Commit 1e40cd38 authored Sep 03, 2005 by Paolo 'Blaisorblade' Giarrusso Committed by Linus Torvalds Sep 05, 2005

[PATCH] uml: fixes performance regression in activate_mm and thus exec()



Normally, activate_mm() is called from exec(), and thus it used to be a
no-op because we use a completely new "MM context" on the host (for
instance, a new process), and so we didn't need to flush any "TLB entries"
(which for us are the set of memory mappings for the host process from the
virtual "RAM" file).

Kernel threads, instead, are usually handled in a different way.  So, when
for AIO we call use_mm(), things used to break and so Benjamin implemented
activate_mm().  However, that is only needed for AIO, and could slow down
exec() inside UML, so be smart: detect being called for AIO (via
PF_BORROWED_MM) and do the full flush only in that situation.

Comment also the caller so that people won't go breaking UML without
noticing.  I also rely on the caller's locks for testing current->flags.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
CC: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent ed1b58d8

fs/aio.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -567,6 +567,10 @@ static void use_mm(struct mm_struct *mm)
		atomic_inc(&mm->mm_count);
		tsk->mm = mm;
		tsk->active_mm = mm;
		/*
		* Note that on UML this requires PF_BORROWED_MM to be set, otherwise
		* it won't work. Update it accordingly if you change it here
		*/
		activate_mm(active_mm, mm);
		task_unlock(tsk);

include/asm-um/mmu_context.h

+9 −1

Original line number	Diff line number	Diff line
		@@ -20,7 +20,15 @@ extern void force_flush_all(void);

		static inline void activate_mm(struct mm_struct old, struct mm_struct new)
		{
		if (old != new)
		/*
		* This is called by fs/exec.c and fs/aio.c. In the first case, for an
		* exec, we don't need to do anything as we're called from userspace
		* and thus going to use a new host PID. In the second, we're called
		* from a kernel thread, and thus need to go doing the mmap's on the
		* host. Since they're very expensive, we want to avoid that as far as
		* possible.
		*/
		if (old != new && (current->flags & PF_BORROWED_MM))
		force_flush_all();
		}

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