Re: [PATCH net-next v1 04/11] xsk: Allocate UMEM chunks through page_pool

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 11/11, Björn Töpel wrote:
> Stanislav Fomichev <sdf.kernel@gmail.com> writes:
> 
> > On 11/10, Björn Töpel wrote:
> 
> >> diff --git a/net/xdp/xsk_pp.c b/net/xdp/xsk_pp.c
> >> new file mode 100644
> >> index 000000000000..fdd55265a6cd
> >> --- /dev/null
> >> +++ b/net/xdp/xsk_pp.c
> >> @@ -0,0 +1,246 @@
> >> +// SPDX-License-Identifier: GPL-2.0
> >> +/* AF_XDP UMEM as a page pool memory provider
> >> + *
> >> + * Each aligned UMEM chunk is a readable net_iov with one owner at a time:
> >> + *
> >> + *   userspace --FILL--> page pool --release--> reuse --alloc--> page pool
> >> + *       ^                   |
> >> + *       +-----RX ring-------+
> >> + */
> >> +
> >> +#include <net/page_pool/helpers.h>
> >> +#include <net/page_pool/memory_provider.h>
> >> +#include <net/xdp_sock_drv.h>
> >> +
> >> +#include "xsk.h"
> >> +#include "xsk_queue.h"
> >> +
> >> +enum xsk_pp_chunk {
> >> +	XSK_PP_USER,	/* owned by userspace */
> >> +	XSK_PP_POOL,	/* owned by a page pool */
> >> +	XSK_PP_REUSE,	/* on the reuse stack */
> >
> > Can you explain more about that reuse/reuse_cnt? When is the chunk reused?
> > How is it different from FILL->RX path?
> 
> 
>             FILL             RX ring
>   USER ----------> POOL ---------------> USER
>                    |  ^
>    release_netmem  |  |  next refill,
>                    v  |  before FILL
>                   REUSE
> 
> POOL is a chunk in the page pool or on a driver ring. XDP_DROP, copied
> XDP_PASS and XDP_TX recycle it inside the page pool, so it stays POOL.
> FILL->RX is the only way back to userspace: an XSK redirect puts the
> chunk on the RX ring, and the flush (patch 7) marks it USER. Userspace
> posts it to FILL again.
> 
> REUSE is a chunk the page pool lets go of without a packet: when the
> pool is destroyed (queue restart, unbind), or its ring is full on a
> recycle. Userspace already gave it up, and the kernel can't post to
> FILL, so the next refill takes it before FILL. Same job as free_list in
> xsk_buff_pool. A FILL entry for a chunk the kernel holds is dropped.
> 
> I'll put that in the comment at the top of xsk_pp.c.

Yes, that helps a lot, thanks!