Re: [PATCH] mm: thp: fix soft dirty for migration when split

[Konstantin Khlebnikov]

On Tue, Dec 11, 2018 at 7:48 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Mon, Dec 10, 2018 at 07:50:52PM +0300, Konstantin Khlebnikov wrote:
> > On Fri, Dec 7, 2018 at 6:34 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Dec 06, 2018 at 04:46:04PM +0800, Peter Xu wrote:
> > > > When splitting a huge migrating PMD, we'll transfer the soft dirty bit
> > > > from the huge page to the small pages.  However we're possibly using a
> > > > wrong data since when fetching the bit we're using pmd_soft_dirty()
> > > > upon a migration entry.  Fix it up.
> > >
> > > Note that if my understanding is correct about the problem then if
> > > without the patch there is chance to lose some of the dirty bits in
> > > the migrating pmd pages (on x86_64 we're fetching bit 11 which is part
> > > of swap offset instead of bit 2) and it could potentially corrupt the
> > > memory of an userspace program which depends on the dirty bit.
> >
> > It seems this code is broken in case of pmd_migraion:
> >
> > old_pmd = pmdp_invalidate(vma, haddr, pmd);
> >
> > #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> > pmd_migration = is_pmd_migration_entry(old_pmd);
> > if (pmd_migration) {
> > swp_entry_t entry;
> >
> > entry = pmd_to_swp_entry(old_pmd);
> > page = pfn_to_page(swp_offset(entry));
> > } else
> > #endif
> > page = pmd_page(old_pmd);
> > VM_BUG_ON_PAGE(!page_count(page), page);
> > page_ref_add(page, HPAGE_PMD_NR - 1);
> > if (pmd_dirty(old_pmd))
> > SetPageDirty(page);
> > write = pmd_write(old_pmd);
> > young = pmd_young(old_pmd);
> > soft_dirty = pmd_soft_dirty(old_pmd);
> >
> > Not just soft_dirt - all bits (dirty, write, young) have diffrent encoding
> > or not present at all for migration entry.
>
> Hi, Konstantin,
>
> Actually I noticed it but I thought it didn't hurt since both
> write/young flags are not used at all when applying to the small pages
> when pmd_migration==true.  But indeed there's at least an unexpected
> side effect of an extra call to SetPageDirty() that I missed.

"write" is used for making smaller migration entry:

swp_entry = make_migration_entry(page + i, write);

>
>
> I'll repost soon.  Thanks!
>
> --
> Peter Xu