[PATCH 62/91] [PATCH] x86, mm: Add __get_user_pages_fast()
From: Willy Tarreau <w@1wt.eu>
Date: 2012-02-05 22:27:38
Also in:
lkml
2.6.27-longterm review patch. If anyone has any objections, please let us know. ------------------ Introduce a gup_fast() variant which is usable from IRQ/NMI context. [ WT: this one is only needed for next patch ] Signed-off-by: Peter Zijlstra <redacted> CC: Nick Piggin <redacted> Cc: Mike Galbraith <redacted> Cc: Paul Mackerras <redacted> Cc: Arnaldo Carvalho de Melo <redacted> LKML-Reference: <new-submission> Signed-off-by: Ingo Molnar <redacted> --- arch/x86/mm/gup.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++ include/linux/mm.h | 6 +++++ 2 files changed, 62 insertions(+), 0 deletions(-) Index: longterm-2.6.27/arch/x86/mm/gup.c ===================================================================
--- longterm-2.6.27.orig/arch/x86/mm/gup.c 2012-02-05 22:34:33.105915236 +0100
+++ longterm-2.6.27/arch/x86/mm/gup.c 2012-02-05 22:34:43.693914996 +0100@@ -219,6 +219,62 @@ return 1; } +/* + * Like get_user_pages_fast() except its IRQ-safe in that it won't fall + * back to the regular GUP. + */ +int __get_user_pages_fast(unsigned long start, int nr_pages, int write, + struct page **pages) +{ + struct mm_struct *mm = current->mm; + unsigned long addr, len, end; + unsigned long next; + unsigned long flags; + pgd_t *pgdp; + int nr = 0; + + start &= PAGE_MASK; + addr = start; + len = (unsigned long) nr_pages << PAGE_SHIFT; + end = start + len; + if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ, + (void __user *)start, len))) + return 0; + + /* + * XXX: batch / limit 'nr', to avoid large irq off latency + * needs some instrumenting to determine the common sizes used by + * important workloads (eg. DB2), and whether limiting the batch size + * will decrease performance. + * + * It seems like we're in the clear for the moment. Direct-IO is + * the main guy that batches up lots of get_user_pages, and even + * they are limited to 64-at-a-time which is not so many. + */ + /* + * This doesn't prevent pagetable teardown, but does prevent + * the pagetables and pages from being freed on x86. + * + * So long as we atomically load page table pointers versus teardown + * (which we do on x86, with the above PAE exception), we can follow the + * address down to the the page and take a ref on it. + */ + local_irq_save(flags); + pgdp = pgd_offset(mm, addr); + do { + pgd_t pgd = *pgdp; + + next = pgd_addr_end(addr, end); + if (pgd_none(pgd)) + break; + if (!gup_pud_range(pgd, addr, next, write, pages, &nr)) + break; + } while (pgdp++, addr = next, addr != end); + local_irq_restore(flags); + + return nr; +} + int get_user_pages_fast(unsigned long start, int nr_pages, int write, struct page **pages) {
Index: longterm-2.6.27/include/linux/mm.h ===================================================================
--- longterm-2.6.27.orig/include/linux/mm.h 2012-02-05 22:34:33.099915218 +0100
+++ longterm-2.6.27/include/linux/mm.h 2012-02-05 22:34:43.701916289 +0100@@ -850,6 +850,12 @@ struct page **pages); /* + * doesn't attempt to fault and will return short. + */ +int __get_user_pages_fast(unsigned long start, int nr_pages, int write, + struct page **pages); + +/* * A callback you can register to apply pressure to ageable caches. * * 'shrink' is passed a count 'nr_to_scan' and a 'gfpmask'. It should