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Date: Tue, 13 Oct 2015 13:48:07 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Martin Schwidefsky <schwidefsky@...ibm.com>
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 09/28/2015 01:49 PM, Martin Schwidefsky wrote:
> On Thu, 24 Sep 2015 17:05:48 +0200
> Vlastimil Babka <vbabka@...e.cz> 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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