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

[Chao Yu]

On 2021/7/6 2:06, Jaegeuk Kim wrote:
> On 07/06, Chao Yu wrote:
> > 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.
>
> We can debug anything tho, this issue is blocking the production, and I'd
> like to get this in this merge windows. Could you please check the patch
> has any holes?

The code looks good to me,

Reviewed-by: Chao Yu <chao@xxxxxxxxxx>

Thanks,

> > Thanks,
> >
> > >