[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>

1 files changed, 4 insertions, 0 deletions

diff --git a/fs/aio.c b/fs/aio.c
index 06d7d4390fe7..4f641abac3c0 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -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);