Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

[Hugh Dickins]

On Thu, 30 Sep 2021, Matthew Wilcox wrote:
> On Wed, Sep 29, 2021 at 10:24:44PM -0700, Hugh Dickins wrote:
> > 
> > Aside from the above page->index mischeck in find_lock_entries(),
> > I now think this bug needs nothing more than simply removing the
> > VM_BUG_ON_PAGE(PageTail(page), page) from truncate_inode_page().
> 
> I don't think that's right.  This bug was also observed when calling
> truncate(), so there's clearly a situation where two concurrent calls
> to truncate_pagecache() leaves a THP in the cache.

I assume you're thinking of one of the fuzzer blkdev ones:
https://lore.kernel.org/linux-mm/CACkBjsbtF_peC7N_4mRfHML_BeiPe+O9DahTfr84puSG_J9rcQ@xxxxxxxxxxxxxx/
or
https://lore.kernel.org/lkml/CACkBjsYwLYLRmX8GpsDpMthagWOjWWrNxqY6ZLNQVr6yx+f5vA@xxxxxxxxxxxxxx/

I haven't started on those ones yet: yes, I imagine one or both of those
will need a further fix (S_ISREG() check somewhere if we're lucky; but
could well be nastier); but for the bug in this thread, I expect
removing the VM_BUG_ON_PAGE(PageTail) to be enough.

If you're thinking of something else, please send a link if you can - thanks.

Hugh