On Thu, Mar 19, 2015 at 12:59 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Thu, 19 Mar 2015 17:54:15 +0200 Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote: > >> On 03/19/2015 03:43 PM, Matthew Wilcox wrote: >> <> >> > >> > Dan missed "Support O_DIRECT to a mapped DAX file". More generally, if we >> > want to be able to do any kind of I/O directly to persistent memory, >> > and I think we do, we need to do one of: >> > >> > 1. Construct struct pages for persistent memory >> > 1a. Permanently >> > 1b. While the pages are under I/O >> > 2. Teach the I/O layers to deal in PFNs instead of struct pages >> > 3. Replace struct page with some other structure that can represent both >> > DRAM and PMEM >> > >> > I'm personally a fan of #3, and I was looking at the scatterlist as >> > my preferred data structure. I now believe the scatterlist as it is >> > currently defined isn't sufficient, so we probably end up needing a new >> > data structure. I think Dan's preferred method of replacing struct >> > pages with PFNs is actually less instrusive, but doesn't give us as >> > much advantage (an entirely new data structure would let us move to an >> > extent based system at the same time, instead of sticking with an array >> > of pages). Clearly Boaz prefers 1a, which works well enough for the >> > 8GB NV-DIMMs, but not well enough for the 400GB NV-DIMMs. >> > >> > What's your preference? I guess option 0 is "force all I/O to go >> > through the page cache and then get copied", but that feels like a nasty >> > performance hit. >> >> Thanks Matthew, you have summarized it perfectly. >> >> I think #1b might have merit, as well. > > It would be interesting to see what a 1b implementation looks like and > how it performs. We already allocate a bunch of temporary things to > support in-flight IO (bio, request) and allocating pageframes on the > same basis seems a fairly logical fit. At least for block-i/o it seems the only place we really need struct page infrastructure is for kmap(). Given we already need a kmap_pfn() solution for option 2 a "dynamic allocation" stop along that development path may just naturally fall out. -- 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
