On Wed, Jan 3, 2018 at 2:04 AM, Jan Kara <jack@xxxxxxx> wrote:
> Hello,
>
> Over the years I have seen so far unexplained crashed in filesystem's
> (ext4, xfs) writeback path due to dirty pages without buffers attached to
> them (see [1] and [2] for relatively recent reports). This was confusing as
> reclaim takes care not to strip buffers from a dirty page and both
> filesystems do add buffers to a page when it is first written to - in
> ->page_mkwrite() and ->write_begin callbacks.
>
> Recently I have come across a code path that is probably leading to this
> inconsistent state and I'd like to discuss how to best fix the problem
> because it's not obvious to me. Consider the following race:
>
> CPU1 CPU2
>
> addr = mmap(file1, MAP_SHARED, ...);
> fd2 = open(file2, O_DIRECT | O_RDONLY);
> read(fd2, addr, len)
> do_direct_IO()
> page = dio_get_page()
> dio_refill_pages()
> iov_iter_get_pages()
> get_user_pages_fast()
> - page fault
> ->page_mkwrite()
> block_page_mkwrite()
> lock_page(page);
> - attaches buffers to page
> - makes sure blocks are allocated
> set_page_dirty(page)
> - install writeable PTE
> unlock_page(page);
> submit_page_section(page)
> - submits bio with 'page' as a buffer
> kswapd reclaims pages:
> ...
> shrink_page_list()
> trylock_page(page) - this is the
> page CPU1 has just faulted in
> try_to_unmap(page)
> pageout(page);
> clear_page_dirty_for_io(page);
> ->writepage()
> - let's assume page got written
> out fast enough, alternatively
> we could get to the same path as
> soon as the page IO completes
> if (page_has_private(page)) {
> try_to_release_page(page)
> - reclaims buffers from the
> page
> __remove_mapping(page)
> - fails as DIO code still
> holds page reference
> ...
>
> eventually read completes
> dio_bio_complete(bio)
> set_page_dirty_lock(page)
> Bummer, we've just marked the page as dirty without having buffers.
> Eventually writeback will find it and filesystem will complain...
>
> Am I missing something?
>
> The problem here is that filesystems fundamentally assume that a page can
> be written to only between ->write_begin - ->write_end (in this interval
> the page is locked), or between ->page_mkwrite - ->writepage and above is
> an example where this does not hold because when a page reference is
> acquired through get_user_pages(), page can get written to by the holder of
> the reference and dirtied even after it has been unmapped from page tables
> and ->writepage has been called. This is not only a cosmetic issue leading
> to assertion failure but it can also lead to data loss, data corruption, or
> other unpleasant surprises as filesystems assume page contents cannot be
> modified until either ->write_begin() or ->page_mkwrite gets called and
> those calls are serialized by proper locking with problematic operations
> such as hole punching etc.
>
> I'm not sure how to fix this problem. We could 'simulate' a writeable page
> fault in set_page_dirty_lock(). It is a bit ugly since we don't have a
> virtual address of the fault, don't hold mmap_sem, etc., possibly
> expensive, but it would make filesystems happy. Data stored by GUP user
> (e.g. read by DIO in the above case) could still get lost if someone e.g.
> punched hole under the buffer or otherwise messed with the underlying
> storage of the page while DIO was running but arguably users could expect
> such outcome.
>
> Another possible solution would be to make sure page is writeably mapped
> until GUP user drops its reference. That would be arguably cleaner but
> probably that would mean we have to track number of writeable GUP page
> references separately (no space space in struct page is a problem here) and
> block page_mkclean() until they are dropped. Also for long term GUP users
> like Infiniband or V4L we'd have to come up with some solution as we should
> not block page_mkclean() for so long.
Do we need to block page_mkclean, or could we defer buffer reclaiming
to the last put of the page?
I think once we have the "register memory with lease" mechanism for
Infiniband we could expand it to the page cache case. The problem is
the regression this would cause with userspace that expects it can
maintain file backed memory registrations indefinitely.
What are the implications of holding off page_mkclean or release
buffers indefinitely?
Is an indefinite / interruptible sleep waiting for the 'put' event of
a get_user_pages() page unacceptable? The current case that the file
contents will not be coherent with respect to in-flight RDMA, perhaps
waiting for that to complete is better than cleaning buffers from the
page prematurely.
Yes, I have more questions than proposals.
>
> As a side note DAX needs some solution for GUP users as well. The problems
> are similar there in nature, just much easier to hit. So at least a
> solution for long-term GUP users can (and I strongly believe should) be
> shared between standard and DAX paths.
In the DAX case we rely on the fact that when the page goes idle we
only need to worry about the filesytem block map changing, the page
won't get reallocated somewhere else. We can't use page idle as an
event in this case, however, if the page reference count is one then
the DIO code can know that it has the page exclusively, so maybe DAX
and non-DAX can share the page count == 1 event notification.
However there's still the matter of how to callback into the
filesystem. The DAX case is currently using a pgmap_radix lookup at
put_page() time to determine when to wakeup waiters. I think this
should move over to a new address_space_operation. That would help
with reusing some of the DAX case machinery for this case, but the DIO
code would need a special case put_page that checks for count == 1 and
synchronization to hold off the DIO submission path to prevent new
page elevations.
...are you sure this is still similar enough to the DAX case that they
can reuse much of the same machinery?
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