linux-kernel - Re: [PATCH v2] mm/page_alloc: fix memalloc_nocma

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Message-Id: <20200723180814.acde28b92ce6adc785a79120@linux-foundation.org>
Date:   Thu, 23 Jul 2020 18:08:14 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     js1304@...il.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@....com, Vlastimil Babka <vbabka@...e.cz>,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/page_alloc: fix memalloc_nocma_{save/restore}
 APIs

On Thu, 23 Jul 2020 10:49:02 +0900 js1304@...il.com wrote:

> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> Currently, memalloc_nocma_{save/restore} API that prevents CMA area
> in page allocation is implemented by using current_gfp_context(). However,
> there are two problems of this implementation.
> 
> First, this doesn't work for allocation fastpath. In the fastpath,
> original gfp_mask is used since current_gfp_context() is introduced in
> order to control reclaim and it is on slowpath. So, CMA area can be
> allocated through the allocation fastpath even if
> memalloc_nocma_{save/restore} APIs are used.

Whoops.

> Currently, there is just
> one user for these APIs and it has a fallback method to prevent actual
> problem.

Shouldn't the patch remove the fallback method?

> Second, clearing __GFP_MOVABLE in current_gfp_context() has a side effect
> to exclude the memory on the ZONE_MOVABLE for allocation target.

More whoops.

Could we please have a description of the end-user-visible effects of
this change?  Very much needed when proposing a -stable backport, I think.

d7fefcc8de9147c is over a year old.  Why did we only just discover
this?  This makes one wonder how serious those end-user-visible effects
are?

> To fix these problems, this patch changes the implementation to exclude
> CMA area in page allocation. Main point of this change is using the
> alloc_flags. alloc_flags is mainly used to control allocation so it fits
> for excluding CMA area in allocation.
>