.kleikamp@xxxxxxxxxx> >>>> --- >>>> >>>> Ryan, >>>> >>>> Please consider incorporating this fix or something similar into your >>>> mm patch in the boot-time pages size patches. >>>> >>>> include/linux/slab.h | 3 ++- >>>> 1 file changed, 2 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/include/linux/slab.h b/include/linux/slab.h >>>> index 9848296ca6ba..a4c7507ab8ec 100644 >>>> --- a/include/linux/slab.h >>>> +++ b/include/linux/slab.h >>>> @@ -685,7 +685,8 @@ static __always_inline unsigned int __kmalloc_index(size_t size, >>>> if (size <= 1024 * 1024) return 20; >>>> if (size <= 2 * 1024 * 1024) return 21; >>>> >>>> - if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant) >>>> + if (!IS_ENABLED(CONFIG_ARM64_BOOT_TIME_PAGE_SIZE) && >>>> + !IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant) >>>> BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()"); >>>> else >>>> BUG(); >>> >> >
