Message-ID: <20181129120253.GR6311@dastard>
Date:   Thu, 29 Nov 2018 23:02:53 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Liu Bo <bo.liu@...ux.alibaba.com>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH RFC] Ext4: fix deadlock on dirty pages between fault and
 writeback

On Thu, Nov 29, 2018 at 09:52:38AM +0100, Jan Kara wrote:
> On Wed 28-11-18 12:11:23, Liu Bo wrote:
> > On Tue, Nov 27, 2018 at 12:42:49PM +0100, Jan Kara wrote:
> > > CCed fsdevel since this may be interesting to other filesystem developers
> > > as well.
> > > 
> > > On Tue 30-10-18 08:22:49, Liu Bo wrote:
> > > > mpage_prepare_extent_to_map() tries to build up a large bio to stuff down
> > > > the pipe.  But if it needs to wait for a page lock, it needs to make sure
> > > > and send down any pending writes so we don't deadlock with anyone who has
> > > > the page lock and is waiting for writeback of things inside the bio.
> > > 
> > > Thanks for report! I agree the current code has a deadlock possibility you
> > > describe. But I think the problem reaches a bit further than what your
> > > patch fixes.  The problem is with pages that are unlocked but have
> > > PageWriteback set.  Page reclaim may end up waiting for these pages and
> > > thus any memory allocation with __GFP_FS set can block on these. So in our
> > > current setting page writeback must not block on anything that can be held
> > > while doing memory allocation with __GFP_FS set. Page lock is just one of
> > > these possibilities, wait_on_page_writeback() in
> > > mpage_prepare_extent_to_map() is another suspect and there mat be more. Or
> > > to say it differently, if there's lock A and GFP_KERNEL allocation can
> > > happen under lock A, then A cannot be taken by the writeback path. This is
> > > actually pretty subtle deadlock possibility and our current lockdep
> > > instrumentation isn't going to catch this.
> > >
> > 
> > Thanks for the nice summary, it's true that a lock A held in both
> > writeback path and memory reclaim can end up with deadlock.
> > 
> > Fortunately, by far there're only deadlock reports of page's lock bit
> > and writeback bit in both ext4 and btrfs[1].  I think
> > wait_on_page_writeback() would be OK as it's been protected by page
> > lock.
> > 
> > [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=01d658f2ca3c85c1ffb20b306e30d16197000ce7
> 
> Yes, but that may just mean that the other deadlocks are just harder to
> hit...
> 
> > > So I see two ways how to fix this properly:
> > > 
> > > 1) Change ext4 code to always submit the bio once we have a full page
> > > prepared for writing. This may be relatively simple but has a higher CPU
> > > overhead for bio allocation & freeing (actual IO won't really differ since
> > > the plugging code should take care of merging the submitted bios). XFS
> > > seems to be doing this.
> > 
> > Seems that that's the safest way to do it, but as you said there's
> > some tradeoff.
> > 
> > (Just took a look at xfs's writepages, xfs also did the page
> > collection if there're adjacent pages in xfs_add_to_ioend(), and since
> > xfs_vm_writepages() is using the generic helper write_cache_pages()
> > which calls lock_page() as well, it's still possible to run into the
> > above kind of deadlock.)
> 
> Originally I thought XFS doesn't have this problem but now when I look
> again, you are right that their ioend may accumulate more pages to write
> and so they are prone to the same deadlock ext4 is. Added XFS list to CC.

I don't think XFS has a problem here, because the deadlock is
dependent on holding a lock that writeback might take and then doing
a GFP_KERNEL allocation. I don't think we do that anywhere in XFS -
the only lock that is of concern here is the ip->i_ilock, and I
think we always do GFP_NOFS allocations inside that lock.

As it is, this sort of lock vs reclaim inversion should be caught by
lockdep - allocations and reclaim contexts are recorded by lockdep
we get reports if we do lock A - alloc and then do reclaim - lock A.
We've always had problems with false positives from lockdep for
these situations where common XFS code can be called from GFP_KERNEL
valid contexts as well as reclaim or GFP_NOFS-only contexts, but I
don't recall ever seeing such a report for the writeback path....

> > > 2) Change the code to unlock the page only when we submit the bio.

> > This sounds doable but not good IMO, the concern is that page locks
> > can be held for too long, or if we do 2), submitting one bio per page
> > in 1) is also needed.
> 
> Hum, you're right that page lock hold times may increase noticeably and
> that's not very good. Ideally we'd need a way to submit whatever we have
> prepared when we are going to sleep but there's no easy way to do that.

XFS unlocks the page after it has been added to the bio and marked
as under writeback, not when the bio is submitted. i.e.

writepage w/ locked page dirty
lock ilock
<mapping, allocation>
unlock ilock
bio_add_page
clear_page_dirty_for_io
set_page_writeback
unlock_page
.....
<gather more dirty pages into bio>
.....
<bio is full or discontiguous page to be written>
submit_bio()

If we switch away which holding a partially built bio, the only page
we have locked is the one we are currently trying to add to the bio.
Lock ordering prevents deadlocks on that one page, and all other
pages in the bio being built are marked as under writeback and are
not locked. Hence anything that wants to modify a page held in the
bio will block waiting for page writeback to clear, not the page
lock.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com

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