Messages in this thread | | | Date | Tue, 11 Jan 2022 00:17:18 -0800 | Subject | Re: Phyr Starter | From | John Hubbard <> |
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On 1/10/22 11:34, Matthew Wilcox wrote: > TLDR: I want to introduce a new data type: > > struct phyr { > phys_addr_t addr; > size_t len; > }; > > and use it to replace bio_vec as well as using it to replace the array > of struct pages used by get_user_pages() and friends. > > ---
This would certainly solve quite a few problems at once. Very compelling.
Zooming in on the pinning aspect for a moment: last time I attempted to convert O_DIRECT callers from gup to pup, I recall wanting very much to record, in each bio_vec, whether these pages were acquired via FOLL_PIN, or some non-FOLL_PIN method. Because at the end of the IO, it is not easy to disentangle which pages require put_page() and which require unpin_user_page*().
And changing the bio_vec for *that* purpose was not really acceptable.
But now that you're looking to change it in a big way (and with some spare bits avaiable...oohh!), maybe I can go that direction after all.
Or, are you looking at a design in which any phyr is implicitly FOLL_PIN'd if it exists at all?
Or any other thoughts in this area are very welcome.
> > There are two distinct problems I want to address: doing I/O to memory > which does not have a struct page and efficiently doing I/O to large > blobs of physically contiguous memory, regardless of whether it has a > struct page. There are some other improvements which I regard as minor. > > There are many types of memory that one might want to do I/O to that do > not have a struct page, some examples: > - Memory on a graphics card (or other PCI card, but gfx seems to be > the primary provider of DRAM on the PCI bus today) > - DAX, or other pmem (there are some fake pages today, but this is > mostly a workaround for the IO problem today) > - Guest memory being accessed from the hypervisor (KVM needs to > create structpages to make this happen. Xen doesn't ...) > All of these kinds of memories can be addressed by the CPU and so also > by a bus master. That is, there is a physical address that the CPU > can use which will address this memory, and there is a way to convert > that to a DMA address which can be programmed into another device. > There's no intent here to support memory which can be accessed by a > complex scheme like writing an address to a control register and then > accessing the memory through a FIFO; this is for memory which can be > accessed by DMA and CPU loads and stores. > > For get_user_pages() and friends, we currently fill an array of struct > pages, each one representing PAGE_SIZE bytes. For an application that > is using 1GB hugepages, writing 2^18 entries is a significant overhead. > It also makes drivers hard to write as they have to recoalesce the > struct pages, even though the VM can tell it whether those 2^18 pages > are contiguous. > > On the minor side, struct phyr can represent any mappable chunk of memory. > A bio_vec is limited to 2^32 bytes, while on 64-bit machines a phyr > can represent larger than 4GB. A phyr is the same size as a bio_vec > on 64 bit (16 bytes), and the same size for 32-bit with PAE (12 bytes). > It is smaller for 32-bit machines without PAE (8 bytes instead of 12). > > Finally, it may be possible to stop using scatterlist to describe the > input to the DMA-mapping operation. We may be able to get struct > scatterlist down to just dma_address and dma_length, with chaining > handled through an enclosing struct. > > I would like to see phyr replace bio_vec everywhere it's currently used. > I don't have time to do that work now because I'm busy with folios. > If someone else wants to take that on, I shall cheer from the sidelines.
I'm starting to wonder if I should jump in here, in order to get this as a way to make the O_DIRECT conversion much cleaner. But let's see.
> What I do intend to do is: > > - Add an interface to gup.c to pin/unpin N phyrs > - Add a sg_map_phyrs() > This will take an array of phyrs and allocate an sg for them > - Whatever else I need to do to make one RDMA driver happy with > this scheme > > At that point, I intend to stop and let others more familiar with this > area of the kernel continue the conversion of drivers. > > P.S. If you've had the Prodigy song running through your head the whole > time you've been reading this email ... I'm sorry / You're welcome. > If people insist, we can rename this to phys_range or something boring, > but I quite like the spelling of phyr with the pronunciation of "fire".
A more conservative or traditional name might look like:
phys_vec (maintains some resemblance to what it's replacing) phys_range phys_addr_range
phyr is rather cool, but it also is awfully too close to "phys" for reading comfort. And there is a lot to be said for self-descriptive names, which phyr is not.
And of course, you're signing for another huge naming debate with Linus if you go with the "cool" name here. :)
thanks, -- John Hubbard NVIDIA
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