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kmap_atomic() disables preemption and pagefaults for historical reasons.
The conversion to kmap_local(), which only disables migration, cannot be
done wholesale because quite some call sites need to be updated to
accommodate with the changed semantics.
On PREEMPT_RT enabled kernels the kmap_atomic() semantics are problematic
due to the implicit disabling of preemption which makes it impossible to
acquire 'sleeping' spinlocks within the kmap atomic sections.
PREEMPT_RT replaces the preempt_disable() with a migrate_disable() for
more than a decade. It could be argued that this is a justification to do
this unconditionally, but PREEMPT_RT covers only a limited number of
architectures and it disables some functionality which limits the coverage
further.
Limit the replacement to PREEMPT_RT for now.
Link: https://lkml.kernel.org/r/20210810091116.pocdmaatdcogvdso@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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adjust_managed_page_count() as called by free_reserved_page() properly
handles pages in a highmem zone, so we can reuse it for
free_highmem_page().
We can now get rid of totalhigh_pages_inc() and simplify
free_reserved_page().
Link: https://lkml.kernel.org/r/20210126182113.19892-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Now that the kmap atomic index is stored in task struct provide a
preemptible variant. On context switch the maps of an outgoing task are
removed and the map of the incoming task are restored. That's obviously
slow, but highmem is slow anyway.
The kmap_local.*() functions can be invoked from both preemptible and
atomic context. kmap local sections disable migration to keep the resulting
virtual mapping address correct, but disable neither pagefaults nor
preemption.
A wholesale conversion of kmap_atomic to be fully preemptible is not
possible because some of the usage sites might rely on the preemption
disable for serialization or on the implicit pagefault disable. Needs to be
done on a case by case basis.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201118204007.468533059@linutronix.de
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Instead of storing the map per CPU provide and use per task storage. That
prepares for local kmaps which are preemptible.
The context switch code is preparatory and not yet in use because
kmap_atomic() runs with preemption disabled. Will be made usable in the
next step.
The context switch logic is safe even when an interrupt happens after
clearing or before restoring the kmaps. The kmap index in task struct is
not modified so any nesting kmap in an interrupt will use unused indices
and on return the counter is the same as before.
Also add an assert into the return to user space code. Going back to user
space with an active kmap local is a nono.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201118204007.372935758@linutronix.de
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Move the gory details of kmap & al into a private header and only document
the interfaces which are usable by drivers.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201103095858.827582066@linutronix.de
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