linux-kernel - Re: find_get_entries_tag regression bisected

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Date:   Tue, 26 Feb 2019 05:16:50 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Balcer, Piotr" <piotr.balcer@...el.com>
Subject: Re: find_get_entries_tag regression bisected

On Tue, Feb 26, 2019 at 04:08:21AM -0800, Matthew Wilcox wrote:
> On Mon, Feb 25, 2019 at 09:03:00PM -0800, Dan Williams wrote:
> > On Sat, Feb 16, 2019 at 1:11 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Sat, Feb 16, 2019 at 09:29:48AM -0800, Matthew Wilcox wrote:
> > > > On Sat, Feb 16, 2019 at 07:35:11AM -0800, Matthew Wilcox wrote:
> > > > > Another way to fix this would be to mask the address in dax_entry_mkclean(),
> > > > > but I think this is cleaner.
> > > >
> > > > That's clearly rubbish, dax_entry_mkclean() can't possibly mask the
> > > > address.  It might be mis-aligned in another process.  But ... if it's
> > > > misaligned in another process, dax_entry_mkclean() will only clean the first
> > > > PTE associated with the PMD; it won't clean the whole thing.  I think we need
> > > > something like this:
> > >
> > > Nope, this isn't enough.  It's _necessary_ to find the processes that
> > > have part of this PMD page mapped, but not the start of it.  But it's
> > > not _sufficient_ because it'll still only mkclean the first PTE.  So we
> > > need a loop.  I'm feeling a bit over my head here.  I may have a go at
> > > a fuller fix, but if someone else wants to have a go at it, be my guest!
> > 
> > Nothing comes to mind outside of pseudo-reverting this conversion by
> > introducing a way to get back to the old semantics. If you don't see a
> > path forward, us mere Xarray-mortals stand no chance.
> 
> This is a pre-existing bug.  Fixing the regression is easy.

To be clear; I sent the regression fix a week ago in
20190216153511.GM12668@...badil.infradead.org

I haven't tested it; I don't have a suitable setup right now.

The pre-existing bug is that if you have two tasks with the same
address range mapped, but one has it mapped with a PMD and the
other has it mapped with PTEs (maybe due to mapping only a subset
of the addresses), dax_entry_mkclean() won't clean the PTEs because
vma_interval_tree_foreach() won't find the VMA.