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

[Yang Shi]

On Sat, Oct 2, 2021 at 10:09 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Sep 30, 2021 at 10:39:14AM -0700, Yang Shi wrote:
> > On Thu, Sep 30, 2021 at 9:49 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > > 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.
>
> I don't see why we should have an S_ISREG() check.  I agree it's not the
> intended usecase, but it ought to work fine.  Unless there's something
> I'm missing?

Check out this bug report:
https://lore.kernel.org/lkml/CACkBjsYwLYLRmX8GpsDpMthagWOjWWrNxqY6ZLNQVr6yx+f5vA@xxxxxxxxxxxxxx/
and the patch from me:
https://lore.kernel.org/linux-mm/20210917205731.262693-1-shy828301@xxxxxxxxx/

I don't think we handle buffers correctly for file THP, right? My
patch is ad hoc, so I thought Hugh's suggestion makes some sense to
me. Why do we have THP collapsed for unintended usecase in the first
place?