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Date: Mon, 28 Sep 2015 13:49:28 +0200
From: Martin Schwidefsky <schwidefsky@...ibm.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Yong Sun <yosun@...e.com>, linux390@...ibm.com,
linux-s390@...r.kernel.org, Zhang Yi <wetpzy@...il.com>,
Mel Gorman <mgorman@...e.de>,
Andrea Arcangeli <aarcange@...hat.com>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Hugh Dickins <hughd@...gle.com>,
Dominik Dingel <dingel@...ux.vnet.ibm.com>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [RFC] futex: prevent endless loop on s390x with emulated
hugepages
On Thu, 24 Sep 2015 17:05:48 +0200
Vlastimil Babka <vbabka@...e.cz> wrote:
> Yong Sun has reported the LTP futex_wake04 test to hang a s390x with our
> kernel based on 3.12. This is reproducible on upstream 4.1.8 as well. 4.2+
> is OK thanks to removal of emulated hugepages, but we should do something
> about the stable kernels here.
>
> The LTP test is a regression test for commit 13d60f4b6a ("futex: Take
> hugepages into account when generating futex_key"), but it turns out that it's
> sufficient to just attempt to wait for a single futex on a tail page of a
> hugetlbfs page:
The software emulated large pages always have been a bit fragile.
> The problem is an endless loop in get_futex_key() when
> CONFIG_TRANSPARENT_HUGEPAGE is enabled and the s390x machine has emulated
> hugepages. The code tries to serialize against __split_huge_page_splitting(),
> but __get_user_pages_fast() fails on the hugetlbfs tail page. This happens
> because pmd_large() is false for emulated hugepages, so the code will proceed
> into gup_pte_range() and fail page_cache_get_speculative() through failing
> get_page_unless_zero() as the tail page count is zero. Failing __gup_fast is
> supposed to be temporary due to a race, so get_futex_key() will try again
> endlessly.
>
> This attempt for a fix is a bandaid solution and probably incomplete.
> Hopefully something better will emerge from the discussion. Fully fixing
> emulated hugepages just for stable backports is unlikely due to them being
> removed. Also THP refcounting redesign should soon remove the trickery from
> get_futex_key().
>
> This patch relies on the fact that s390x with emulated hugepages returns false
> in has_transparent_hugepage(), so we don't need to do the serialization
> trickery and just use the code for !CONFIG_TRANSPARENT_HUGEPAGE. We just need
> an extra variable to cache the result of has_transparent_hugepage(), which is
> __init and potentially expensive on some architectures.
>
> 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. For 3.12 a patch would look like this (needs more testing
though):
--
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;
}
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
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