Re: [PATCH, RFC] check for frozen filesystems in the mmap path

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> >> Index: linux-2.6/mm/memory.c
> >> ===================================================================
> >> --- linux-2.6.orig/mm/memory.c
> >> +++ linux-2.6/mm/memory.c
> >> @@ -1944,6 +1944,7 @@ static int do_wp_page(struct mm_struct *
> >>  		 * read-only shared pages can get COWed by
> >>  		 * get_user_pages(.write=1, .force=1).
> >>  		 */
> >> +		vfs_check_frozen(old_page->mapping->host->i_sb, SB_FREEZE_WRITE);
> >>  		if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
> >>  			struct vm_fault vmf;
> >>  			int tmp;
> > 
> > it seems strage.
> > 
> > 1. it seems to have a race
> > 
> > 	CPU0				CPU1
> > 	----------------------------------------------------
> > 	do_wp_page
> > 	 vfs_check_frozen
> > 					ioctl_fsfreeze
> > 					  freeze_bdev
> > 					    __fsync_super
> >         process touch mem
> > 
> > vfs_check_frozen only wait to unfreeze, but not prevent new
> > new freeze request starting.
> 
> Well, I think that is ok.  I don't *think* that any IO can actually
> happen to the filesystem even if it gets dirtied via mmap, so if a bit
> of mmap-dirtied memory sneaks in before it's actually frozen, I'm not
> sure that's really a problem.  The goal was to prevent massive amounts
> of memory from getting dirtied, backed by the frozen filesystem.  This
> would potentially lead to a situation where the un-freezing thread was
> stuck waiting for memory to free up, stuck behind waiting for the
> filesystem to unfreeze for writeout, and we can't unfreeze.

Ah, I see.
one another question.

Why dirty limit don't works properly in this case?



> > 2. this logic kill multi thread application.
> > 
> > this logic mean mmap_sem grabbing until unfreeze.
> > it mean othrer thread in the same process can't page-fault although
> > it don't touch frozen-sb.
> > it seems strange.
> 
> Hm, I hadn't thought about this ... On the one hand, ->page_mkwrite can
> already sleep, though a userspace freeze/unfreeze could potentially take
> much much longer.  freeze/unfreeze *should* happen very quickly, but
> nothing enforces that.
> 
> Do you have any suggestions?