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SubjectRe: [PATCH 00/29] RFC: iov_iter: Switch to using an ops table
On Sat, Nov 21, 2020 at 02:13:21PM +0000, David Howells wrote:
> I had a go switching the iov_iter stuff away from using a type bitmask to
> using an ops table to get rid of the if-if-if-if chains that are all over
> the place. After I pushed it, someone pointed me at Pavel's two patches.
>
> I have another iterator class that I want to add - which would lengthen the
> if-if-if-if chains. A lot of the time, there's a conditional clause at the
> beginning of a function that just jumps off to a type-specific handler or
> to reject the operation for that type. An ops table can just point to that
> instead.

So, given the performance problem, how about turning this inside out?

struct iov_step {
union {
void *kaddr;
void __user *uaddr;
};
unsigned int len;
bool user_addr;
bool kmap;
struct page *page;
};

bool iov_iterate(struct iov_step *step, struct iov_iter *i, size_t max)
{
if (step->page)
kunmap(page)
else if (step->kmap)
kunmap_atomic(step->kaddr);

if (max == 0)
return false;

if (i->type & ITER_IOVEC) {
step->user_addr = true;
step->uaddr = i->iov.iov_base + i->iov_offset;
return true;
}
if (i->type & ITER_BVEC) {
... get the page ...
} else if (i->type & ITER_KVEC) {
... get the page ...
} else ...

kmap or kmap_atomic as appropriate ...
...set kaddr & len ...

return true;
}

size_t copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)
{
struct iov_step step = {};

while (iov_iterate(&step, i, bytes)) {
if (user_addr)
copy_from_user(addr, step.uaddr, step.len);
else
memcpy(addr, step.kaddr, step.len);
bytes -= step.len;
}
}

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