linux-kernel - Re: find_get_entries_tag regression bisected

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Message-ID: <20190226120820.GB11592@bombadil.infradead.org>
Date:   Tue, 26 Feb 2019 04:08:21 -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 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.