Re: Is shmem page accounting wrong on split?

[Matthew Wilcox <willy@infradead.org>]

On Fri, Aug 28, 2020 at 06:31:22PM +0100, Matthew Wilcox wrote:
> On Fri, Aug 28, 2020 at 10:08:52AM -0700, Hugh Dickins wrote:
> > It's a good observation of an oddity that I probably didn't think of,
> > but you haven't said which kind of shmem page accounting goes wrong here
> > (vm_enough_memory? df of filesystem? du of filesystem? memcg charge?
> > all of the above? observed in practice?), and what needs solving.

Oh, I forgot to say which 

> The problem I've observed in practice is following the same pattern in
> truncate_inode_pages_range().  The call to delete_from_page_cache_batch()
> trips the assertion that the page hasn't already been deleted from the
> page cache.  I think the solution is obvious -- don't add the page to
> locked_pvec if page->mapping is NULL.

Here's the change I'm currently testing.  It's survived about eight
minutes of xfstests so far, which is far longer than it was surviving
before.

-       /* Try to split huge page, so we can truly punch the hole or truncate */
-       return split_huge_page(page) >= 0;
+       /*
+        * split_huge_page may remove the page itself; if so, we should
+        * not attempt to remove it again.
+        */
+       return split_huge_page(page) >= 0 && page->mapping;