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

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Date: Tue, 19 Mar 2024 14:40:34 +0100
From: Michal Hocko <mhocko@...e.com>
To: Barry Song <21cnbao@...il.com>
Cc: liuhailong@...o.com, 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 Tue 19-03-24 19:09:18, Barry Song wrote:
> On Tue, Mar 19, 2024 at 4:56 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > 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.
> 
> I assume the regular LRU implementation you are talking about is the LRU
> without skip_cma()?

No, I mean standard LRU reclaim implementation rather than MGLRU.

-- 
Michal Hocko
SUSE Labs