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Date:   Thu, 29 Apr 2021 21:16:34 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-kernel@vger.kernel.org
Subject: Re: BUG_ON(!mapping_empty(&inode->i_data))
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On Fri, 2 Apr 2021 14:16:04 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:

> On Fri, 2 Apr 2021, Hugh Dickins wrote:
> > 
> > There is a "Put holes back where they were" xas_store(&xas, NULL) on
> > the failure path, which I think we would expect to delete empty nodes.
> > But it only goes as far as nr_none.  Is it ok to xas_store(&xas, NULL)
> > where there was no non-NULL entry before?  I should try that, maybe
> > adjusting the !nr_none break will give a very simple fix.
> 
> No, XArray did not like that:
> xas_update() XA_NODE_BUG_ON(node, !list_empty(&node->private_list)).
> 
> But also it's the wrong thing for collapse_file() to do, from a file
> integrity point of view. So far as there is a non-NULL page in the list,
> or nr_none is non-zero, those subpages are frozen at the src end, and
> THP head locked and not Uptodate at the dst end. But go beyond nr_none,
> and a racing task could be adding new pages, which THP collapse failure
> has no right to delete behind its back.
> 
> Not an issue for READ_ONLY_THP_FOR_FS, but important for shmem and future.
> 
> > 
> > Or, if you remove the "static " from xas_trim(), maybe that provides
> > the xas_prune_range() you proposed, or the cleanup pass I proposed.
> > To be called on collapse_file() failure, or when eviction finds
> > !mapping_empty().
> 
> Something like this I think.
> 

I'm not sure this ever was resolved?

Is it the case that the series "Remove nrexceptional tracking v2" at
least exposed this bug?

IOW, what the heck should I do with

mm-introduce-and-use-mapping_empty.patch
mm-stop-accounting-shadow-entries.patch
dax-account-dax-entries-as-nrpages.patch
mm-remove-nrexceptional-from-inode.patch

Thanks.