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

[Barry Song <21cnbao@gmail.com>]

On Tue, Mar 19, 2024 at 4:56 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Fri 15-03-24 16:18:03, liuhailong@oppo.com wrote:
> > From: "Hailong.Liu" <liuhailong@oppo.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
>