linux-kernel - Re: [PATCH v2] Revert "mm: skip CMA pages when they are not available"

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Message-ID: <ZflTCY-Oaxm0U70u@tiehlicka>
Date: Tue, 19 Mar 2024 09:55:37 +0100
From: Michal Hocko <mhocko@...e.com>
To: liuhailong@...o.com
Cc: akpm@...ux-foundation.org, nathan@...nel.org, ndesaulniers@...gle.com,
	trix@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, surenb@...gle.com, zhaoyang.huang@...soc.com,
	quic_charante@...cinc.com, yuzhao@...gle.com
Subject: Re: [PATCH v2] Revert "mm: skip CMA pages when they are not
 available"

On Fri 15-03-24 16:18:03, liuhailong@...o.com wrote:
> From: "Hailong.Liu" <liuhailong@...o.com>
> 
> This reverts
> commit b7108d66318a ("Multi-gen LRU: skip CMA pages when they are not eligible")
> commit 5da226dbfce3 ("mm: skip CMA pages when they are not available")
> 
> skip_cma may cause system not responding. if cma pages is large in lru_list
> and system is in lowmemory, many tasks would direct reclaim and waste
> cpu time to isolate_lru_pages and return.
> 
> Test this patch on android-5.15 8G device
> reproducer:
> - cma_declare_contiguous 3G pages
> - set /proc/sys/vm/swappiness 0 to enable direct_reclaim reclaim file
>   only.
> - run a memleak process in userspace

Does this represent a sane configuration? CMA memory is unusable for
kernel allocations and memleak process is also hard to reclaim due to
swap suppression. Isn't such a system doomed to struggle to reclaim any
memory? Btw. how does the same setup behave with the regular LRU
implementation? My guess would be that it would struggle as well.
-- 
Michal Hocko
SUSE Labs