linux-kernel - Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG

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Message-ID: <alpine.LRH.2.02.1804232006540.2299@file01.intranet.prod.int.rdu2.redhat.com>
Date:   Mon, 23 Apr 2018 20:25:15 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Matthew Wilcox <willy@...radead.org>,
        David Miller <davem@...emloft.net>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        eric.dumazet@...il.com, edumazet@...gle.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        mst@...hat.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org, dm-devel@...hat.com,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM



On Mon, 23 Apr 2018, Michal Hocko wrote:

> On Mon 23-04-18 10:06:08, Mikulas Patocka wrote:
> 
> > > > He didn't want to fix vmalloc(GFP_NOIO)
> > > 
> > > I don't remember that conversation, so I don't know whether I agree with
> > > his reasoning or not.  But we are supposed to be moving away from GFP_NOIO
> > > towards marking regions with memalloc_noio_save() / restore.  If you do
> > > that, you won't need vmalloc(GFP_NOIO).
> > 
> > He said the same thing a year ago. And there was small progress. 6 out of 
> > 27 __vmalloc calls were converted to memalloc_noio_save in a year - 5 in 
> > infiniband and 1 in btrfs. (the whole discussion is here 
> > http://lkml.iu.edu/hypermail/linux/kernel/1706.3/04681.html )
> 
> Well this is not that easy. It requires a cooperation from maintainers.
> I can only do as much. I've posted patches in the past and actively
> bringing up this topic at LSFMM last two years...

You're right - but you have chosen the uneasy path. Fixing __vmalloc code 
is easy and it doesn't require cooperation with maintainers.

> > He refuses 15-line patch to fix GFP_NOIO bug because he believes that in 4 
> > years, the kernel will be refactored and GFP_NOIO will be eliminated. Why 
> > does he have veto over this part of the code? I'd much rather argue with 
> > people who have constructive comments about fixing bugs than with him.
> 
> I didn't NACK the patch AFAIR. I've said it is not a good idea longterm.
> I would be much more willing to change my mind if you would back your
> patch by a real bug report. Hacks are acceptable when we have a real
> issue in hands. But if we want to fix potential issue then better make
> it properly.

Developers should fix bugs in advance, not to wait until a crash hapens, 
is analyzed and reported.

What's the problem with 15-line hack? Is the problem that kernel 
developers would feel depressed when looking the source code? Other than 
harming developers' feelings, I don't see what kind of damange could that 
piece of code do.

Mikulas