On 10/11/24 1:03 PM, Bernd Schubert wrote: >> >> Curious, what's all the space needed for? > > The basic fuse header: struct fuse_in_header -> current 40B > and per request header headers, I think current max is 64. > > And then some extra compat space for both, so that they can be safely > extended in the future (which is currently an issue). So that's 104b, and regular CQE stuff too I presume, so that's 104+16 == 120 bytes. That'd fit in a 128b CQE, and 256b would be pleeeeenty? Just squeeze a version field or something in there so you know what the version is for future proofing? I would strongly recommend making it as large as you need it for those things, but no longer just for compat/future reasons. Eg 128b over 256b is a win for sure, and 256b over 512b is a REALLY nice win. >>>> Since this obviously means larger CQ rings, one nice side effect is that >>>> since 6.10 we don't need contig pages to map any of the rings. So should >>>> work just fine regardless of memory fragmentation, where previously that >>>> would've been a concern. >>>> >>> >>> Out of interest, what is the change? Up to fuse-io-uring rfc2 I was >>> vmalloced buffers for fuse that got mmaped - was working fine. Miklos just >>> wants to avoid that kernel allocates large chunks of memory on behalf of >>> users. >> >> It was the change that got rid of remap_pfn_range() for mapping, and >> switched to vm_insert_page(s) instead. Memory overhead should generally >> not be too bad, it's all about sizing the rings appropriately. The much >> bigger concern is needing contig memory, as that can become scarce after >> longer uptimes, even with plenty of memory free. This is particularly >> important if you need 512b CQEs, obviously. >> > > For sure, I was just curious what you had changed. I think I had looked into > that io-uring code around 2 years ago. Going to look into the update > io-uring code, thanks for the hint. > For fuse I was just using remap_vmalloc_range(). > > https://lore.kernel.org/all/20240529-fuse-uring-for-6-9-rfc2-out-v1-7-d149476b1d65@xxxxxxx/ That's the one to use, io_uring was just stuck with using the wrong API for quite a while, but that got sorted. -- Jens Axboe
