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

[Eric Sandeen <sandeen@redhat.com>]

KOSAKI Motohiro wrote:
> 
>> 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.

> 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?

Thanks,
-Eric