Re: [PATCH] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD_NULL

[Kiryl Shutsemau <kirill@shutemov.name>]

On Fri, Sep 12, 2025 at 07:13:48PM +0200, David Hildenbrand wrote:
> On 12.09.25 18:58, Kiryl Shutsemau wrote:
> > From: Kiryl Shutsemau <kas@kernel.org>
> > 
> > MADV_COLLAPSE on a file mapping behaves inconsistently depending on if
> > PMD page table is installed or not.
> > 
> > Consider following example:
> > 
> > 	p = mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
> > 		 MAP_SHARED, fd, 0);
> > 	err = madvise(p, 2UL << 20, MADV_COLLAPSE);
> > 
> > fd is a populated tmpfs file.
> > 
> > The result depends on the address that the kernel returns on mmap().
> > If it is located in an existing PMD table, the madvise() will succeed.
> > However, if the table does not exist, it will fail with -EINVAL.
> > 
> > This occurs because find_pmd_or_thp_or_none() returns SCAN_PMD_NULL when
> > a page table is missing, which causes collapse_pte_mapped_thp() to fail.
> > 
> > SCAN_PMD_NULL and SCAN_PMD_NONE should be treated the same in
> > collapse_pte_mapped_thp(): install the PMD leaf entry and allocate page
> > tables as needed.
> > 
> > Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> > ---
> 
> Makes sense to me.
> 
> Is this something we want a Fixes: or even Cc: stable for?

I am not sure if it is stable@ matter.

I believe it is there from the start:

Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")

-- 
  Kiryl Shutsemau / Kirill A. Shutemov