Re: [RFC PATCH v1 04/57] mm/page_alloc: Make page_frag_cache boot-time page size compatible

Ryan Roberts
2024-11-14T09:38:41Z

On 14/11/2024 08:23, Vlastimil Babka wrote:
> On 10/14/24 12:58, Ryan Roberts wrote:
>> "struct page_frag_cache" has some optimizations that depend on page
>> size. Let's refactor it a bit so that those optimizations can be
>> determined at run-time for the case where page size is a boot-time
>> parameter. For compile-time page size, the compiler should dead code
>> strip and the result is very similar to before.
>>
>> One wrinkle is that we don't know if we need the size member until
>> runtime. So remove the ifdeffery and always define offset as u32 (needed
>> if PAGE_SIZE is >= 64K) and size as u16 (only used when PAGE_SIZE <=
>> 32K). We move the members around a bit so that the overall size of the
>> struct remains the same; 24 bytes for 64-bit and 16 bytes on 32 bit.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>
> Looks ok, but ideally the PAGE_FRAG_CACHE_MAX_ORDER #define should also be
> replaced by some variable that's populated just once. It can be static local
> to page_alloc.c as nothing else seems to use it.

I can certainly do that, but wouldn't that be penalizing a compile-time page
size configuration? My current change means that PAGE_FRAG_CACHE_MAX_ORDER still
resolves to a compile-time constant in that situation and the compiler can
eliminate conditional branches it knows will never be taken. Or perhaps you're
suggesting I conditionally make it a variable if PAGE_SIZE_MIN != PAGE_SIZE_MAX?

Thanks,
Ryan

>
>>
>> page_alloc
>> ---
>>
>> ***NOTE***
>> Any confused maintainers may want to read the cover note here for context:
>> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
>>
>> include/linux/mm_types.h | 13 ++++++-------
>> mm/page_alloc.c | 31 ++++++++++++++++++-------------
>> 2 files changed, 24 insertions(+), 20 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 4854249792545..0844ed7cfaa53 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -544,16 +544,15 @@ static inline void *folio_get_private(struct folio *folio)
>>
>> struct page_frag_cache {
>> void * va;
>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> - __u16 offset;
>> - __u16 size;
>> -#else
>> - __u32 offset;
>> -#endif
>> /* we maintain a pagecount bias, so that we dont dirty cache line
>> * containing page->_refcount every time we allocate a fragment.
>> */
>> - unsigned int pagecnt_bias;
>> + unsigned int pagecnt_bias;
>> + __u32 offset;
>> + /* size only used when PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE, in which
>> + * case PAGE_FRAG_CACHE_MAX_SIZE is 32K and 16 bits is sufficient.
>> + */
>> + __u16 size;
>> bool pfmemalloc;
>> };
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 91ace8ca97e21..8678103b1b396 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4822,13 +4822,18 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
>> struct page *page = NULL;
>> gfp_t gfp = gfp_mask;
>>
>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> - gfp_mask = (gfp_mask & ~__GFP_DIRECT_RECLAIM) | __GFP_COMP |
>> - __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC;
>> - page = alloc_pages_node(NUMA_NO_NODE, gfp_mask,
>> - PAGE_FRAG_CACHE_MAX_ORDER);
>> - nc->size = page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE;
>> -#endif
>> + if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) {
>> + gfp_mask = (gfp_mask & ~__GFP_DIRECT_RECLAIM) | __GFP_COMP |
>> + __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC;
>> + page = alloc_pages_node(NUMA_NO_NODE, gfp_mask,
>> + PAGE_FRAG_CACHE_MAX_ORDER);
>> + /*
>> + * Cast to silence warning due to 16-bit nc->size. Not real
>> + * because PAGE_SIZE only less than PAGE_FRAG_CACHE_MAX_SIZE
>> + * when PAGE_FRAG_CACHE_MAX_SIZE is 32K.
>> + */
>> + nc->size = (__u16)(page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE);
>> + }
>> if (unlikely(!page))
>> page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
>>
>> @@ -4870,10 +4875,10 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>> if (!page)
>> return NULL;
>>
>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> /* if size can vary use size else just use PAGE_SIZE */
>> - size = nc->size;
>> -#endif
>> + if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> + size = nc->size;
>> +
>> /* Even if we own the page, we do not use atomic_set().
>> * This would break get_page_unless_zero() users.
>> */
>> @@ -4897,10 +4902,10 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>> goto refill;
>> }
>>
>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> /* if size can vary use size else just use PAGE_SIZE */
>> - size = nc->size;
>> -#endif
>> + if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> + size = nc->size;
>> +
>> /* OK, page count is 0, we can safely set it */
>> set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
>>
>