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Date: Mon, 01 Jul 2024 16:27:48 -0700
From: Alexander H Duyck <alexander.duyck@...il.com>
To: Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
kuba@...nel.org, pabeni@...hat.com
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Andrew Morton
<akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH net-next v9 03/13] mm: page_frag: use initial zero
offset for page_frag_alloc_align()
On Tue, 2024-06-25 at 21:52 +0800, Yunsheng Lin wrote:
> We are above to use page_frag_alloc_*() API to not just
"about to use", not "above to use"
> 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.
>
> As offsets is added due to alignment requirement before
> actually checking if the cache is enough, which might make
> it exploitable if caller passes a align value bigger than
> 32K mistakenly. As we are allowing order 3 page allocation
> to fail easily under low memory condition, align value bigger
> than PAGE_SIZE is not really allowed, so add a 'align >
> PAGE_SIZE' checking in page_frag_alloc_va_align() to catch
> that.
>
> 1. https://lore.kernel.org/all/f4abe71b3439b39d17a6fb2d410180f367cadf5c.camel@gmail.com/
>
> CC: Alexander Duyck <alexander.duyck@...il.com>
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
> include/linux/page_frag_cache.h | 2 +-
> include/linux/skbuff.h | 4 ++--
> mm/page_frag_cache.c | 26 +++++++++++---------------
> 3 files changed, 14 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
> index 3a44bfc99750..b9411f0db25a 100644
> --- a/include/linux/page_frag_cache.h
> +++ b/include/linux/page_frag_cache.h
> @@ -32,7 +32,7 @@ static inline void *page_frag_alloc_align(struct page_frag_cache *nc,
> unsigned int fragsz, gfp_t gfp_mask,
> unsigned int align)
> {
> - WARN_ON_ONCE(!is_power_of_2(align));
> + WARN_ON_ONCE(!is_power_of_2(align) || align > PAGE_SIZE);
> return __page_frag_alloc_align(nc, fragsz, gfp_mask, -align);
> }
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index eb8ae8292c48..d1fea23ec386 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3320,7 +3320,7 @@ static inline void *netdev_alloc_frag(unsigned int fragsz)
> static inline void *netdev_alloc_frag_align(unsigned int fragsz,
> unsigned int align)
> {
> - WARN_ON_ONCE(!is_power_of_2(align));
> + WARN_ON_ONCE(!is_power_of_2(align) || align > PAGE_SIZE);
> return __netdev_alloc_frag_align(fragsz, -align);
> }
>
> @@ -3391,7 +3391,7 @@ static inline void *napi_alloc_frag(unsigned int fragsz)
> static inline void *napi_alloc_frag_align(unsigned int fragsz,
> unsigned int align)
> {
> - WARN_ON_ONCE(!is_power_of_2(align));
> + WARN_ON_ONCE(!is_power_of_2(align) || align > PAGE_SIZE);
> return __napi_alloc_frag_align(fragsz, -align);
> }
>
> diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
> index 88f567ef0e29..da244851b8a4 100644
> --- a/mm/page_frag_cache.c
> +++ b/mm/page_frag_cache.c
> @@ -72,10 +72,6 @@ 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.
> */
> @@ -84,11 +80,16 @@ 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;
> +#endif
> +
> + offset = __ALIGN_KERNEL_MASK(nc->offset, ~align_mask);
> + if (unlikely(offset + fragsz > size)) {
The fragsz check below could be moved to here.
> page = virt_to_page(nc->va);
>
> if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
> @@ -99,17 +100,13 @@ 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)) {
> + offset = 0;
> + if (unlikely(fragsz > PAGE_SIZE)) {
Since we aren't taking advantage of the flag that is left after the
subtraction we might just want to look at moving this piece up to just
after the offset + fragsz check. That should prevent us from trying to
refill if we have a request that is larger than a single page. In
addition we could probably just drop the 3 PAGE_SIZE checks above as
they would be redundant.
> /*
> * The caller is trying to allocate a fragment
> * with fragsz > PAGE_SIZE but the cache isn't big
> @@ -124,8 +121,7 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
> }
>
> nc->pagecnt_bias--;
> - offset &= align_mask;
> - nc->offset = offset;
> + nc->offset = offset + fragsz;
>
> return nc->va + offset;
> }
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