Re: [PATCH] x86/mm: Fix pmd_modify() dropping the dirty bit

[Vernon Yang <vernon2gm@gmail.com>]

On Wed, Sep 02, 2026 at 09:10:36PM -0700, Andrew Morton wrote:
> On Thu,  3 Sep 2026 11:16:08 +0800 Vernon Yang <vernon2gm@gmail.com> wrote:
>
> > From: Vernon Yang <yanglincheng@kylinos.cn>
> >
> > pmd_modify() masks the old value with (_HPAGE_CHG_MASK & ~_PAGE_DIRTY),
> > silently discarding the hardware dirty bit.  The subsequent
> > pmd_mksaveddirty() call is supposed to transfer _PAGE_DIRTY into
> > _PAGE_SAVED_DIRTY when write-protecting, but the dirty bit was already
> > stripped from the value, so there is nothing left to transfer.
> >
> > Contrast with pte_modify(), which keeps _PAGE_DIRTY_BITS in its mask,
> > and pud_modify(), which keeps _HPAGE_CHG_MASK untouched: pmd_modify()
> > is the odd one out.  Any pmd_modify() on a writable, dirty PMD loses
> > the dirty state.
> >
> > One visible consequence is data loss with MADV_FREE on PMD-mapped THP:
> >
> >     memset(buf, 0x5A, size);          // PMD-mapped THP, PMD dirty
> >     madvise(buf, size, MADV_FREE);    // PMD cleaned but left writable,
> >                                       // folio marked lazyfree
> >     memset(buf, 0x5A, size);          // hardware sets _PAGE_DIRTY again
> >     mprotect(buf, size, PROT_READ);   // pmd_modify() drops the dirty bit
> >     mprotect(buf, size, PROT_READ|PROT_WRITE);
> >     // ... memory pressure ...
> >
> > Reclaim (e.g. under memcg pressure) then finds the lazyfree folio with
> > no dirty bit set anywhere and frees it in
> > __discard_anon_folio_pmd_locked(), even though the data was rewritten
> > after MADV_FREE; subsequent reads fault in fresh zero pages.  NUMA
> > hinting alone can trigger the same loss, as do_huge_pmd_numa_page()
> > restores the PMD through pmd_modify() as well.
> >
> > PMD-mapped file THPs are affected too: mprotect()/NUMA hinting dropping
> > the dirty bit means rewritten data is never written back.
> >
> > Fix it by keeping _PAGE_DIRTY in the preserved mask, exactly like
> > pte_modify() and pud_modify() do.  The existing
> > pmd_mksaveddirty()/pmd_clear_saveddirty() pair then performs the
> > hardware-dirty <-> saved-dirty transition based on the write bit,
> > preserving the shadow-stack encoding rules.
>
> Yeah, this is exactly what I came up with, using chatgpt.
>
> > Closes: https://lore.kernel.org/r/CAJxLxMUGu1-L+O_nAONOwOXnS=cNbNApCWqdthRjd76LThtSPg@mail.gmail.com/
>
> We definitely want a Reported-by: Orson here.  He obviously did a ton
> of work on this, and it's the least we can do to thank him.  I'll add it.

Yeah, I missed adding Reported-by: Orson, Sorry. Thank you for adding
it.

--
Cheers,
Vernon