Re: BUG_ON(!mapping_empty(&inode->i_data))

[Hugh Dickins <hughd@google.com>]

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.

Hugh
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