linux-ext4 - Re: [REPORT] kernel BUG at fs/ext4/inode.c:2620

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Date:   Fri, 18 Feb 2022 01:03:13 -0500
From:   "Theodore Ts'o" <tytso@....edu>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Lee Jones <lee.jones@...aro.org>, linux-ext4@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Dave Chinner <dchinner@...hat.com>,
        Goldwyn Rodrigues <rgoldwyn@...e.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Bob Peterson <rpeterso@...hat.com>,
        Damien Le Moal <damien.lemoal@....com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johannes Thumshirn <jth@...nel.org>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, cluster-devel@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [REPORT] kernel BUG at fs/ext4/inode.c:2620 - page_buffers()

On Fri, Feb 18, 2022 at 04:24:20AM +0000, Matthew Wilcox wrote:
> On Thu, Feb 17, 2022 at 09:54:30PM -0500, Theodore Ts'o wrote:
> > process_vm_writev() uses [un]pin_user_pages_remote() which is the same
> > interface uses for RDMA.  But it's not clear this is ever supposed to
> > work for memory which is mmap'ed region backed by a file.
> > pin_user_pages_remote() appears to assume that it is an anonymous
> > region, since the get_user_pages functions in mm/gup.c don't call
> > read_page() to read data into any pages that might not be mmaped in.
> 
> ... it doesn't end up calling handle_mm_fault() in faultin_page()?

Ah yes, sorry, I missed that.  This is what happens when a syzbot bug
is thrown to a file system developer, who then has to wade theough
mm code for which he is not understand....

	   	    	     	    - Ted