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Message-ID: <20250502215019.822-2-arkamar@atlas.cz>
Date: Fri, 2 May 2025 23:50:19 +0200
From: Petr Vaněk <arkamar@...as.cz>
To: linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Ryan Roberts <ryan.roberts@....com>,
xen-devel@...ts.xenproject.org,
x86@...nel.org,
stable@...r.kernel.org,
Petr Vaněk <arkamar@...as.cz>
Subject: [PATCH v2 1/1] mm: fix folio_pte_batch() on XEN PV
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.
Fixes: f8d937761d65 ("mm/memory: optimize fork() with PTE-mapped THP")
Cc: stable@...r.kernel.org
Co-developed-by: David Hildenbrand <david@...hat.com>
Signed-off-by: David Hildenbrand <david@...hat.com>
Signed-off-by: Petr Vaněk <arkamar@...as.cz>
---
mm/internal.h | 27 +++++++++++----------------
1 file changed, 11 insertions(+), 16 deletions(-)
diff --git a/mm/internal.h b/mm/internal.h
index e9695baa5922..25a29872c634 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -248,11 +248,9 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
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 folio *folio, unsigned long addr,
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 folio *folio, unsigned long addr,
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 folio *folio, unsigned long addr,
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);
}
/**
--
2.48.1
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