Re: [PATCH] mm: fix trying to reclaim unevicable LRU page

[Michal Hocko]

On Thu 30-05-19 11:42:29, Minchan Kim wrote:
> On Tue, May 28, 2019 at 05:14:07PM +0200, Michal Hocko wrote:
> > [Cc Pankaj Suryawanshi who has reported a similar problem
> > http://lkml.kernel.org/r/SG2PR02MB309806967AE91179CAFEC34BE84B0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]
> > 
> > On Fri 24-05-19 16:11:14, Minchan Kim wrote:
> > > There was below bugreport from Wu Fangsuo.
> > > 
> > > 7200 [  680.491097] c4 7125 (syz-executor) page:ffffffbf02f33b40 count:86 mapcount:84 mapping:ffffffc08fa7a810 index:0x24
> > > 7201 [  680.531186] c4 7125 (syz-executor) flags: 0x19040c(referenced|uptodate|arch_1|mappedtodisk|unevictable|mlocked)
> > > 7202 [  680.544987] c0 7125 (syz-executor) raw: 000000000019040c ffffffc08fa7a810 0000000000000024 0000005600000053
> > > 7203 [  680.556162] c0 7125 (syz-executor) raw: ffffffc009b05b20 ffffffc009b05b20 0000000000000000 ffffffc09bf3ee80
> > > 7204 [  680.566860] c0 7125 (syz-executor) page dumped because: VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page))
> > > 7205 [  680.578038] c0 7125 (syz-executor) page->mem_cgroup:ffffffc09bf3ee80
> > > 7206 [  680.585467] c0 7125 (syz-executor) ------------[ cut here ]------------
> > > 7207 [  680.592466] c0 7125 (syz-executor) kernel BUG at /home/build/farmland/adroid9.0/kernel/linux/mm/vmscan.c:1350!
> > > 7223 [  680.603663] c0 7125 (syz-executor) Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
> > > 7224 [  680.611436] c0 7125 (syz-executor) Modules linked in:
> > > 7225 [  680.616769] c0 7125 (syz-executor) CPU: 0 PID: 7125 Comm: syz-executor Tainted: G S              4.14.81 #3
> > > 7226 [  680.626826] c0 7125 (syz-executor) Hardware name: ASR AQUILAC EVB (DT)
> > > 7227 [  680.633623] c0 7125 (syz-executor) task: ffffffc00a54cd00 task.stack: ffffffc009b00000
> > > 7228 [  680.641917] c0 7125 (syz-executor) PC is at shrink_page_list+0x1998/0x3240
> > > 7229 [  680.649144] c0 7125 (syz-executor) LR is at shrink_page_list+0x1998/0x3240
> > > 7230 [  680.656303] c0 7125 (syz-executor) pc : [<ffffff90083a2158>] lr : [<ffffff90083a2158>] pstate: 60400045
> > > 7231 [  680.666086] c0 7125 (syz-executor) sp : ffffffc009b05940
> > > ..
> > > 7342 [  681.671308] c0 7125 (syz-executor) [<ffffff90083a2158>] shrink_page_list+0x1998/0x3240
> > > 7343 [  681.679567] c0 7125 (syz-executor) [<ffffff90083a3dc0>] reclaim_clean_pages_from_list+0x3c0/0x4f0
> > > 7344 [  681.688793] c0 7125 (syz-executor) [<ffffff900837ed64>] alloc_contig_range+0x3bc/0x650
> > > 7347 [  681.717421] c0 7125 (syz-executor) [<ffffff90084925cc>] cma_alloc+0x214/0x668
> > > 7348 [  681.724892] c0 7125 (syz-executor) [<ffffff90091e4d78>] ion_cma_allocate+0x98/0x1d8
> > > 7349 [  681.732872] c0 7125 (syz-executor) [<ffffff90091e0b20>] ion_alloc+0x200/0x7e0
> > > 7350 [  681.740302] c0 7125 (syz-executor) [<ffffff90091e154c>] ion_ioctl+0x18c/0x378
> > > 7351 [  681.747738] c0 7125 (syz-executor) [<ffffff90084c6824>] do_vfs_ioctl+0x17c/0x1780
> > > 7352 [  681.755514] c0 7125 (syz-executor) [<ffffff90084c7ed4>] SyS_ioctl+0xac/0xc0
> > > 
> > > Wu found it's due to [1]. Before that, unevictable page goes to cull_mlocked
> > > routine so that it couldn't reach the VM_BUG_ON_PAGE line.
> > > 
> > > To fix the issue, this patch filter out unevictable LRU pages
> > > from the reclaim_clean_pages_from_list in CMA.
> > 
> > The changelog is rather modest on details and I have to confess I have
> > little bit hard time to understand it. E.g. why do not we need to handle
> > the regular reclaim path?
> 
> No need to pass unevictable pages into regular reclaim patch if we are
> able to know in advance.

I am sorry to be dense here. So what is the difference in the CMA path?
Am I right that the pfn walk (CMA) rather than LRU isolation (reclaim)
is the key differentiator?

-- 
Michal Hocko
SUSE Labs