Re: [PATCH v7 07/22] Replace the XIP page fault handler with the DAX page fault handler

[Matthew Wilcox]

On Mon, Aug 11, 2014 at 04:35:00PM +0200, Jan Kara wrote:
> On Mon 11-08-14 10:13:08, Matthew Wilcox wrote:
> > On Mon, Aug 11, 2014 at 10:51:47AM +0200, Jan Kara wrote:
> > > So I'm afraid we'll have to find some other way to synchronize
> > > page faults and truncate / punch hole in DAX.
> > 
> > What if we don't?  If we hit the race (which is vanishingly unlikely with
> > real applications), the consequence is simply that after a truncate, a
> > file may be left with one or two blocks allocated somewhere after i_size.
> > As I understand it, that's not a real problem; they're temporarily
> > unavailable for allocation but will be freed on file removal or the next
> > truncation of that file.
>   You mean if you won't have any locking between page fault and truncate?
> You can have:
> a) extending truncate making forgotten blocks with non-zeros visible
> b) filesystem corruption due to doubly used blocks (block will be freed
> from the truncated file and thus can be reallocated but it will still be
> accessible via mmap from the truncated file).
> 
>   So not a good idea.

Not *no* locking ... just no locking around get_block, like in v7.
So check i_size, call get_block, lock i_mmap_mutex, re-check i_size,
insert mapping if i_size is OK, drop i_mmap_mutex.  As long as get_block()
has enough locking of its own against set_size and concurrent calls
to get_block(), I don't think we can get visible non-zeroes or double
allocation.

> > I'm also still considering the possibility of having truncate-down block
> > until all mmaps that extend after the new i_size have been removed ...
>   Hum, I'm not sure how you would do that with current locking scheme and
> wait for all page faults on that range to finish but maybe you have some
> good idea :)

While it can be blocked with i_dio_count currently, this would be a more
complicated thing to do ...
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