On Mon, Oct 12, 2020 at 05:44:38PM +0100, Matthew Wilcox wrote:
On Mon, Oct 12, 2020 at 09:28:29AM -0700, Dave Hansen wrote:
kmap_atomic() is always preferred over kmap()/kmap_thread(). kmap_atomic() is _much_ more lightweight since its TLB invalidation is always CPU-local and never broadcast.
So, basically, unless you *must* sleep while the mapping is in place, kmap_atomic() is preferred.
But kmap_atomic() disables preemption, so the _ideal_ interface would map it only locally, then on preemption make it global. I don't even know if that _can_ be done. But this email makes it seem like kmap_atomic() has no downsides.
And that is IIUC what Thomas was trying to solve.
Also, Linus brought up that kmap_atomic() has quirks in nesting.[1]
From what I can see all of these discussions support the need to have something
between kmap() and kmap_atomic().
However, the reason behind converting call sites to kmap_thread() are different between Thomas' patch set and mine. Both require more kmap granularity. However, they do so with different reasons and underlying implementations but with the _same_ resulting semantics; a thread local mapping which is preemptable.[2] Therefore they each focus on changing different call sites.
While this patch set is huge I think it serves a valuable purpose to identify a large number of call sites which are candidates for this new semantic.
Ira
[1] https://lore.kernel.org/lkml/CAHk-=wgbmwsTOKs23Z=71EBTrULoeaH2U3TNqT2atHEWvk... [2] It is important to note these implementations are not incompatible with each other. So I don't see yet another 'kmap_something()' being required.