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KILL_LEVEL=7.0 tests=BSF_BESS_OUTBOUND X-BESS-BRTS-Status:1 On 10/30/25 19:06, Pavel Begunkov wrote: > On 10/29/25 18:37, Joanne Koong wrote: >> On Wed, Oct 29, 2025 at 7:01 AM Pavel Begunkov wrote: >>> >>> On 10/27/25 22:28, Joanne Koong wrote: >>>> Add an API for fetching the registered buffer associated with a >>>> io_uring cmd. This is useful for callers who need access to the buffer >>>> but do not have prior knowledge of the buffer's user address or length. >>> >>> Joanne, is it needed because you don't want to pass {offset,size} >>> via fuse uapi? It's often more convenient to allocate and register >>> one large buffer and let requests to use subchunks. Shouldn't be >>> different for performance, but e.g. if you try to overlay it onto >>> huge pages it'll be severely overaccounted. >>> >> >> Hi Pavel, >> >> Yes, I was thinking this would be a simpler interface than the >> userspace caller having to pass in the uaddr and size on every >> request. Right now the way it is structured is that userspace >> allocates a buffer per request, then registers all those buffers. On >> the kernel side when it fetches the buffer, it'll always fetch the >> whole buffer (eg offset is 0 and size is the full size). >> >> Do you think it is better to allocate one large buffer and have the >> requests use subchunks? > > I think so, but that's general advice, I don't know the fuse > implementation details, and it's not a strong opinion. It'll be great > if you take a look at what other server implementations might want and > do, and if whether this approach is flexible enough, and how amendable > it is if you change it later on. E.g. how many registered buffers it > might need? io_uring caps it at some 1000s. How large buffers are? > Each separate buffer has memory footprint. And because of the same > footprint there might be cache misses as well if there are too many. > Can you always predict the max number of buffers to avoid resizing > the table? Do you ever want to use huge pages while being > restricted by mlock limits? And so on. > > In either case, I don't have a problem with this patch, just > found it a bit off. Maybe we could address that later on, so far I don't like the idea of a single buffer size for all ring entries. Maybe it would make sense to introduce buffer pools of different sizes and let ring entries use a needed buffer size dynamically. The part I'm still not too happy about is the need for fuse server changes - my alternative patch didn't need that at all. Thanks, Bernd