[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>

diff --git a/fs/aio.c b/fs/aio.c
index 06d7d4390fe7dd8562e8433ce3a29b2b9ee4e241..4f641abac3c0927f2036f81f3f70106a57a09e22 100644 (file)
--- 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);
 

diff --git a/include/asm-um/mmu_context.h b/include/asm-um/mmu_context.h
index 095bb627b96a1d52be4cc1291f48c308449f65a1..2edb4f1f789cdb96e0208da409622db6f1eaa70b 100644 (file)
--- a/include/asm-um/mmu_context.h
+++ b/include/asm-um/mmu_context.h
@@ -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();
 }