* Ingo Molnar <mingo@xxxxxxxxxx> wrote: > [...] > > For anything more complex, that maps any of this storage to > user-space, or exposes it to higher level struct page based APIs, > etc., where references matter and it's more of a cache with > potentially multiple users, not an IO space, the natural API is > struct page. Let me walk back on this: > I'd say that this particular series mostly addresses the 'pfn as > sector_t' side of the equation, where persistent memory is IO space, > not memory space, and as such it is the more natural and thus also > the cheaper/faster approach. ... but that does not appear to be the case: this series replaces a 'struct page' interface with a pure pfn interface for the express purpose of being able to DMA to/from 'memory areas' that are not struct page backed. > Linus probably disagrees? :-) [ and he'd disagree rightfully ;-) ] So what this patch set tries to achieve is (sector_t -> sector_t) IO between storage devices (i.e. a rare and somewhat weird usecase), and does it by squeezing one device's storage address into our formerly struct page backed descriptor, via a pfn. That looks like a layering violation and a mistake to me. If we want to do direct (sector_t -> sector_t) IO, with no serialization worries, it should have its own (simple) API - which things like hierarchical RAID or RDMA APIs could use. If what we want to do is to support say an mmap() of a file on persistent storage, and then read() into that file from another device via DMA, then I think we should have allocated struct page backing at mmap() time already, and all regular syscall APIs would 'just work' from that point on - far above what page-less, pfn-based APIs can do. The temporary struct page backing can then be freed at munmap() time. And if the usage is pure fd based, we don't really have fd-to-fd APIs beyond the rarely used splice variants (and even those don't do pure cross-IO, they use a pipe as an intermediary), so there's no problem to solve I suspect. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
