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@xxxxxxx> > > 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@xxxxxxx/ >> >> 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); >> >
