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

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Date: Tue, 19 Mar 2024 19:09:18 +0800
From: Barry Song <21cnbao@...il.com>
To: Michal Hocko <mhocko@...e.com>, 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 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()?

I remember Hailong mentioned something like " it also trigger memory psi
event to allow admin do something to release memory" and " without
patch the devices would kill camera process".  So it seems the difference
is if a killing will occur.

Hailong, would you like to provide more detail?

> --
> Michal Hocko
> SUSE Labs
>