Re: Phyr Starter

On 2022-01-11 1:17 a.m., John Hubbard wrote:

On 1/10/22 11:34, Matthew Wilcox wrote:

quoted

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.

I agree.

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?

I'd also second being able to store a handful of flags in each phyr. My
userspace P2PDMA patchset needs to add a flag to each sgl to indicate
whether it was mapped as a bus address or not (which would be necessary
for the DMA mapped side dma_map_phyr).

Though, it's not immediately obvious where to put the flags without
increasing the size of the structure :(

Logan