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Thu, 23 Jan 2025 11:15:42 -0800 (PST) Received: from [10.67.48.245] ([192.19.223.252]) by smtp.gmail.com with ESMTPSA id 586e51a60fabf-2b28f48b2f4sm131010fac.45.2025.01.23.11.15.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Jan 2025 11:15:41 -0800 (PST) Message-ID: <2ef8b7d4-7fae-4a08-9db1-4a33cd56ec9c@broadcom.com> Date: Thu, 23 Jan 2025 11:15:39 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v23 3/7] mm: page_frag: use initial zero offset for page_frag_alloc_align() To: Yunsheng Lin , davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, Eric Dumazet Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Duyck , Andrew Morton , Linux-MM , Alexander Duyck References: <20241028115343.3405838-1-linyunsheng@huawei.com> <20241028115343.3405838-4-linyunsheng@huawei.com> Content-Language: en-US From: Florian Fainelli Autocrypt: addr=florian.fainelli@broadcom.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi Yunsheng, On 10/28/24 04:53, Yunsheng Lin wrote: > 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@gmail.com/ > > CC: Alexander Duyck > CC: Andrew Morton > CC: Linux-MM > Signed-off-by: Yunsheng Lin > Reviewed-by: Alexander Duyck Sorry for the late feedback, this patch causes the bgmac driver in is .ndo_open() function to return -ENOMEM, the call trace looks like this: bgmac_open -> bgmac_dma_init -> bgmac_dma_rx_skb_for_slot -> netdev_alloc_frag BGMAC_RX_ALLOC_SIZE = 10048 and PAGE_FRAG_CACHE_MAX_SIZE = 32768. Eventually we land into __page_frag_alloc_align() with the following parameters across multiple successive calls: __page_frag_alloc_align: fragsz=10048, align_mask=-1, size=32768, offset=0 __page_frag_alloc_align: fragsz=10048, align_mask=-1, size=32768, offset=10048 __page_frag_alloc_align: fragsz=10048, align_mask=-1, size=32768, offset=20096 __page_frag_alloc_align: fragsz=10048, align_mask=-1, size=32768, offset=30144 So in that case we do indeed have offset + fragsz (40192) > size (32768) and so we would eventually return NULL. Any idea on how to best fix that within the bgmac driver? Thanks! > --- > 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; > } -- Florian