From 70b5928f5cd289b2ccf34384ca83b1d9ee7a0fad Mon Sep 17 00:00:00 2001 From: Ben Hutchings Date: Mon, 24 Jan 2022 16:11:18 +0100 Subject: [PATCH] mips,s390,sh,sparc: gup: Work around the "COW can break either way" issue In Linux 4.14 and 4.19 these architectures still have their own implementations of get_user_pages_fast(). These also need to force the write flag on when taking the fast path. Fixes: 407faed92b4a ("gup: document and work around "COW can break either way" issue") Fixes: 5e24029791e8 ("gup: document and work around "COW can break either way" issue") Signed-off-by: Ben Hutchings --- arch/mips/mm/gup.c | 9 ++++++++- arch/s390/mm/gup.c | 9 ++++++++- arch/sh/mm/gup.c | 9 ++++++++- arch/sparc/mm/gup.c | 9 ++++++++- 4 files changed, 32 insertions(+), 4 deletions(-) diff --git a/arch/mips/mm/gup.c b/arch/mips/mm/gup.c index 1e4658eee13fb..5a535220d7cd8 100644 --- a/arch/mips/mm/gup.c +++ b/arch/mips/mm/gup.c @@ -272,7 +272,14 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write, next = pgd_addr_end(addr, end); if (pgd_none(pgd)) goto slow; - if (!gup_pud_range(pgd, addr, next, write, pages, &nr)) + /* + * The FAST_GUP case requires FOLL_WRITE even for pure reads, + * because get_user_pages() may need to cause an early COW in + * order to avoid confusing the normal COW routines. So only + * targets that are already writable are safe to do by just + * looking at the page tables. + */ + if (!gup_pud_range(pgd, addr, next, 1, pages, &nr)) goto slow; } while (pgdp++, addr = next, addr != end); local_irq_enable(); diff --git a/arch/s390/mm/gup.c b/arch/s390/mm/gup.c index 9bce54eac0b07..f26ca7fbd44ef 100644 --- a/arch/s390/mm/gup.c +++ b/arch/s390/mm/gup.c @@ -285,7 +285,14 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write, might_sleep(); start &= PAGE_MASK; - nr = __get_user_pages_fast(start, nr_pages, write, pages); + /* + * The FAST_GUP case requires FOLL_WRITE even for pure reads, + * because get_user_pages() may need to cause an early COW in + * order to avoid confusing the normal COW routines. So only + * targets that are already writable are safe to do by just + * looking at the page tables. + */ + nr = __get_user_pages_fast(start, nr_pages, 1, pages); if (nr == nr_pages) return nr; diff --git a/arch/sh/mm/gup.c b/arch/sh/mm/gup.c index 8045b5bb7075e..4abfc37dd7fe8 100644 --- a/arch/sh/mm/gup.c +++ b/arch/sh/mm/gup.c @@ -240,7 +240,14 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write, next = pgd_addr_end(addr, end); if (pgd_none(pgd)) goto slow; - if (!gup_pud_range(pgd, addr, next, write, pages, &nr)) + /* + * The FAST_GUP case requires FOLL_WRITE even for pure reads, + * because get_user_pages() may need to cause an early COW in + * order to avoid confusing the normal COW routines. So only + * targets that are already writable are safe to do by just + * looking at the page tables. + */ + if (!gup_pud_range(pgd, addr, next, 1, pages, &nr)) goto slow; } while (pgdp++, addr = next, addr != end); local_irq_enable(); diff --git a/arch/sparc/mm/gup.c b/arch/sparc/mm/gup.c index 5335ba3c850ed..50593ca021449 100644 --- a/arch/sparc/mm/gup.c +++ b/arch/sparc/mm/gup.c @@ -262,7 +262,14 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write, next = pgd_addr_end(addr, end); if (pgd_none(pgd)) goto slow; - if (!gup_pud_range(pgd, addr, next, write, pages, &nr)) + /* + * The FAST_GUP case requires FOLL_WRITE even for pure reads, + * because get_user_pages() may need to cause an early COW in + * order to avoid confusing the normal COW routines. So only + * targets that are already writable are safe to do by just + * looking at the page tables. + */ + if (!gup_pud_range(pgd, addr, next, 1, pages, &nr)) goto slow; } while (pgdp++, addr = next, addr != end); -- 2.47.3