[Intel-wired-lan] [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap_thread()
ira.weiny at intel.com
Mon Oct 12 19:53:54 UTC 2020
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.
>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. 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.
 It is important to note these implementations are not incompatible with
each other. So I don't see yet another 'kmap_something()' being required.
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