Messages in this thread | | | Date | Fri, 30 Oct 2020 13:06:27 +0000 | From | Matthew Wilcox <> | Subject | Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends |
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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.
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