Re: [PATCH 10/16] fuse: Implement writepages callback

[Maxim Patlasov <mpatlasov@parallels.com>]

09/03/2013 02:31 PM, Miklos Szeredi D?D,N?DuN?:
> On Fri, Aug 30, 2013 at 06:50:18PM +0400, Maxim Patlasov wrote:
>> Hi Miklos,
>>
>> 08/30/2013 02:12 PM, Miklos Szeredi D?D,N?DuN?:
>>> On Fri, Aug 09, 2013 at 07:02:12PM +0400, Maxim Patlasov wrote:
>>>> 08/06/2013 08:25 PM, Miklos Szeredi D?D,N?DuN?:
>>>>> Hmm.  Direct IO on an mmaped file will do get_user_pages() which will
>>>>> do the necessary page fault magic and ->page_mkwrite() will be called.
>>>>> At least AFAICS.
>>>> Yes, I agree.
>>>>
>>>>> The page cannot become dirty through a memory mapping without first
>>>>> switching the pte from read-only to read-write first.  Page accounting
>>>>> logic relies on this too.  The other way the page can become dirty is
>>>>> through write(2) on the fs.  But we do get notified about that too.
>>>> Yes, that's correct, but I don't understand why you disregard two
>>>> other cases of marking page dirty (both related to direct AIO read
>>> >from a file to a memory region mmap-ed to a fuse file):
>>>> 1. dio_bio_submit() -->
>>>>        bio_set_pages_dirty() -->
>>>>          set_page_dirty_lock()
>>>>
>>>> 2. dio_bio_complete() -->
>>>>        bio_check_pages_dirty() -->
>>>>           bio_dirty_fn() -->
>>>>              bio_set_pages_dirty() -->
>>>>                 set_page_dirty_lock()
>>>>
>>>> As soon as a page became dirty through a memory mapping (exactly as
>>>> you explained), nothing would prevent it to be written-back. And
>>>> fuse will call end_page_writeback almost immediately after copying
>>>> the real page to a temporary one. Then dio_bio_submit may re-dirty
>>>> page speculatively w/o notifying fuse. And again, since then nothing
>>>> would prevent it to be written-back once more. Hence we can end up
>>>> in more then one temporary page in fuse write-back. And similar
>>>> concern for dio_bio_complete() re-dirty.
>>>>
>>>> This make me think that we do need fuse_page_is_writeback() in
>>>> fuse_writepages_fill(). But it shouldn't be harmful because it will
>>>> no-op practically always due to waiting for fuse writeback in
>>>> ->page_mkwrite() and in course of handling write(2).
>>> The problem is: if we need it in ->writepages, we need it in ->writepage too.
>>> And that's where we can't have it because it would deadlock in reclaim.
>> I thought we're protected from the deadlock by the following chunk
>> (in the very beginning of fuse_writepage):
>>
>>> +	if (fuse_page_is_writeback(inode, page->index)) {
>>> +		if (wbc->sync_mode != WB_SYNC_ALL) {
>>> +			redirty_page_for_writepage(wbc, page);
>>> +			return 0;
>>> +		}
>>> +		fuse_wait_on_page_writeback(inode, page->index);
>>> +	}
>> Because reclaimer will never call us with WB_SYNC_ALL. Did I miss
>> something?
> Yeah, we could have that in ->writepage() too.  And apparently that would work,
> reclaim would just leave us alone.
>
> Then there's sync(2) which does do WB_SYNC_ALL. Yet for an unprivileged fuse
> mount we don't want ->writepages() to block because that's a quite clear DoS
> issue.

Yes, I agree, but those cases (when sync(2) coincides with a page under 
fuse writeback originated by flusher coinciding with those direct AIO 
read redirty) should be very rare. I'd suggest to go on and put up with 
it for now: unprivileged users won't be able to use writeback_cache 
option until sysad enables allow_wbcache in fusermount.

>
> So we are left with this:

Yes. May we implement it as a separate fix after inclusion of this 
patch-set?

>
>>> There's a way to work around this:
>>>
>>>     - if the request is still in queue, just update it with the contents of
>>>       the new page
>>>
>>>     - if the request already in userspace, create a new reqest, but only let
>>>       userspace have it once the previous request for the same page
>>>       completes, so the ordering is not messed up
>>>
>>> But that's a lot of hairy code.
>> Is it exactly how NFS solves similar problem?
> NFS will apparently just block if there's a request outstanding and we are in
> WB_SYNC_ALL mode.  Which is somewhat simpler.

Yes, indeed.

Thanks,
Maxim

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