On 2024/4/8 1:52, Alexander H Duyck wrote: > On Sun, 2024-04-07 at 21:08 +0800, Yunsheng Lin wrote: >> We are above 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 coaleasing 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 aovid 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 | 31 ++++++++++++++----------------- >> 1 file changed, 14 insertions(+), 17 deletions(-) >> >> diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c >> index a0f90ba25200..3e3e88d9af90 100644 >> --- a/mm/page_frag_cache.c >> +++ b/mm/page_frag_cache.c >> @@ -67,9 +67,8 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc, >> unsigned int fragsz, gfp_t gfp_mask, >> unsigned int align_mask) >> { >> - unsigned int size = PAGE_SIZE; >> + unsigned int size, offset; >> struct page *page; >> - int offset; >> >> if (unlikely(!nc->va)) { >> refill: >> @@ -77,10 +76,6 @@ 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 >> /* Even if we own the page, we do not use atomic_set(). >> * This would break get_page_unless_zero() users. >> */ >> @@ -89,11 +84,18 @@ 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)) { >> +#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) >> + /* if size can vary use size else just use PAGE_SIZE */ >> + size = nc->size; >> +#else >> + size = PAGE_SIZE; >> +#endif >> + >> + offset = ALIGN(nc->offset, -align_mask); >> + if (unlikely(offset + fragsz > size)) { > > Rather than using "ALIGN" with a negative value it would probably make > more sense to use __ALIGN_KERNEL_MASK with ~align_mask. I am not sure > how well the compiler sorts out the use of negatives to flip values > that are then converted to masks with the "(a) - 1". The next patch will remove the '-' in '-align_mask' as the 'ALIGN' operation is done in the inline helper. I am not sure that matter much to use __ALIGN_KERNEL_MASK with ~align_mask? >
