linux-kernel - Re: [PATCH] mm: be more verbose for alloc_contig

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Date:   Thu, 18 Feb 2021 08:10:30 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, david@...hat.com,
        joaodias@...gle.com
Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures

On Thu, Feb 18, 2021 at 09:56:18AM +0100, Michal Hocko wrote:
> On Wed 17-02-21 08:36:03, Minchan Kim wrote:
> > alloc_contig_range is usually used on cma area or movable zone.
> > It's critical if the page migration fails on those areas so
> > dump more debugging message like memory_hotplug unless user
> > specifiy __GFP_NOWARN.
> 
> I agree with David that this has a potential to generate a lot of output
> and it is not really clear whether it is worth it. Page isolation code
> already has REPORT_FAILURE mode which currently used only for the memory
> hotplug because this was just too noisy from the CMA path - d381c54760dc
> ("mm: only report isolation failures when offlining memory").
> 
> Maybe migration failures are less likely to fail but still. Doesn't CMA
> allocator provide some useful error reporting on its own?

Unfortunately, it's very useless. :(

```
in cma.c
                pr_debug("%s(): memory range at %p is busy, retrying\n",
                         __func__, pfn_to_page(pfn));

```
even, the pfn is not failed page. 
Originally, I thought to deal with it from cma.c to minimize changes
but it was tough because cma area couldn't get the failed page list.