> >> 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?
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