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Date:   Thu, 29 Nov 2018 14:00:02 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jan Kara <jack@...e.cz>, 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 29-11-18 23:02:53, Dave Chinner wrote:
> 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....

I think for A == page lock, XFS may have the problem (and lockdep won't
notice because it does not track page locks). There are some parts of
kernel which do GFP_KERNEL allocations under page lock - pte_alloc_one() is
one such function which allocates page tables with GFP_KERNEL and gets
called with the faulted page locked. And I believe there are others.

So direct reclaim from pte_alloc_one() can wait for writeback on page B
while holding lock on page A. And if B is just prepared (added to bio,
under writeback, unlocked) but not submitted in xfs_writepages() and we
block on lock_page(A), we have a deadlock.

Generally deadlocks like these will be invisible to lockdep because it does
not track either PageWriteback or PageLocked as a dependency.

> > > > 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()

Yes, ext4 works the same way. But thanks for confirmation.

> 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.

Yes, and the blocking on writeback of such page in direct reclaim is
exactly one link in the deadlock chain...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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