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

[Yang Shi]

On Thu, Sep 30, 2021 at 9:49 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> 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

Makes sense to me. We should be able to check S_ISREG() in khugepaged,
if it is not a regular file, just bail out. Sounds not that nasty to
me AFAIU.

> 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
>