Re: [RFC] futex: prevent endless loop on s390x with emulated hugepages

Vlastimil Babka <vbabka@xxxxxxx> · Tue, 13 Oct 2015 13:48:07 +0200

On 09/28/2015 01:49 PM, Martin Schwidefsky wrote:
On Thu, 24 Sep 2015 17:05:48 +0200
Vlastimil Babka <vbabka@xxxxxxx> wrote:

[...]

However, __get_user_pages_fast() is still broken. The get_user_pages_fast()
wrapper will hide this in the common case. The other user of the __ variant
is kvm, which is mentioned as the reason for removal of emulated hugepages.
The call of page_cache_get_speculative() looks also broken in this scenario
on debug builds because of VM_BUG_ON_PAGE(PageTail(page), page). With
CONFIG_TINY_RCU enabled, there's plain atomic_inc(&page->_count) which also
probably shouldn't happen for a tail page...

It boils down to __get_user_pages_fast being broken for emulated large pages,
doesn't it? My preferred fix would be to get __get_user_page_fast to work
in this case.

I agree, but didn't know enough of the architecture to attempt such fix 
:) Thanks!

For 3.12 a patch would look like this (needs more testing
though):

FWIW it works for me in the particular LTP test, but as you said, it 
needs more testing and breaking stable would suck.

--

diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
index bb0c157..5948b7f 100644
--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -370,7 +370,7 @@ static inline int is_module_addr(void *addr)
  #define _SEGMENT_ENTRY_EMPTY	(_SEGMENT_ENTRY_INVALID)

  #define _SEGMENT_ENTRY_LARGE	0x400	/* STE-format control, large page   */
-#define _SEGMENT_ENTRY_CO	0x100	/* change-recording override   */
+#define _SEGMENT_ENTRY_SWLARGE	0x100	/* SW large page bit */
  #define _SEGMENT_ENTRY_SPLIT	0x001	/* THP splitting bit */
  #define _SEGMENT_ENTRY_YOUNG	0x002	/* SW segment young bit */
  #define _SEGMENT_ENTRY_NONE	_SEGMENT_ENTRY_YOUNG
@@ -391,8 +391,8 @@ static inline int is_module_addr(void *addr)
  #define _SEGMENT_ENTRY_SPLIT_BIT 0	/* THP splitting bit number */

  /* Set of bits not changed in pmd_modify */
-#define _SEGMENT_CHG_MASK	(_SEGMENT_ENTRY_ORIGIN | _SEGMENT_ENTRY_LARGE \
-				 | _SEGMENT_ENTRY_SPLIT | _SEGMENT_ENTRY_CO)
+#define _SEGMENT_CHG_MASK (_SEGMENT_ENTRY_ORIGIN | _SEGMENT_ENTRY_LARGE \
+			   | _SEGMENT_ENTRY_SPLIT | _SEGMENT_ENTRY_SWLARGE)

  /* Page status table bits for virtualization */
  #define PGSTE_ACC_BITS	0xf000000000000000UL
@@ -563,12 +563,25 @@ static inline int pmd_none(pmd_t pmd)
  static inline int pmd_large(pmd_t pmd)
  {
  #ifdef CONFIG_64BIT
-	return (pmd_val(pmd) & _SEGMENT_ENTRY_LARGE) != 0;
+	return (pmd_val(pmd) &
+		(_SEGMENT_ENTRY_LARGE | _SEGMENT_ENTRY_SWLARGE)) != 0;
  #else
  	return 0;
  #endif
  }

+static inline pmd_t pmd_swlarge_deref(pmd_t pmd)
+{
+	unsigned long origin;
+
+	if (pmd_val(pmd) & _SEGMENT_ENTRY_SWLARGE) {
+		origin = pmd_val(pmd) & _SEGMENT_ENTRY_ORIGIN;
+		pmd_val(pmd) &= ~_SEGMENT_ENTRY_ORIGIN;
+		pmd_val(pmd) |= *(unsigned long *) origin;
+	}
+	return pmd;
+}
+
  static inline int pmd_prot_none(pmd_t pmd)
  {
  	return (pmd_val(pmd) & _SEGMENT_ENTRY_INVALID) &&
@@ -578,8 +591,10 @@ static inline int pmd_prot_none(pmd_t pmd)
  static inline int pmd_bad(pmd_t pmd)
  {
  #ifdef CONFIG_64BIT
-	if (pmd_large(pmd))
+	if (pmd_large(pmd)) {
+		pmd = pmd_swlarge_deref(pmd);
  		return (pmd_val(pmd) & ~_SEGMENT_ENTRY_BITS_LARGE) != 0;
+	}
  #endif
  	return (pmd_val(pmd) & ~_SEGMENT_ENTRY_BITS) != 0;
  }
@@ -1495,8 +1510,6 @@ static inline int pmd_trans_splitting(pmd_t pmd)
  static inline void set_pmd_at(struct mm_struct *mm, unsigned long addr,
  			      pmd_t *pmdp, pmd_t entry)
  {
-	if (!(pmd_val(entry) & _SEGMENT_ENTRY_INVALID) && MACHINE_HAS_EDAT1)
-		pmd_val(entry) |= _SEGMENT_ENTRY_CO;
  	*pmdp = entry;
  }

diff --git a/arch/s390/mm/dump_pagetables.c b/arch/s390/mm/dump_pagetables.c
index 46d517c..e159735 100644
--- a/arch/s390/mm/dump_pagetables.c
+++ b/arch/s390/mm/dump_pagetables.c
@@ -129,7 +129,7 @@ static void walk_pte_level(struct seq_file *m, struct pg_state *st,
  }

  #ifdef CONFIG_64BIT
-#define _PMD_PROT_MASK (_SEGMENT_ENTRY_PROTECT | _SEGMENT_ENTRY_CO)
+#define _PMD_PROT_MASK (_SEGMENT_ENTRY_PROTECT)
  #else
  #define _PMD_PROT_MASK 0
  #endif
@@ -138,7 +138,7 @@ static void walk_pmd_level(struct seq_file *m, struct pg_state *st,
  			   pud_t *pud, unsigned long addr)
  {
  	unsigned int prot;
-	pmd_t *pmd;
+	pmd_t *pmd, pmd_val;
  	int i;

  	for (i = 0; i < PTRS_PER_PMD && addr < max_addr; i++) {
@@ -146,7 +146,8 @@ static void walk_pmd_level(struct seq_file *m, struct pg_state *st,
  		pmd = pmd_offset(pud, addr);
  		if (!pmd_none(*pmd)) {
  			if (pmd_large(*pmd)) {
-				prot = pmd_val(*pmd) & _PMD_PROT_MASK;
+				pmd_val = pmd_swlarge_deref(*pmd);
+				prot = pmd_val(pmd_val) & _PMD_PROT_MASK;
  				note_page(m, st, prot, 3);
  			} else
  				walk_pte_level(m, st, pmd, addr);
diff --git a/arch/s390/mm/gup.c b/arch/s390/mm/gup.c
index 5d758db..8dd86a5 100644
--- a/arch/s390/mm/gup.c
+++ b/arch/s390/mm/gup.c
@@ -48,8 +48,9 @@ static inline int gup_pte_range(pmd_t *pmdp, pmd_t pmd, unsigned long addr,
  	return 1;
  }

-static inline int gup_huge_pmd(pmd_t *pmdp, pmd_t pmd, unsigned long addr,
-		unsigned long end, int write, struct page **pages, int *nr)
+static inline int gup_huge_pmd(pmd_t *pmdp, pmd_t pmd_orig, pmd_t pmd,
+		unsigned long addr, unsigned long end,
+		int write, struct page **pages, int *nr)
  {
  	unsigned long mask, result;
  	struct page *head, *page, *tail;
@@ -78,7 +79,7 @@ static inline int gup_huge_pmd(pmd_t *pmdp, pmd_t pmd, unsigned long addr,
  		return 0;
  	}

-	if (unlikely(pmd_val(pmd) != pmd_val(*pmdp))) {
+	if (unlikely(pmd_val(pmd_orig) != pmd_val(*pmdp))) {
  		*nr -= refs;
  		while (refs--)
  			put_page(head);
@@ -103,7 +104,7 @@ static inline int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr,
  		unsigned long end, int write, struct page **pages, int *nr)
  {
  	unsigned long next;
-	pmd_t *pmdp, pmd;
+	pmd_t *pmdp, pmd, pmd_orig;

  	pmdp = (pmd_t *) pudp;
  #ifdef CONFIG_64BIT
@@ -112,7 +113,7 @@ static inline int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr,
  	pmdp += pmd_index(addr);
  #endif
  	do {
-		pmd = *pmdp;
+		pmd = pmd_orig = *pmdp;
  		barrier();
  		next = pmd_addr_end(addr, end);
  		/*
@@ -127,8 +128,9 @@ static inline int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr,
  		if (pmd_none(pmd) || pmd_trans_splitting(pmd))
  			return 0;
  		if (unlikely(pmd_large(pmd))) {
-			if (!gup_huge_pmd(pmdp, pmd, addr, next,
-					  write, pages, nr))
+			if (!gup_huge_pmd(pmdp, pmd_orig,
+					  pmd_swlarge_deref(pmd),
+					  addr, next, write, pages, nr))
  				return 0;
  		} else if (!gup_pte_range(pmdp, pmd, addr, next,
  					  write, pages, nr))
diff --git a/arch/s390/mm/hugetlbpage.c b/arch/s390/mm/hugetlbpage.c
index 99a68d5..d4a17d9 100644
--- a/arch/s390/mm/hugetlbpage.c
+++ b/arch/s390/mm/hugetlbpage.c
@@ -98,22 +98,18 @@ void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
  	if (!MACHINE_HAS_HPAGE) {
  		pmd_val(pmd) &= ~_SEGMENT_ENTRY_ORIGIN;
  		pmd_val(pmd) |= pte_page(pte)[1].index;
+		pmd_val(pmd) |= _SEGMENT_ENTRY_SWLARGE;
  	} else
-		pmd_val(pmd) |= _SEGMENT_ENTRY_LARGE | _SEGMENT_ENTRY_CO;
+		pmd_val(pmd) |= _SEGMENT_ENTRY_LARGE;
  	*(pmd_t *) ptep = pmd;
  }

  pte_t huge_ptep_get(pte_t *ptep)
  {
-	unsigned long origin;
  	pmd_t pmd;

  	pmd = *(pmd_t *) ptep;
-	if (!MACHINE_HAS_HPAGE && pmd_present(pmd)) {
-		origin = pmd_val(pmd) & _SEGMENT_ENTRY_ORIGIN;
-		pmd_val(pmd) &= ~_SEGMENT_ENTRY_ORIGIN;
-		pmd_val(pmd) |= *(unsigned long *) origin;
-	}
+	pmd = pmd_swlarge_deref(pmd);
  	return __pmd_to_pte(pmd);
  }

diff --git a/arch/s390/mm/vmem.c b/arch/s390/mm/vmem.c
index bcfb70b..a4f6f2f 100644
--- a/arch/s390/mm/vmem.c
+++ b/arch/s390/mm/vmem.c
@@ -235,8 +235,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node)
  				if (!new_page)
  					goto out;
  				pmd_val(*pm_dir) = __pa(new_page) |
-					_SEGMENT_ENTRY | _SEGMENT_ENTRY_LARGE |
-					_SEGMENT_ENTRY_CO;
+					_SEGMENT_ENTRY | _SEGMENT_ENTRY_LARGE;
  				address = (address + PMD_SIZE) & PMD_MASK;
  				continue;
  			}


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