Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends

[Matthew Wilcox <willy@infradead.org>]

On Thu, Oct 29, 2020 at 11:18:06PM +0100, Thomas Gleixner wrote:
> This series provides kmap_local.* iomap_local variants which only disable
> migration to keep the virtual mapping address stable accross preemption,
> but do neither disable pagefaults nor preemption. The new functions can be
> used in any context, but if used in atomic context the caller has to take
> care of eventually disabling pagefaults.

Could I ask for a CONFIG_KMAP_DEBUG which aliases all the kmap variants
to vmap()?  I think we currently have a problem in iov_iter on HIGHMEM
configs:

copy_page_to_iter() calls page_copy_sane() which checks:

        head = compound_head(page);
        if (likely(n <= v && v <= page_size(head)))
                return true;

but then:

                void *kaddr = kmap_atomic(page);
                size_t wanted = copy_to_iter(kaddr + offset, bytes, i);
                kunmap_atomic(kaddr);

so if offset to offset+bytes is larger than PAGE_SIZE, this is going to
work for lowmem pages and fail miserably for highmem pages.  I suggest
vmap() because vmap has a PAGE_SIZE gap between each allocation.

Alternatively if we could have a kmap_atomic_compound(), that would
be awesome, but probably not realistic to implement.  I've more
or less resigned myself to having to map things one page at a time.