Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends | Date | Fri, 30 Oct 2020 20:35:18 +0100 |
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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
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