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Message-ID: <df826acf-8867-7eb6-e7f0-962c106bc28b@huawei.com>
Date: Tue, 9 Apr 2024 15:59:46 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Alexander Duyck <alexander.duyck@...il.com>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Andrew Morton
<akpm@...ux-foundation.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH net-next v1 02/12] mm: page_frag: use initial zero offset
for page_frag_alloc_align()
On 2024/4/9 0:11, Alexander Duyck wrote:
> On Mon, Apr 8, 2024 at 6:39 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>>
>> 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@gmail.com/
>>>>
>>>> CC: Alexander Duyck <alexander.duyck@...il.com>
>>>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
>>>> ---
>>>> 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?
>
> It is a matter of making the negations more obvious. Basically you
> could achieve the same alignment by doing:
> (offset + (~align_mask)) & ~(~align_mask)
> rather than:
> (offset + ((-align_mask) - 1)) & ~((-align_mask) - 1)
>
> I'm not sure the compiler will pick up on the fact that the two are
> identical and can save a number of operations. Also my suggested
> approach is closer to how it used to work. Technically the one you are
> using only works if align_mask is a negative power of 2.
In patch 3, we have below, so the above trick is not really needed after
patch 3:
@@ -94,7 +93,7 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
size = PAGE_SIZE;
#endif
- offset = ALIGN(nc->offset, -align_mask);
+ offset = nc->offset;
if (unlikely(offset + fragsz > size)) {
page = virt_to_page(nc->va);
@@ -131,7 +130,7 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
return nc->va + offset;
}
-EXPORT_SYMBOL(__page_frag_alloc_align);
+EXPORT_SYMBOL(page_frag_alloc);
..
+static inline void *__page_frag_alloc_align(struct page_frag_cache *nc,
+ unsigned int fragsz, gfp_t gfp_mask,
+ unsigned int align)
+{
+ nc->offset = ALIGN(nc->offset, align);
+
+ return page_frag_alloc(nc, fragsz, gfp_mask);
+}
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));
- return __page_frag_alloc_align(nc, fragsz, gfp_mask, -align);
-}
> .
>
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