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SubjectRe: [f2fs-dev] [PATCH] f2fs: initialize page->private when using for our internal use
On 07/05, 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.

Hmm, I can see it in 4.14 and 5.10 kernel.

The trace is on:

30875 [ 1065.118750] c3 87 f2fs_migrate_page+0x354/0x45c
30876 [ 1065.123872] c3 87 move_to_new_page+0x70/0x30c
30877 [ 1065.128813] c3 87 migrate_pages+0x3a0/0x964
30878 [ 1065.133583] c3 87 compact_zone+0x608/0xb04
30879 [ 1065.138257] c3 87 kcompactd+0x378/0x4ec
30880 [ 1065.142664] c3 87 kthread+0x11c/0x12c
30881 [ 1065.146897] c3 87 ret_from_fork+0x10/0x18

It seems compaction_alloc() gets a free page which doesn't reset the fields?

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