On Thu, May 7, 2015 at 7:42 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote: > > * 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. I'm wrapped around the idea that __pfn_t *is* that simple api for the tiered storage driver use case. For RDMA I think we need struct page because I assume that would be coordinated through a filesystem an truncate() is back in play. What does an alternative API look like? > 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. Yes, passing around mmap()'d (DAX) persistent memory will need more than a __pfn_t. -- 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
