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Date:   Thu, 21 Oct 2021 02:36:49 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Yang Shi <shy828301@gmail.com>
Cc:     Hugh Dickins <hughd@google.com>, Song Liu <song@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hao Sun <sunhao.th@gmail.com>, Linux MM <linux-mm@kvack.org>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Song Liu <songliubraving@fb.com>
Subject: Re: [PATCH] fs: buffer: check huge page size instead of single page
 for invalidatepage
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On Wed, Oct 20, 2021 at 05:24:09PM -0700, Yang Shi wrote:
> On Wed, Oct 20, 2021 at 4:51 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Wed, Oct 20, 2021 at 04:38:49PM -0700, Yang Shi wrote:
> > > > However, it still doesn't make too much sense to have thp_size passed
> > > > to do_invalidatepage(), then have PAGE_SIZE hardcoded in a BUG
> > > > assertion IMHO. So it seems this patch is still useful because
> > > > block_invalidatepage() is called by a few filesystems as well, for
> > > > example, ext4. Or I'm wondering whether we should call
> > > > do_invalidatepage() for each subpage of THP in truncate_cleanup_page()
> > > > since private is for each subpage IIUC.
> > >
> > > Seems no interest?
> >
> > No.  I have changes in this area as part of the folio patchset (where
> > we end up converting this to invalidate_folio).  I'm not really
> > interested in doing anything before that, since this shouldn't be
> > reachable today.
> 
> Understood. But this is definitely reachable unless Hugh's patch
> (skipping non-regular file) is applied.

Right.  That's the bug that needs to be fixed.  Seeing THPs here is
a symptom.  Getting rid of the error just makes the problem silent.

> > > Anyway the more I was staring at the code the more I thought calling
> > > do_invalidatepage() for each subpage made more sense. So, something
> > > like the below makes sense?
> >
> > Definitely not.  We want to invalidate the entire folio at once.
> 
> I didn't look at the folio patch (on each individual patch level), but
> I'm supposed it still needs to invalidate buffer for each subpage,
> right?

No.  Buffers are tracked for the entire folio, not on each subpage.

Actually, the filesystem people currently believe that the O(n^2) nature
of buffer-head handling mean that it's a bad idea to create multi-page
folios for bufferhead based filesystems (which includes block devices),
and the correct path forward is to migrate away from buffer-heads.
That may change, but it's the current plan.