On Tue, May 08, 2018 at 07:51:01PM +0900, Minchan Kim wrote: > On Mon, May 07, 2018 at 10:28:29AM -0700, Joel Fernandes wrote: > > On Mon, May 07, 2018 at 11:15:37PM +0900, Minchan Kim wrote: > > > binder_update_page_range needs down_write of mmap_sem because > > > vm_insert_page need to change vma->vm_flags to VM_MIXEDMAP unless > > > it is set. However, when I profile binder working, it seems > > > every binder buffers should be mapped in advance by binder_mmap. > > > It means we could set VM_MIXEDMAP in binder_mmap time which is > > > already hold a mmap_sem as down_write so binder_update_page_range > > > doesn't need to hold a mmap_sem as down_write. > > > Please use proper API down_read. It would help mmap_sem contention > > > problem as well as fixing down_write abuse. > > > > > > Ganesh Mahendran tested app launching and binder throughput test > > > and he said he couldn't find any problem and I did binder latency > > > test per Greg KH request(Thanks Martijn to teach me how I can do) > > > I cannot find any problem, too. > > > > > > Cc: Ganesh Mahendran <[email protected]> > > > Cc: Joe Perches <[email protected]> > > > Cc: Arve Hjønnevåg <[email protected]> > > > Cc: Todd Kjos <[email protected]> > > > Cc: Greg Kroah-Hartman <[email protected]> > > > Reviewed-by: Martijn Coenen <[email protected]> > > > Signed-off-by: Minchan Kim <[email protected]> > > > --- > > > drivers/android/binder.c | 4 +++- > > > drivers/android/binder_alloc.c | 6 +++--- > > > 2 files changed, 6 insertions(+), 4 deletions(-) > > > > > > diff --git a/drivers/android/binder.c b/drivers/android/binder.c > > > index 4eab5be3d00f..7b8e96f60719 100644 > > > --- a/drivers/android/binder.c > > > +++ b/drivers/android/binder.c > > > @@ -4730,7 +4730,9 @@ static int binder_mmap(struct file *filp, struct > > > vm_area_struct *vma) > > > failure_string = "bad vm_flags"; > > > goto err_bad_arg; > > > } > > > - vma->vm_flags = (vma->vm_flags | VM_DONTCOPY) & ~VM_MAYWRITE; > > > + vma->vm_flags |= VM_DONTCOPY | VM_MIXEDMAP; > > > + vma->vm_flags &= ~VM_MAYWRITE; > > > + > > > vma->vm_ops = &binder_vm_ops; > > > vma->vm_private_data = proc; > > > > > > diff --git a/drivers/android/binder_alloc.c > > > b/drivers/android/binder_alloc.c > > > index 5a426c877dfb..4f382d51def1 100644 > > > --- a/drivers/android/binder_alloc.c > > > +++ b/drivers/android/binder_alloc.c > > > @@ -219,7 +219,7 @@ static int binder_update_page_range(struct > > > binder_alloc *alloc, int allocate, > > > mm = alloc->vma_vm_mm; > > > > > > if (mm) { > > > - down_write(&mm->mmap_sem); > > > + down_read(&mm->mmap_sem); > > > > > > Nice. Is there a need to hold the reader-lock at all here? Just curious what > > else is it protecting (here or in vm_insert_page). > > It should protect vm_area_struct. IOW, when we try insert page into virtual > address area, > vma shouldn't be changed(ie, unmap/collapse/split).
When you say unmap, are you talking about pages being unmapped from the VMA or something else? For the collapse/split part, the binder VMA (vm_area_struct) itself isn't changed after the initial mmap (AFAIK) so I don't see a need for protection there. Again, I'm not against the patch but thought its good to clarify for myself what the usage of the lock is, here. thanks, - Joel
