linux-kernel - Re: [RFC PATCH 1/4] mm: pagewalk: add the ability to install PTEs

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Message-ID: <09af9d3f-2165-4580-8cf3-9278ccd349d1@lucifer.local>
Date: Tue, 15 Oct 2024 08:27:14 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
        "Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Muchun Song <muchun.song@...ux.dev>,
        Richard Henderson <richard.henderson@...aro.org>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...db.de>,
        linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
        Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
        linux-kselftest@...r.kernel.org,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>
Subject: Re: [RFC PATCH 1/4] mm: pagewalk: add the ability to install PTEs

On Mon, Oct 14, 2024 at 11:47:58PM -0700, Christoph Hellwig wrote:
> Hi Lorenzo,
>
> sorry for only replying to this so late.

No worries, and thanks for taking a look! :)

>
> On Fri, Sep 27, 2024 at 01:51:11PM +0100, Lorenzo Stoakes wrote:
> > The existing generic pagewalk logic permits the walking of page tables,
> > invoking callbacks at individual page table levels via user-provided
> > mm_walk_ops callbacks.
> >
> > This is useful for traversing existing page table entries, but precludes
> > the ability to establish new ones.
> >
> > Existing mechanism for performing a walk which also installs page table
> > entries if necessary are heavily duplicated throughout the kernel, each
> > with semantic differences from one another and largely unavailable for use
> > elsewhere.
>
> I do like the idea of having common code for installing page tables!
>

Awesome.

> Minor nits below:
>
> > +int walk_page_range_mm(struct mm_struct *mm, unsigned long start,
> >  		unsigned long end, const struct mm_walk_ops *ops,
> >  		void *private)
>
> It would be good to have a minimum level of documentation for this
> function, including how it differs from walk_page_range and why
> it should remain internal.

Will add on respin!

>
> > +	/* For internal use only. */
> > +	if (ops->install_pte)
> > +		return -EINVAL;
>
> And this should probably be expanded a bit, including that no exported
> symbol should allow inserting arbitrary PTEs.  Maybe best done with
> a helper to share that comment with the other places that have this
> check.
>

Yeah a helper makes sense actually, a more general 'are these ops valid?'
thing. Will update on next respin with some explanation.

The next iteration I plan to un-RFC as seems generally the concept is
unopposed for this series, will make these changes then.

Thanks!