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

[Vladimir Zapolskiy]

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@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxxxxx> # 5.5+
> > > Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
>
> 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 :)

> > 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 :)

--
Best wishes,
Vladimir