We are about to use page_frag_alloc_*() API to not just allocate memory for skb->data, but also use them to do the memory allocation for skb frag too. Currently the implementation of page_frag in mm subsystem is running the offset as a countdown rather than count-up value, there may have several advantages to that as mentioned in [1], but it may have some disadvantages, for example, it may disable skb frag coalescing and more correct cache prefetching We have a trade-off to make in order to have a unified implementation and API for page_frag, so use a initial zero offset in this patch, and the following patch will try to make some optimization to avoid the disadvantages as much as possible. 1. https://lore.kernel.org/all/f4abe71b3439b39d17a6fb2d410180f367cadf5c.camel@xxxxxxxxx/ CC: Alexander Duyck <alexander.duyck@xxxxxxxxx> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx> --- mm/page_frag_cache.c | 46 ++++++++++++++++++++++---------------------- 1 file changed, 23 insertions(+), 23 deletions(-) diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c index 609a485cd02a..4c8e04379cb3 100644 --- a/mm/page_frag_cache.c +++ b/mm/page_frag_cache.c @@ -63,9 +63,13 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc, unsigned int fragsz, gfp_t gfp_mask, unsigned int align_mask) { +#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) + unsigned int size = nc->size; +#else unsigned int size = PAGE_SIZE; +#endif + unsigned int offset; struct page *page; - int offset; if (unlikely(!nc->va)) { refill: @@ -85,11 +89,24 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc, /* reset page count bias and offset to start of new frag */ nc->pfmemalloc = page_is_pfmemalloc(page); nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1; - nc->offset = size; + nc->offset = 0; } - offset = nc->offset - fragsz; - if (unlikely(offset < 0)) { + offset = __ALIGN_KERNEL_MASK(nc->offset, ~align_mask); + if (unlikely(offset + fragsz > size)) { + if (unlikely(fragsz > PAGE_SIZE)) { + /* + * The caller is trying to allocate a fragment + * with fragsz > PAGE_SIZE but the cache isn't big + * enough to satisfy the request, this may + * happen in low memory conditions. + * We don't release the cache page because + * it could make memory pressure worse + * so we simply return NULL here. + */ + return NULL; + } + page = virt_to_page(nc->va); if (!page_ref_sub_and_test(page, nc->pagecnt_bias)) @@ -100,33 +117,16 @@ 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 /* OK, page count is 0, we can safely set it */ set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1); /* reset page count bias and offset to start of new frag */ nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1; - offset = size - fragsz; - if (unlikely(offset < 0)) { - /* - * The caller is trying to allocate a fragment - * with fragsz > PAGE_SIZE but the cache isn't big - * enough to satisfy the request, this may - * happen in low memory conditions. - * We don't release the cache page because - * it could make memory pressure worse - * so we simply return NULL here. - */ - return NULL; - } + offset = 0; } nc->pagecnt_bias--; - offset &= align_mask; - nc->offset = offset; + nc->offset = offset + fragsz; return nc->va + offset; } -- 2.33.0
