The patch titled Subject: mm: fix folio_pte_batch() on XEN PV has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-fix-folio_pte_batch-on-xen-pv.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Petr Van��k arkamar@atlas.cz Subject: mm: fix folio_pte_batch() on XEN PV Date: Fri, 2 May 2025 23:50:19 +0200

On XEN PV, folio_pte_batch() can incorrectly batch beyond the end of a folio due to a corner case in pte_advance_pfn(). Specifically, when the PFN following the folio maps to an invalidated MFN,

expected_pte = pte_advance_pfn(expected_pte, nr);

produces a pte_none(). If the actual next PTE in memory is also pte_none(), the pte_same() succeeds,

if (!pte_same(pte, expected_pte)) break;

the loop is not broken, and batching continues into unrelated memory.

For example, with a 4-page folio, the PTE layout might look like this:

[ 53.465673] [ T2552] folio_pte_batch: printing PTE values at addr=0x7f1ac9dc5000 [ 53.465674] [ T2552] PTE[453] = 000000010085c125 [ 53.465679] [ T2552] PTE[454] = 000000010085d125 [ 53.465682] [ T2552] PTE[455] = 000000010085e125 [ 53.465684] [ T2552] PTE[456] = 000000010085f125 [ 53.465686] [ T2552] PTE[457] = 0000000000000000 <-- not present [ 53.465689] [ T2552] PTE[458] = 0000000101da7125

pte_advance_pfn(PTE[456]) returns a pte_none() due to invalid PFN->MFN mapping. The next actual PTE (PTE[457]) is also pte_none(), so the loop continues and includes PTE[457] in the batch, resulting in 5 batched entries for a 4-page folio. This triggers the following warning:

[ 53.465751] [ T2552] page: refcount:85 mapcount:20 mapping:ffff88813ff4f6a8 index:0x110 pfn:0x10085c [ 53.465754] [ T2552] head: order:2 mapcount:80 entire_mapcount:0 nr_pages_mapped:4 pincount:0 [ 53.465756] [ T2552] memcg:ffff888003573000 [ 53.465758] [ T2552] aops:0xffffffff8226fd20 ino:82467c dentry name(?):"libc.so.6" [ 53.465761] [ T2552] flags: 0x2000000000416c(referenced|uptodate|lru|active|private|head|node=0|zone=2) [ 53.465764] [ T2552] raw: 002000000000416c ffffea0004021f08 ffffea0004021908 ffff88813ff4f6a8 [ 53.465767] [ T2552] raw: 0000000000000110 ffff888133d8bd40 0000005500000013 ffff888003573000 [ 53.465768] [ T2552] head: 002000000000416c ffffea0004021f08 ffffea0004021908 ffff88813ff4f6a8 [ 53.465770] [ T2552] head: 0000000000000110 ffff888133d8bd40 0000005500000013 ffff888003573000 [ 53.465772] [ T2552] head: 0020000000000202 ffffea0004021701 000000040000004f 00000000ffffffff [ 53.465774] [ T2552] head: 0000000300000003 8000000300000002 0000000000000013 0000000000000004 [ 53.465775] [ T2552] page dumped because: VM_WARN_ON_FOLIO((_Generic((page + nr_pages - 1), const struct page *: (const struct folio *)_compound_head(page + nr_pages - 1), struct page *: (struct folio *)_compound_head(page + nr_pages - 1))) != folio)

Original code works as expected everywhere, except on XEN PV, where pte_advance_pfn() can yield a pte_none() after balloon inflation due to MFNs invalidation. In XEN, pte_advance_pfn() ends up calling __pte()->xen_make_pte()->pte_pfn_to_mfn(), which returns pte_none() when mfn == INVALID_P2M_ENTRY.

The pte_pfn_to_mfn() documents that nastiness:

If there's no mfn for the pfn, then just create an empty non-present pte. Unfortunately this loses information about the original pfn, so pte_mfn_to_pfn is asymmetric.

While such hacks should certainly be removed, we can do better in folio_pte_batch() and simply check ahead of time how many PTEs we can possibly batch in our folio.

This way, we can not only fix the issue but cleanup the code: removing the pte_pfn() check inside the loop body and avoiding end_ptr comparison + arithmetic.

Link: https://lkml.kernel.org/r/20250502215019.822-2-arkamar@atlas.cz Fixes: f8d937761d65 ("mm/memory: optimize fork() with PTE-mapped THP") Co-developed-by: David Hildenbrand david@redhat.com Signed-off-by: David Hildenbrand david@redhat.com Signed-off-by: Petr Van��k arkamar@atlas.cz Cc: Ryan Roberts ryan.roberts@arm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/internal.h | 27 +++++++++++---------------- 1 file changed, 11 insertions(+), 16 deletions(-)

--- a/mm/internal.h~mm-fix-folio_pte_batch-on-xen-pv +++ a/mm/internal.h @@ -248,11 +248,9 @@ static inline int folio_pte_batch(struct pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags, bool *any_writable, bool *any_young, bool *any_dirty) { - unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio); - const pte_t *end_ptep = start_ptep + max_nr; pte_t expected_pte, *ptep; bool writable, young, dirty; - int nr; + int nr, cur_nr;

if (any_writable) *any_writable = false; @@ -265,11 +263,15 @@ static inline int folio_pte_batch(struct VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio); VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio);

+ /* Limit max_nr to the actual remaining PFNs in the folio we could batch. */ + max_nr = min_t(unsigned long, max_nr, + folio_pfn(folio) + folio_nr_pages(folio) - pte_pfn(pte)); + nr = pte_batch_hint(start_ptep, pte); expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags); ptep = start_ptep + nr;

- while (ptep < end_ptep) { + while (nr < max_nr) { pte = ptep_get(ptep); if (any_writable) writable = !!pte_write(pte); @@ -282,14 +284,6 @@ static inline int folio_pte_batch(struct if (!pte_same(pte, expected_pte)) break;

- /* - * Stop immediately once we reached the end of the folio. In - * corner cases the next PFN might fall into a different - * folio. - */ - if (pte_pfn(pte) >= folio_end_pfn) - break; - if (any_writable) *any_writable |= writable; if (any_young) @@ -297,12 +291,13 @@ static inline int folio_pte_batch(struct if (any_dirty) *any_dirty |= dirty;

- nr = pte_batch_hint(ptep, pte); - expected_pte = pte_advance_pfn(expected_pte, nr); - ptep += nr; + cur_nr = pte_batch_hint(ptep, pte); + expected_pte = pte_advance_pfn(expected_pte, cur_nr); + ptep += cur_nr; + nr += cur_nr; }

- return min(ptep - start_ptep, max_nr); + return min(nr, max_nr); }

/** _

Patches currently in -mm which might be from arkamar@atlas.cz are

mm-fix-folio_pte_batch-on-xen-pv.patch