linux-kernel - Re: [PATCH 1/2] mm: KSM: fix ksm

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Message-ID: <CA+CK2bBqFr+_jxr2KU4DWv7GmOztfP8Lc+dKjy=YeUDvotyoBQ@mail.gmail.com>
Date:   Thu, 15 Jul 2021 14:57:37 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Zhansaya Bagdauletkyzy <zhansayabagdaulet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: KSM: fix ksm_run data type

On Thu, Jul 15, 2021 at 2:30 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Jul 15, 2021 at 02:21:21PM -0400, Pavel Tatashin wrote:
> > On Thu, Jul 15, 2021 at 2:18 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Fri, Jul 16, 2021 at 12:01:01AM +0600, Zhansaya Bagdauletkyzy wrote:
> > > > +++ b/mm/ksm.c
> > > > @@ -289,7 +289,7 @@ static int ksm_nr_node_ids = 1;
> > > >  #define KSM_RUN_MERGE        1
> > > >  #define KSM_RUN_UNMERGE      2
> > > >  #define KSM_RUN_OFFLINE      4
> > > > -static unsigned long ksm_run = KSM_RUN_STOP;
> > > > +static unsigned int ksm_run = KSM_RUN_STOP;
> > >
> > > Should this be an enum instead?
> >
> > I think "unsigned int" is OK here, as it is exposed as uint to users:
> > Documentation/ABI/testing/sysfs-kernel-mm-ksm
> >
> > /sys/kernel/mm/ksm/run
> >
> > run: write 0 to disable ksm, read 0 while ksm is disabled.
> >
> > - write 1 to run ksm, read 1 while ksm is running.
> > - write 2 to disable ksm and unmerge all its pages.
>
> The document is out of date then as it does not mention 'offline'.

The offline mode cannot be set externally.

In run_store()
   if (flags > KSM_RUN_UNMERGE)
      return -EINVAL;

>
> Also, why does the call to kstrtouint() specify base 10?  If it is a
> bitmap, then permitting 0x [1] is more natural.  I would expect to see
> base 0 there.

Users can only write 0, 1, or 2, it is not a bitmap from the user's
perspective as the user cannot write: '3' . But, I think it is
somewhat weird that ksm_run is used as a bitmap internally with
KSM_RUN_OFFLINE = 4.

Imo, KSM_RUN_OFFLINE should be placed in a separate boolean from "ksm_run".

>
> [1] or even 0b, although I see that _parse_integer_fixup_radix does not
> support the 0b notation.