Re: [f2fs-dev] [PATCH] f2fs: initialize page->private when using for our internal use

[Chao Yu]

On 2021/7/5 19:47, Matthew Wilcox wrote:
> On Mon, Jul 05, 2021 at 07:33:35PM +0800, Chao Yu wrote:
> > On 2021/7/5 16:56, Jaegeuk Kim wrote:
> > > On 07/05, Chao Yu wrote:
> > > > On 2021/7/5 13:22, Jaegeuk Kim wrote:
> > > > > We need to guarantee it's initially zero. Otherwise, it'll hurt entire flag
> > > > > operations.
> > > >
> > > > Oops, I didn't get the point, shouldn't .private be zero after page was
> > > > just allocated by filesystem? What's the case we will encounter stall
> > > > private data left in page?
> > >
> > > I'm seeing f2fs_migrate_page() has the newpage with some value without Private
> > > flag. That causes a kernel panic later due to wrong private flag used in f2fs.
> >
> > I'm not familiar with that part of codes, so Cc mm mailing list for help.
> >
> > My question is newpage in .migrate_page() may contain non-zero value in .private
> > field but w/o setting PagePrivate flag, is it a normal case?
>
> I think freshly allocated pages have a page->private of 0.  ie this
> code in mm/page_alloc.c:
>
>                  page = rmqueue(ac->preferred_zoneref->zone, zone, order,
>                                  gfp_mask, alloc_flags, ac->migratetype);
>                  if (page) {
>                          prep_new_page(page, order, gfp_mask, alloc_flags);
>
> where prep_new_page() calls post_alloc_hook() which contains:
>          set_page_private(page, 0);
>
> Now, I do see in __buffer_migrate_page() (mm/migrate.c):
>
>          attach_page_private(newpage, detach_page_private(page));
>
> but as far as I can tell, f2fs doesn't call any of the
> buffer_migrate_page() paths.  So I'm not sure why you're seeing
> a non-zero page->private.

Well, that's strange.

Jaegeuk, let's add a BUGON in f2fs to track the call path where newpage
has non-zero private value? if this issue is reproducible.

Thanks,

>