* Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote: > On 05/07/2015 10:42 AM, Dan Williams wrote: > > On Thu, May 7, 2015 at 10:36 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote: > >> * Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > >> > >> So is there anything fundamentally wrong about creating struct > >> page backing at mmap() time (and making sure aliased mmaps share > >> struct page arrays)? > > > > Something like "get_user_pages() triggers memory hotplug for > > persistent memory", so they are actual real struct pages? Can we > > do memory hotplug at that granularity? > > We've traditionally limited them to SECTION_SIZE granularity, which > is 128MB IIRC. There are also assumptions in places that you can do > page++ within a MAX_ORDER block if !CONFIG_HOLES_IN_ZONE. I really don't think that's very practical: memory hotplug is slow, it's really not on the same abstraction level as mmap(), and the zone data structures are also fundamentally very coarse: not just because RAM ranges are huge, but also so that the pfn->page transformation stays relatively simple and fast. > But, in all practicality, a lot of those places are in code like the > buddy allocator. If your PTEs all have _PAGE_SPECIAL set and we're > not ever expecting these fake 'struct page's to hit these code > paths, it probably doesn't matter. > > You can probably get away with just allocating PAGE_SIZE worth of > 'struct page' (which is 64) and mapping it in to vmemmap[]. The > worst case is that you'll eat 1 page of space for each outstanding > page of I/O. That's a lot better than 2MB of temporary 'struct > page' space per page of I/O that it would take with a traditional > hotplug operation. So I think the main value of struct page is if everyone on the system sees the same struct page for the same pfn - not just the temporary IO instance. The idea of having very temporary struct page arrays misses the point I think: if struct page is used as essentially an IO sglist then most of the synchronization properties are lost: then we might as well use the real deal in that case and skip the dynamic allocation and use pfns directly and avoid the dynamic allocation overhead. Stable, global page-struct descriptors are a given for real RAM, where we allocate a struct page for every page in nice, large, mostly linear arrays. We'd really need that for pmem too, to get the full power of struct page: and that means allocating them in nice, large, predictable places - such as on the device itself ... It might even be 'scattered' across the device, with 64 byte struct page size we can pack 64 descriptors into a single page, so every 65 pages we could have a page-struct page. Finding a pmem page's struct page would thus involve rounding it modulo 65 and reading that page. The problem with that is fourfold: - that we now turn a very kernel internal API and data structure into an ABI. If struct page grows beyond 64 bytes it's a problem. - on bootup (or device discovery time) we'd have to initialize all the page structs. We could probably do this in a hierarchical way, by dividing continuous pmem ranges into power-of-two groups of blocks, and organizing them like the buddy allocator does. - 1.5% of storage space lost. - will wear-leveling properly migrate these 'hot' pages around? The alternative would be some global interval-rbtree of struct page backed pmem ranges. Beyond the synchronization problems of such a data structure (which looks like a nightmare) I don't think it's even feasible: especially if there's a filesystem on the pmem device then the block allocations could be physically fragmented (and there's no fundamental reason why they couldn't be fragmented), so a continuous mmap() of a file on it will yield wildly fragmented device-pfn ranges, exploding the rbtree. Think 1 million node interval-rbtree with an average depth of 20: cachemiss country for even simple lookups - not to mention the freeing/recycling complexity of unused struct pages to not allow it to grow too large. I might be wrong though about all this :) 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
