linux-kernel - Re: [PATCH v2] mm: Introduce new function vm_insert_kmem

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Date:   Fri, 5 Oct 2018 17:41:39 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        robin@...tonic.nl, stefanr@...6.in-berlin.de, hjc@...k-chips.com,
        Heiko Stuebner <heiko@...ech.de>, airlied@...ux.ie,
        robin.murphy@....com, iamjoonsoo.kim@....com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kees Cook <keescook@...omium.org>, treding@...dia.com,
        Michal Hocko <mhocko@...e.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>, aryabinin@...tuozzo.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kate Stewart <kstewart@...uxfoundation.org>, tchibo@...gle.com,
        riel@...hat.com, Minchan Kim <minchan@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Huang, Ying" <ying.huang@...el.com>, ak@...ux.intel.com,
        rppt@...ux.vnet.ibm.com, linux@...inikbrodowski.net,
        Arnd Bergmann <arnd@...db.de>, cpandya@...eaurora.org,
        hannes@...xchg.org, Joe Perches <joe@...ches.com>,
        mcgrof@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        linux1394-devel@...ts.sourceforge.net,
        dri-devel@...ts.freedesktop.org,
        linux-rockchip@...ts.infradead.org, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2] mm: Introduce new function vm_insert_kmem_page

On Fri, Oct 5, 2018 at 4:19 PM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> Hi Souptick,
>
> On Fri, Oct 5, 2018 at 12:01 PM Souptick Joarder <jrdr.linux@...il.com> wrote:
> >
> > The final goal is to remove vm_insert_page by converting it to
> > vmf_insert_page. But to do that we have to first introduce the
> > new API which is similar to vm_insert_page  (for non #PF). I tried this by
> > introducing vm_insert_kmem_page ( * identical as vm_insert_page
> > except API name *) in this patch. But this looks like a bad approach.
>
> We are going in circles here. That you want to convert vm_insert_page
> to vmf_insert_page for the PF case is fine and understood. However,
> you don't *need* to introduce a new name for the remaining non-PF
> cases if the function is going to be the exact same thing as before.
> You say "The final goal is to remove vm_insert_page", but you haven't
> justified *why* you need to remove that name.
>
> Now, if we want to rename the function for some reason (e.g. avoid
> confusion with vmf_insert_page), that is fine but is another topic. It
> may be or not a good idea, but it is orthogonal to the vmf_ work.
> Matthew, on this regard, told you that you shouldn't duplicate
> functions. If you want a rename, do so; but don't copy the code. In
> other words: nobody said introducing the vm_insert_kmem_page name is a
> bad idea -- what Matthew told you is that *duplicating* vm_insert_page
> just for that is bad.
>
> Further, you are copying the code (if I understand your thought
> process) because you want to change the callers of non-PF first, and
> then do the "full conversion from vm_* to vmf_*". However, that is
> confusing, because there is no need to change non-PF callers of
> vm_insert_page since they don't care about the new vmf_* functions.
>
> Instead, the proper way of doing this is:
>
>   1. Introduce the vmf_* API
>   2. Change all PF-users users to that (leaving all non-PF ones
> untouched!) -- if this is too big, you can split this patch into
> several patches, one per subsystem, etc.

We are done with step 2. All the PF-users are converted to use
vmf_insert_page. ( Ref - linux-next-20181005)

>   3. Remove the vm_* functions (except the ones that are still used in
> non-PF contexts, e.g. vm_insert_page)

Step 3 is part of step 2. Already done.

>
> Then, optionally, if you want to rename the function for the remaining
> non-PF users:
>
>   4. Rename vm_insert_page (justifying why the current name is
> confusing *on its own merits*).
>
> Otherwise, if you want to pursue Matthew's idea:
>
>   4. Introduce the vm_insert_range (possibly leveraging
> vm_insert_page, or not; you have to see what is best).
>   5. Replace those callers that can take advantage of vm_insert_range
>   6. Remove vm_insert_page and replace callers with vm_insert_range
> (only if it is not worth to keep vm_insert_range, again justifying it
> *on its own merits*)

Step 4 to 6, going to do it.  It is part of plan now :-)

>
> As you see, these are all logical step-by-step improvements, without
> duplicating functions temporarily, leaving temporary changes or
> changing current callers to new APIs for unrelated reasons (i.e. no
> need to introduce vm_insert_kmem_page simply to do a "conversion" to
> vmf_).
>
> Cheers,
> Miguel