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

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Date:   Fri, 5 Oct 2018 12:49:41 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>, linux@...linux.org.uk,
        Robin van der Gracht <robin@...tonic.nl>,
        stefanr@...6.in-berlin.de, hjc@...k-chips.com,
        Heiko Stübner <heiko@...ech.de>,
        Dave Airlie <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,
        mhocko@...e.com, Dan Williams <dan.j.williams@...el.com>,
        kirill.shutemov@...ux.intel.com,
        Mark Rutland <mark.rutland@....com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kate Stewart <kstewart@...uxfoundation.org>, tchibo@...gle.com,
        riel@...hat.com, minchan@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, ying.huang@...el.com,
        Andi Kleen <ak@...ux.intel.com>, rppt@...ux.vnet.ibm.com,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Arnd Bergmann <arnd@...db.de>, cpandya@...eaurora.org,
        hannes@...xchg.org, Joe Perches <joe@...ches.com>,
        mcgrof@...nel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <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

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

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*)

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