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

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Message-ID: <CAHbLzkosZr_ugNAMbXreUumSyAUMH1hPhTaiXXXRUNx88J9Xsw@mail.gmail.com>
Date:   Mon, 4 Oct 2021 11:28:45 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Song Liu <song@...nel.org>,
        Rongwei Wang <rongwei.wang@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        William Kucharski <william.kucharski@...cle.com>
Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

On Sat, Oct 2, 2021 at 10:09 AM Matthew Wilcox <willy@...radead.org> 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@...gle.com> 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@mail.gmail.com/
> > > or
> > > https://lore.kernel.org/lkml/CACkBjsYwLYLRmX8GpsDpMthagWOjWWrNxqY6ZLNQVr6yx+f5vA@mail.gmail.com/
> > >
> > > 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@mail.gmail.com/
and the patch from me:
https://lore.kernel.org/linux-mm/20210917205731.262693-1-shy828301@gmail.com/

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?