On Tue, May 7, 2024 at 9:24 AM Christoph Hellwig hch@infradead.org wrote:
On Tue, May 07, 2024 at 01:18:57PM -0300, Jason Gunthorpe wrote:
On Tue, May 07, 2024 at 05:05:12PM +0100, Pavel Begunkov wrote:
even in tree if you give them enough rope, and they should not have that rope when the only sensible options are page/folio based kernel memory (incuding large/huge folios) and dmabuf.
I believe there is at least one deep confusion here, considering you previously mentioned Keith's pre-mapping patches. The "hooks" are not that about in what format you pass memory, it's arguably the least interesting part for page pool, more or less it'd circulate whatever is given. It's more of how to have a better control over buffer lifetime and implement a buffer pool passing data to users and empty buffers back.
Isn't that more or less exactly what dmabuf is? Why do you need another almost dma-buf thing for another project?
That's the exact point I've been making since the last round of the series. We don't need to reinvent dmabuf poorly in every subsystem, but instead fix the odd parts in it and make it suitable for everyone.
FWIW the change Christoph is requesting is straight forward from my POV and doesn't really hurt the devmem use case. I'd basically remove the ops and add an if statement in the slow path where the ops are being used to alloc/free from dmabuf instead of alloc_pages(). Something like (very rough, doesn't compile):
diff --git a/net/core/page_pool.c b/net/core/page_pool.c index 92be1aaf18ccc..2cc986455bce6 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c @@ -557,8 +557,8 @@ netmem_ref page_pool_alloc_netmem(struct page_pool *pool, gfp_t gfp) return netmem;
/* Slow-path: cache empty, do real allocation */ - if (static_branch_unlikely(&page_pool_mem_providers) && pool->mp_ops) - netmem = pool->mp_ops->alloc_pages(pool, gfp); + if (page_pool_is_dmabuf(pool)) + netmem = mp_dmabuf_devmem_alloc_pages(): else netmem = __page_pool_alloc_pages_slow(pool, gfp); return netmem;
The folks that will be negatively impacted by this are Jakub/Pavel/David. I think all were planning to extend the hooks for io_uring or other memory types.
Pavel/David, AFAICT you have these options here (but maybe you can think of more):
1. Align with devmem TCP to use udmabuf for your io_uring memory. I think in the past you said it's a uapi you don't link but in the face of this pushback you may want to reconsider.
2. Follow the example of devmem TCP and add another if statement to alloc from io_uring, so something like:
diff --git a/net/core/page_pool.c b/net/core/page_pool.c index 92be1aaf18ccc..3545bb82c7d05 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c @@ -557,8 +557,10 @@ netmem_ref page_pool_alloc_netmem(struct page_pool *pool, gfp_t gfp) return netmem;
/* Slow-path: cache empty, do real allocation */ - if (static_branch_unlikely(&page_pool_mem_providers) && pool->mp_ops) - netmem = pool->mp_ops->alloc_pages(pool, gfp); + if (page_pool_is_dmabuf(pool)) + netmem = mp_dmabuf_devmem_alloc_pages(): + else if (page_pool_is_io_uring(pool)) + netmem = mp_io_uring_alloc_pages(): else netmem = __page_pool_alloc_pages_slow(pool, gfp); return netmem;
Note that Christoph/Jason may not like you adding non-dmabuf io_uring backing memory in the first place, so there may be pushback against this approach.
3. Pushback on the nack on this thread. It seems you're already discussing this. I'll see what happens.
To be honest the GVE queue-API has just been merged I think, so I'm now unblocked on sending non-RFCs of this work and I'm hoping to send the next version soon. I may apply these changes on the next version for more discussion or leave as is and carry the nack until the conversation converges.