linux-kernel - Re: [PATCH v5 2/2] mm,thp: Add experimental config option RO_EXEC_FILEMAP_HUGE_FAULT

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Message-ID: <20190903125150.GW14028@dhcp22.suse.cz>
Date:   Tue, 3 Sep 2019 14:51:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     William Kucharski <william.kucharski@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Song Liu <songliubraving@...com>,
        Bob Kasten <robert.a.kasten@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Chad Mynhier <chad.mynhier@...cle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <jweiner@...com>
Subject: Re: [PATCH v5 2/2] mm,thp: Add experimental config option
 RO_EXEC_FILEMAP_HUGE_FAULT_THP

On Tue 03-09-19 05:22:08, Matthew Wilcox wrote:
> On Tue, Sep 03, 2019 at 02:14:24PM +0200, Michal Hocko wrote:
> > On Mon 02-09-19 03:23:41, William Kucharski wrote:
> > > Add filemap_huge_fault() to attempt to satisfy page
> > > faults on memory-mapped read-only text pages using THP when possible.
> > 
> > This deserves much more description of how the thing is implemented and
> > expected to work. For one thing it is not really clear to me why you
> > need CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP at all. You need a support
> > from the filesystem anyway. So who is going to enable/disable this
> > config?
> 
> There are definitely situations in which enabling this code will crash
> the kernel.  But we want to get filesystems to a point where they can
> start working on their support for large pages.  So our workaround is
> to try to get the core pieces merged under a CONFIG_I_KNOW_WHAT_IM_DOING
> flag and let people play with it.  Then continue to work on the core
> to eliminate those places that are broken.

I am not sure I understand. Each fs has to opt in to the feature
anyway. If it doesn't then there should be no risk of regression, right?
I do not expect any fs would rush an implementation in while not being
sure about the correctness. So how exactly does a config option help
here.
 
> > I cannot really comment on fs specific parts but filemap_huge_fault
> > sounds convoluted so much I cannot wrap my head around it. One thing
> > stand out though. The generic filemap_huge_fault depends on ->readpage
> > doing the right thing which sounds quite questionable to me. If nothing
> > else  I would expect ->readpages to do the job.
> 
> Ah, that's because you're not a filesystem person ;-)  ->readpages is
> really ->readahead.  It's a crappy interface and should be completely
> redesigned.

OK, the interface looked like the right fit for this purpose. Thanks for
clarifying.
-- 
Michal Hocko
SUSE Labs