Re: [PATCH 1/4] erofs: fix setting up pcluster for temporary pages

[Gao Xiang <hsiangkao@redhat.com>]

On Fri, Oct 30, 2020 at 03:32:55PM +0200, Vladimir Zapolskiy wrote:
> Hi Gao Xiang,
> 
> On 10/30/20 2:47 PM, Gao Xiang wrote:
> > Hi Vladimir,
> > 
> > On Fri, Oct 30, 2020 at 02:20:31PM +0200, Vladimir Zapolskiy wrote:
> > > Hello Gao Xiang,
> > > 
> > > On 10/22/20 5:57 PM, Gao Xiang via Linux-erofs wrote:
> > > > From: Gao Xiang <hsiangkao@redhat.com>
> > > > 
> > > > pcluster should be only set up for all managed pages instead of
> > > > temporary pages. Since it currently uses page->mapping to identify,
> > > > the impact is minor for now.
> > > > 
> > > > Fixes: 5ddcee1f3a1c ("erofs: get rid of __stagingpage_alloc helper")
> > > > Cc: <stable@vger.kernel.org> # 5.5+
> > > > Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
> > > 
> > > I was looking exactly at this problem recently, my change is one-to-one
> > > to your fix, thus I can provide a tag:
> > > 
> > > Tested-by: Vladimir Zapolskiy <vladimir@tuxera.com>
> > 
> > Many thanks for confirming this!
> > I found this when I was killing magical stagingpage page->mapping,
> > it's somewhat late :-)
> > 
> 
> sure, for me it was an exciting immersion into the filesystem code :)

Thanks for your effort on this!

You could also post related kernel message in advance and
I will definitly look into that as well. :)

> 
> > > 
> > > 
> > > The fixed problem is minor, but the kernel log becomes polluted, if
> > > a page allocation debug option is enabled:
> > > 
> > >      % md5sum ~/erofs/testfile
> > >      BUG: Bad page state in process kworker/u9:0  pfn:687de
> > >      page:0000000057b8bcb4 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x687de
> > >      flags: 0x4000000000002000(private)
> > >      raw: 4000000000002000 dead000000000100 dead000000000122 0000000000000000
> > >      raw: 0000000000000000 ffff888066758690 00000000ffffffff 0000000000000000
> > >      page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
> > >      Modules linked in:
> > >      CPU: 1 PID: 602 Comm: kworker/u9:0 Not tainted 5.9.1 #2
> > >      Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> > >      Workqueue: erofs_unzipd z_erofs_decompressqueue_work
> > >      Call Trace:
> > >       dump_stack+0x84/0xba
> > >       bad_page.cold+0xac/0xb1
> > >       check_free_page_bad+0xb0/0xc0
> > >       free_pcp_prepare+0x2c8/0x2d0
> > >       free_unref_page+0x18/0xf0
> > >       put_pages_list+0x11a/0x120
> > >       z_erofs_decompressqueue_work+0xc9/0x110
> > >       ? z_erofs_decompress_pcluster.isra.0+0xf10/0xf10
> > >       ? read_word_at_a_time+0x12/0x20
> > >       ? strscpy+0xc7/0x1a0
> > >       process_one_work+0x30c/0x730
> > >       worker_thread+0x91/0x640
> > >       ? __kasan_check_read+0x11/0x20
> > >       ? rescuer_thread+0x8a0/0x8a0
> > >       kthread+0x1dd/0x200
> > >       ? kthread_unpark+0xa0/0xa0
> > >       ret_from_fork+0x1f/0x30
> > >      Disabling lock debugging due to kernel taint
> > 
> > Yeah, I can make a pull-request to Linus if you need this to be in master
> > now, or I can post it for v5.11-rc1 since 5.4 LTS isn't effected (and it
> > would be only a print problem with debugging option.)
> > 
> 
> As for myself I don't utterly need this fix on the master branch ASAP, however
> it might be reasonable to get it included right into the next v5.10 release,
> because I believe it'll be an LTS. Eventually it's up to you to make a decision,
> from my side I won't urge you, the fixed issue is obviously a non-critical one.
> 
> Thank you for the original fix and taking my opinion into consideration :)

Yeah, v5.10 is a LTS version, and you are right, I will try to make a
pull-request after I get Chao's RVB.

Thanks,
Gao Xiang

> 
> --
> Best wishes,
> Vladimir
>