linux-kernel - Re:Re: [PATCH v4] net: ethernet: mtk_eth_soc: fix misuse of mem alloc interface netdev[napi]_alloc

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Date:   Wed, 8 Jun 2022 20:43:31 +0800 (CST)
From:   "Chen Lin" <chen45464546@....com>
To:     "Jakub Kicinski" <kuba@...nel.org>
Cc:     nbd@....name, john@...ozen.org, sean.wang@...iatek.com,
        Mark-MC.Lee@...iatek.com, davem@...emloft.net, edumazet@...gle.com,
        pabeni@...hat.com, matthias.bgg@...il.com, netdev@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        alexander.duyck@...il.com
Subject: Re:Re: [PATCH v4] net: ethernet: mtk_eth_soc: fix misuse of mem
 alloc interface netdev[napi]_alloc_frag

At 2022-06-08 07:14:13, "Jakub Kicinski" <kuba@...nel.org> wrote:
>On Tue,  7 Jun 2022 07:39:11 +0800 Chen Lin wrote:
>> +static inline void *mtk_max_lro_buf_alloc(gfp_t gfp_mask)
>
>No need for inline, compiler will inline this anyway.
>
>> +{
>> +	void *data;
>
>unsigned long data; then you can move the cast from the long line to
>the return statement, saving us from the strange indentation.
>
>> +	data = (void *)__get_free_pages(gfp_mask |
>> +			  __GFP_COMP | __GFP_NOWARN,
>> +			  get_order(mtk_max_frag_size(MTK_MAX_LRO_RX_LENGTH)));
>> +
>> +	return data;
>> +}

I'll do it like below :
+static void *mtk_max_lro_buf_alloc(gfp_t gfp_mask)
+{
+       unsigned long data;
+       unsigned int size = mtk_max_frag_size(MTK_MAX_LRO_RX_LENGTH);
+
+       data = __get_free_pages(gfp_mask | __GFP_COMP | __GFP_NOWARN,
+                               get_order(size));
+
+       return (void *)data;
+}

Through analysis of the ASM code from objdump, I confirmed that 
the inline is not necessary. Thanks for your tips.

Also, I confirmed that create a new local variable 'size'
will not affect the generation of a constant 'order' parameter at compile time.


ASM code of calling 'mtk_max_lro_buf_alloc':

'mtk_max_lro_buf_alloc' inlined and 'order'(w1) is a constant 0x2
0000000000004530 <mtk_napi_rx>:
...
4a98:       52854400        mov     w0, #0x2a20                     // #10784
4a9c:       52800041        mov     w1, #0x2                        // #2
4aa0:       72a00080        movk    w0, #0x4, lsl #16
4aa4:       94000000        bl      0 <__get_free_pages>
4aa8:       f90033e0        str     x0, [sp, #96]

0000000000000730 <mtk_rx_alloc>:
...
7fc:       2a1703e0        mov     w0, w23
800:       52800041        mov     w1, #0x2                        // #2
804:       7140047f        cmp     w3, #0x1, lsl #12
808:       54fffe49        b.ls    7d0 <mtk_rx_alloc+0xa0>  // b.plast
80c:       94000000        bl      0 <__get_free_pages>

The compiler is smart.