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

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, Oct 30 2020 at 13:06, Matthew Wilcox wrote:
> 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:

For kmap() that would work, but for kmap_atomic() not so much when it is
called in non-preemptible context because vmap() might sleep.

> 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.

On 32bit highmem the kmap_atomic() case is easy: Double the number of
mapping slots and only use every second one, which gives you a guard
page between the maps.

For 64bit we could do something ugly: Enable the highmem kmap_atomic()
crud and enforce an alias mapping (at least on the architectures where
this is reasonable). Then you get the same as for 32bit.

> 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.

That might be horribly awesome on 32bit :)

Thanks,

        tglx