Re: [PATCH vhost 3/6] virtio_net: replace private by pp struct inside page

Re: [PATCH vhost 3/6] virtio_net: replace private by pp struct inside page

[Jesper Dangaard Brouer]

On 17/04/2024 10.20, Xuan Zhuo wrote:
> On Wed, 17 Apr 2024 12:08:10 +0800, Jason Wang <jasowang@redhat.com> wrote:
>> On Wed, Apr 17, 2024 at 9:38 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>>
>>> On Tue, 16 Apr 2024 11:24:53 +0800, Jason Wang <jasowang@redhat.com> wrote:
>>>> On Mon, Apr 15, 2024 at 5:04 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>>>>
>>>>> On Mon, 15 Apr 2024 16:56:45 +0800, Jason Wang <jasowang@redhat.com> wrote:
>>>>>> On Mon, Apr 15, 2024 at 4:50 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>>>>>>
>>>>>>> On Mon, 15 Apr 2024 14:43:24 +0800, Jason Wang <jasowang@redhat.com> wrote:
>>>>>>>> On Mon, Apr 15, 2024 at 10:35 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>>>>>>>>
>>>>>>>>> On Fri, 12 Apr 2024 13:49:12 +0800, Jason Wang <jasowang@redhat.com> wrote:
>>>>>>>>>> On Fri, Apr 12, 2024 at 1:39 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>> On Fri, 12 Apr 2024 12:47:55 +0800, Jason Wang <jasowang@redhat.com> wrote:
>>>>>>>>>>>> On Thu, Apr 11, 2024 at 10:51 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> Now, we chain the pages of big mode by the page's private variable.
>>>>>>>>>>>>> But a subsequent patch aims to make the big mode to support
>>>>>>>>>>>>> premapped mode. This requires additional space to store the dma addr.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Within the sub-struct that contains the 'private', there is no suitable
>>>>>>>>>>>>> variable for storing the DMA addr.
>>>>>>>>>>>>>
>>>>>>>>>>>>>                  struct {        /* Page cache and anonymous pages */
>>>>>>>>>>>>>                          /**
>>>>>>>>>>>>>                           * @lru: Pageout list, eg. active_list protected by
>>>>>>>>>>>>>                           * lruvec->lru_lock.  Sometimes used as a generic list
>>>>>>>>>>>>>                           * by the page owner.
>>>>>>>>>>>>>                           */
>>>>>>>>>>>>>                          union {
>>>>>>>>>>>>>                                  struct list_head lru;
>>>>>>>>>>>>>
>>>>>>>>>>>>>                                  /* Or, for the Unevictable "LRU list" slot */
>>>>>>>>>>>>>                                  struct {
>>>>>>>>>>>>>                                          /* Always even, to negate PageTail */
>>>>>>>>>>>>>                                          void *__filler;
>>>>>>>>>>>>>                                          /* Count page's or folio's mlocks */
>>>>>>>>>>>>>                                          unsigned int mlock_count;
>>>>>>>>>>>>>                                  };
>>>>>>>>>>>>>
>>>>>>>>>>>>>                                  /* Or, free page */
>>>>>>>>>>>>>                                  struct list_head buddy_list;
>>>>>>>>>>>>>                                  struct list_head pcp_list;
>>>>>>>>>>>>>                          };
>>>>>>>>>>>>>                          /* See page-flags.h for PAGE_MAPPING_FLAGS */
>>>>>>>>>>>>>                          struct address_space *mapping;
>>>>>>>>>>>>>                          union {
>>>>>>>>>>>>>                                  pgoff_t index;          /* Our offset within mapping. */
>>>>>>>>>>>>>                                  unsigned long share;    /* share count for fsdax */
>>>>>>>>>>>>>                          };
>>>>>>>>>>>>>                          /**
>>>>>>>>>>>>>                           * @private: Mapping-private opaque data.
>>>>>>>>>>>>>                           * Usually used for buffer_heads if PagePrivate.
>>>>>>>>>>>>>                           * Used for swp_entry_t if PageSwapCache.
>>>>>>>>>>>>>                           * Indicates order in the buddy system if PageBuddy.
>>>>>>>>>>>>>                           */
>>>>>>>>>>>>>                          unsigned long private;
>>>>>>>>>>>>>                  };
>>>>>>>>>>>>>
>>>>>>>>>>>>> But within the page pool struct, we have a variable called
>>>>>>>>>>>>> dma_addr that is appropriate for storing dma addr.
>>>>>>>>>>>>> And that struct is used by netstack. That works to our advantage.
>>>>>>>>>>>>>
>>>>>>>>>>>>>                  struct {        /* page_pool used by netstack */
>>>>>>>>>>>>>                          /**
>>>>>>>>>>>>>                           * @pp_magic: magic value to avoid recycling non
>>>>>>>>>>>>>                           * page_pool allocated pages.
>>>>>>>>>>>>>                           */
>>>>>>>>>>>>>                          unsigned long pp_magic;
>>>>>>>>>>>>>                          struct page_pool *pp;
>>>>>>>>>>>>>                          unsigned long _pp_mapping_pad;
>>>>>>>>>>>>>                          unsigned long dma_addr;
>>>>>>>>>>>>>                          atomic_long_t pp_ref_count;
>>>>>>>>>>>>>                  };
>>>>>>>>>>>>>
>>>>>>>>>>>>> On the other side, we should use variables from the same sub-struct.
>>>>>>>>>>>>> So this patch replaces the "private" with "pp".
>>>>>>>>>>>>>
>>>>>>>>>>>>> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
>>>>>>>>>>>>> ---
>>>>>>>>>>>>
>>>>>>>>>>>> Instead of doing a customized version of page pool, can we simply
>>>>>>>>>>>> switch to use page pool for big mode instead? Then we don't need to
>>>>>>>>>>>> bother the dma stuffs.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> The page pool needs to do the dma by the DMA APIs.
>>>>>>>>>>> So we can not use the page pool directly.
>>>>>>>>>>
>>>>>>>>>> I found this:
>>>>>>>>>>
>>>>>>>>>> define PP_FLAG_DMA_MAP         BIT(0) /* Should page_pool do the DMA
>>>>>>>>>>                                          * map/unmap
>>>>>>>>>>
>>>>>>>>>> It seems to work here?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I have studied the page pool mechanism and believe that we cannot use it
>>>>>>>>> directly. We can make the page pool to bypass the DMA operations.
>>>>>>>>> This allows us to handle DMA within virtio-net for pages allocated from the page
>>>>>>>>> pool. Furthermore, we can utilize page pool helpers to associate the DMA address
>>>>>>>>> to the page.
>>>>>>>>>
>>>>>>>>> However, the critical issue pertains to unmapping. Ideally, we want to return
>>>>>>>>> the mapped pages to the page pool and reuse them. In doing so, we can omit the
>>>>>>>>> unmapping and remapping steps.
>>>>>>>>>
>>>>>>>>> Currently, there's a caveat: when the page pool cache is full, it disconnects
>>>>>>>>> and releases the pages. When the pool hits its capacity, pages are relinquished
>>>>>>>>> without a chance for unmapping.

Could Jakub's memory provider for PP help your use-case?

See: [1] 
https://lore.kernel.org/all/20240403002053.2376017-3-almasrymina@google.com/
Or: [2]
https://lore.kernel.org/netdev/f8270765-a27b-6ccf-33ea-cda097168d79@redhat.com/T/


[...]
>>>>>>
>>>>>> Adding Jesper for some comments.
>>>>>>
>>>>>>>
>>>>>>> Back to this patch set, I think we should keep the virtio-net to manage
>>>>>>> the pages.
>>>>>>>

For context the patch:
  [3] 
https://lore.kernel.org/all/20240411025127.51945-4-xuanzhuo@linux.alibaba.com/

>>>>>>> What do you think?
>>>>>>
>>>>>> I might be wrong, but I think if we need to either
>>>>>>
>>>>>> 1) seek a way to manage the pages by yourself but not touching page
>>>>>> pool metadata (or Jesper is fine with this)
>>>>>
>>>>> Do you mean working with page pool or not?
>>>>>
>>>>
>>>> I meant if Jesper is fine with reusing page pool metadata like this patch.
>>>>
>>>>> If we manage the pages by self(no page pool), we do not care the metadata is for
>>>>> page pool or not. We just use the space of pages like the "private".
>>>>
>>>> That's also fine.
>>>>

I'm not sure it is "fine" to, explicitly choosing not to use page pool,
and then (ab)use `struct page` member (pp) that intended for page_pool
for other stuff. (In this case create a linked list of pages).

  +#define page_chain_next(p)	((struct page *)((p)->pp))
  +#define page_chain_add(p, n)	((p)->pp = (void *)n)

I'm not sure that I (as PP maintainer) can make this call actually, as I
think this area belong with the MM "page" maintainers (Cc MM-list +
people) to judge.

Just invention new ways to use struct page fields without adding your
use-case to struct page, will make it harder for MM people to maintain
(e.g. make future change).

--Jesper

Re: [PATCH vhost 3/6] virtio_net: replace private by pp struct inside page

[Matthew Wilcox]

On Thu, Apr 18, 2024 at 10:19:33PM +0200, Jesper Dangaard Brouer wrote:
> I'm not sure it is "fine" to, explicitly choosing not to use page pool,
> and then (ab)use `struct page` member (pp) that intended for page_pool
> for other stuff. (In this case create a linked list of pages).
> 
>  +#define page_chain_next(p)	((struct page *)((p)->pp))
>  +#define page_chain_add(p, n)	((p)->pp = (void *)n)
> 
> I'm not sure that I (as PP maintainer) can make this call actually, as I
> think this area belong with the MM "page" maintainers (Cc MM-list +
> people) to judge.
> 
> Just invention new ways to use struct page fields without adding your
> use-case to struct page, will make it harder for MM people to maintain
> (e.g. make future change).

I can't really follow what's being proposed; the quoting is quite deep.

Here's the current plan for struct page:

 - The individual users are being split off.  This has already happened
   for struct folio, struct slab and struct pgdesc.  Others are hopefully
   coming.
 - At some point, struct page will become:

   struct page {
     unsigned long flags;
     unsigned long data[5];
     unsigned int data2[2];
     ... some other bits and pieces ...
   };

 - After that, we will turn struct page into:

  struct page {
    unsigned long memdesc;
  };

Users like pagepool will allocate a struct ppdesc that will be
referred to by the memdesc.  The bottom 4 bits will identify it as a
ppdesc.  You can put anything you like in a struct ppdesc, it just has
to be allocated from a slab with a 16 byte alignment.

More details here:
https://kernelnewbies.org/MatthewWilcox/Memdescs

This is all likely to land in 2025.  The goal for 2024 is to remove
mapping & index from 'struct page'.  This has been in progress since
2019 so I'm really excited that we're so close!  If you want to
turn struct ppdesc into its own struct like folio, slab & ptdesc,
I'm happy to help.  I once had a patchset for that:

https://lore.kernel.org/netdev/20221130220803.3657490-1-willy@infradead.org/

but I'm sure it's truly bitrotted.

Re: [PATCH vhost 3/6] virtio_net: replace private by pp struct inside page

[Xuan Zhuo]

On Thu, 18 Apr 2024 22:19:33 +0200, Jesper Dangaard Brouer <hawk@kernel.org> wrote:
>
>
> On 17/04/2024 10.20, Xuan Zhuo wrote:
> > On Wed, 17 Apr 2024 12:08:10 +0800, Jason Wang <jasowang@redhat.com> wrote:
> >> On Wed, Apr 17, 2024 at 9:38 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >>>
> >>> On Tue, 16 Apr 2024 11:24:53 +0800, Jason Wang <jasowang@redhat.com> wrote:
> >>>> On Mon, Apr 15, 2024 at 5:04 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >>>>>
> >>>>> On Mon, 15 Apr 2024 16:56:45 +0800, Jason Wang <jasowang@redhat.com> wrote:
> >>>>>> On Mon, Apr 15, 2024 at 4:50 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >>>>>>>
> >>>>>>> On Mon, 15 Apr 2024 14:43:24 +0800, Jason Wang <jasowang@redhat.com> wrote:
> >>>>>>>> On Mon, Apr 15, 2024 at 10:35 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >>>>>>>>>
> >>>>>>>>> On Fri, 12 Apr 2024 13:49:12 +0800, Jason Wang <jasowang@redhat.com> wrote:
> >>>>>>>>>> On Fri, Apr 12, 2024 at 1:39 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >>>>>>>>>>>
> >>>>>>>>>>> On Fri, 12 Apr 2024 12:47:55 +0800, Jason Wang <jasowang@redhat.com> wrote:
> >>>>>>>>>>>> On Thu, Apr 11, 2024 at 10:51 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Now, we chain the pages of big mode by the page's private variable.
> >>>>>>>>>>>>> But a subsequent patch aims to make the big mode to support
> >>>>>>>>>>>>> premapped mode. This requires additional space to store the dma addr.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Within the sub-struct that contains the 'private', there is no suitable
> >>>>>>>>>>>>> variable for storing the DMA addr.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>                  struct {        /* Page cache and anonymous pages */
> >>>>>>>>>>>>>                          /**
> >>>>>>>>>>>>>                           * @lru: Pageout list, eg. active_list protected by
> >>>>>>>>>>>>>                           * lruvec->lru_lock.  Sometimes used as a generic list
> >>>>>>>>>>>>>                           * by the page owner.
> >>>>>>>>>>>>>                           */
> >>>>>>>>>>>>>                          union {
> >>>>>>>>>>>>>                                  struct list_head lru;
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>                                  /* Or, for the Unevictable "LRU list" slot */
> >>>>>>>>>>>>>                                  struct {
> >>>>>>>>>>>>>                                          /* Always even, to negate PageTail */
> >>>>>>>>>>>>>                                          void *__filler;
> >>>>>>>>>>>>>                                          /* Count page's or folio's mlocks */
> >>>>>>>>>>>>>                                          unsigned int mlock_count;
> >>>>>>>>>>>>>                                  };
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>                                  /* Or, free page */
> >>>>>>>>>>>>>                                  struct list_head buddy_list;
> >>>>>>>>>>>>>                                  struct list_head pcp_list;
> >>>>>>>>>>>>>                          };
> >>>>>>>>>>>>>                          /* See page-flags.h for PAGE_MAPPING_FLAGS */
> >>>>>>>>>>>>>                          struct address_space *mapping;
> >>>>>>>>>>>>>                          union {
> >>>>>>>>>>>>>                                  pgoff_t index;          /* Our offset within mapping. */
> >>>>>>>>>>>>>                                  unsigned long share;    /* share count for fsdax */
> >>>>>>>>>>>>>                          };
> >>>>>>>>>>>>>                          /**
> >>>>>>>>>>>>>                           * @private: Mapping-private opaque data.
> >>>>>>>>>>>>>                           * Usually used for buffer_heads if PagePrivate.
> >>>>>>>>>>>>>                           * Used for swp_entry_t if PageSwapCache.
> >>>>>>>>>>>>>                           * Indicates order in the buddy system if PageBuddy.
> >>>>>>>>>>>>>                           */
> >>>>>>>>>>>>>                          unsigned long private;
> >>>>>>>>>>>>>                  };
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> But within the page pool struct, we have a variable called
> >>>>>>>>>>>>> dma_addr that is appropriate for storing dma addr.
> >>>>>>>>>>>>> And that struct is used by netstack. That works to our advantage.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>                  struct {        /* page_pool used by netstack */
> >>>>>>>>>>>>>                          /**
> >>>>>>>>>>>>>                           * @pp_magic: magic value to avoid recycling non
> >>>>>>>>>>>>>                           * page_pool allocated pages.
> >>>>>>>>>>>>>                           */
> >>>>>>>>>>>>>                          unsigned long pp_magic;
> >>>>>>>>>>>>>                          struct page_pool *pp;
> >>>>>>>>>>>>>                          unsigned long _pp_mapping_pad;
> >>>>>>>>>>>>>                          unsigned long dma_addr;
> >>>>>>>>>>>>>                          atomic_long_t pp_ref_count;
> >>>>>>>>>>>>>                  };
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> On the other side, we should use variables from the same sub-struct.
> >>>>>>>>>>>>> So this patch replaces the "private" with "pp".
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> >>>>>>>>>>>>> ---
> >>>>>>>>>>>>
> >>>>>>>>>>>> Instead of doing a customized version of page pool, can we simply
> >>>>>>>>>>>> switch to use page pool for big mode instead? Then we don't need to
> >>>>>>>>>>>> bother the dma stuffs.
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> The page pool needs to do the dma by the DMA APIs.
> >>>>>>>>>>> So we can not use the page pool directly.
> >>>>>>>>>>
> >>>>>>>>>> I found this:
> >>>>>>>>>>
> >>>>>>>>>> define PP_FLAG_DMA_MAP         BIT(0) /* Should page_pool do the DMA
> >>>>>>>>>>                                          * map/unmap
> >>>>>>>>>>
> >>>>>>>>>> It seems to work here?
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> I have studied the page pool mechanism and believe that we cannot use it
> >>>>>>>>> directly. We can make the page pool to bypass the DMA operations.
> >>>>>>>>> This allows us to handle DMA within virtio-net for pages allocated from the page
> >>>>>>>>> pool. Furthermore, we can utilize page pool helpers to associate the DMA address
> >>>>>>>>> to the page.
> >>>>>>>>>
> >>>>>>>>> However, the critical issue pertains to unmapping. Ideally, we want to return
> >>>>>>>>> the mapped pages to the page pool and reuse them. In doing so, we can omit the
> >>>>>>>>> unmapping and remapping steps.
> >>>>>>>>>
> >>>>>>>>> Currently, there's a caveat: when the page pool cache is full, it disconnects
> >>>>>>>>> and releases the pages. When the pool hits its capacity, pages are relinquished
> >>>>>>>>> without a chance for unmapping.
>
> Could Jakub's memory provider for PP help your use-case?
>
> See: [1]
> https://lore.kernel.org/all/20240403002053.2376017-3-almasrymina@google.com/
> Or: [2]
> https://lore.kernel.org/netdev/f8270765-a27b-6ccf-33ea-cda097168d79@redhat.com/T/


It can not. That make the pp can get page by the callbacks.

Here we talk about the map/unmap.

The virtio-net has the different DMA APIs.

	dma_addr_t virtqueue_dma_map_single_attrs(struct virtqueue *_vq, void *ptr, size_t size,
						  enum dma_data_direction dir, unsigned long attrs);
	void virtqueue_dma_unmap_single_attrs(struct virtqueue *_vq, dma_addr_t addr,
					      size_t size, enum dma_data_direction dir,
					      unsigned long attrs);
	dma_addr_t virtqueue_dma_map_page_attrs(struct virtqueue *_vq, struct page *page,
						size_t offset, size_t size,
						enum dma_data_direction dir,
						unsigned long attrs);
	void virtqueue_dma_unmap_page_attrs(struct virtqueue *_vq, dma_addr_t addr,
					    size_t size, enum dma_data_direction dir,
					    unsigned long attrs);
	int virtqueue_dma_mapping_error(struct virtqueue *_vq, dma_addr_t addr);

	bool virtqueue_dma_need_sync(struct virtqueue *_vq, dma_addr_t addr);
	void virtqueue_dma_sync_single_range_for_cpu(struct virtqueue *_vq, dma_addr_t addr,
						     unsigned long offset, size_t size,
						     enum dma_data_direction dir);
	void virtqueue_dma_sync_single_range_for_device(struct virtqueue *_vq, dma_addr_t addr,
							unsigned long offset, size_t size,
							enum dma_data_direction dir);


Thanks.

>
>
> [...]
> >>>>>>
> >>>>>> Adding Jesper for some comments.
> >>>>>>
> >>>>>>>
> >>>>>>> Back to this patch set, I think we should keep the virtio-net to manage
> >>>>>>> the pages.
> >>>>>>>
>
> For context the patch:
>   [3]
> https://lore.kernel.org/all/20240411025127.51945-4-xuanzhuo@linux.alibaba.com/
>
> >>>>>>> What do you think?
> >>>>>>
> >>>>>> I might be wrong, but I think if we need to either
> >>>>>>
> >>>>>> 1) seek a way to manage the pages by yourself but not touching page
> >>>>>> pool metadata (or Jesper is fine with this)
> >>>>>
> >>>>> Do you mean working with page pool or not?
> >>>>>
> >>>>
> >>>> I meant if Jesper is fine with reusing page pool metadata like this patch.
> >>>>
> >>>>> If we manage the pages by self(no page pool), we do not care the metadata is for
> >>>>> page pool or not. We just use the space of pages like the "private".
> >>>>
> >>>> That's also fine.
> >>>>
>
> I'm not sure it is "fine" to, explicitly choosing not to use page pool,
> and then (ab)use `struct page` member (pp) that intended for page_pool
> for other stuff. (In this case create a linked list of pages).
>
>   +#define page_chain_next(p)	((struct page *)((p)->pp))
>   +#define page_chain_add(p, n)	((p)->pp = (void *)n)
>
> I'm not sure that I (as PP maintainer) can make this call actually, as I
> think this area belong with the MM "page" maintainers (Cc MM-list +
> people) to judge.
>
> Just invention new ways to use struct page fields without adding your
> use-case to struct page, will make it harder for MM people to maintain
> (e.g. make future change).
>
> --Jesper
>
>