linux-kernel - Re: find_get_entries_tag regression bisected

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Message-ID: <CAPcyv4j8G0S4Z2ngzrz_OhFjp8+4tqCtpRRE6Eq33uczh0QBRA@mail.gmail.com>
Date:   Mon, 25 Feb 2019 21:03:00 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
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 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.