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Date:   Tue, 1 Oct 2019 20:06:13 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Nikolay Borisov <nborisov@suse.com>
Cc:     Josef Bacik <josef@toxicpanda.com>, kernel-team@fb.com,
        linux-btrfs@vger.kernel.org, Zdenek Sojka <zsojka@seznam.cz>
Subject: Re: [PATCH] btrfs: nofs inode allocations
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On Mon, Sep 09, 2019 at 05:28:11PM +0300, Nikolay Borisov wrote:
> > This is because btrfs_new_inode() calls new_inode() under the
> > transaction.  We could probably move the new_inode() outside of this but
> > for now just wrap it in memalloc_nofs_save().
> 
> If I'm understanding correctly what happens here is that symlinking
> wants to instantiate a new inode
> (new_inode->btrfs_alloc_inode->kmem_cache_alloc with GFP_KERNEL) but it
> triggers background reclaim, hence goes to sleep, while holding a
> transaction. At the same time background reclaim joins the same
> transaction which is now blocked by the symlinking thread, hence it's
> prone to deadlock ?

Yes, that's the pattern, GFP_KERNEL inside
start_transaction/commit_transaction. The more generic fix is in the works.