mirror of
https://github.com/torvalds/linux.git
synced 2024-11-29 23:51:37 +00:00
57efa1fe59
Since commit70e806e4e6("mm: Do early cow for pinned pages during fork() for ptes") pages under a FOLL_PIN will not be write protected during COW for fork. This means that pages returned from pin_user_pages(FOLL_WRITE) should not become write protected while the pin is active. However, there is a small race where get_user_pages_fast(FOLL_PIN) can establish a FOLL_PIN at the same time copy_present_page() is write protecting it: CPU 0 CPU 1 get_user_pages_fast() internal_get_user_pages_fast() copy_page_range() pte_alloc_map_lock() copy_present_page() atomic_read(has_pinned) == 0 page_maybe_dma_pinned() == false atomic_set(has_pinned, 1); gup_pgd_range() gup_pte_range() pte_t pte = gup_get_pte(ptep) pte_access_permitted(pte) try_grab_compound_head() pte = pte_wrprotect(pte) set_pte_at(); pte_unmap_unlock() // GUP now returns with a write protected page The first attempt to resolve this by using the write protect caused problems (and was missing a barrrier), see commitf3c64eda3e("mm: avoid early COW write protect games during fork()") Instead wrap copy_p4d_range() with the write side of a seqcount and check the read side around gup_pgd_range(). If there is a collision then get_user_pages_fast() fails and falls back to slow GUP. Slow GUP is safe against this race because copy_page_range() is only called while holding the exclusive side of the mmap_lock on the src mm_struct. [akpm@linux-foundation.org: coding style fixes] Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@mail.gmail.com Link: https://lkml.kernel.org/r/2-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com Fixes:f3c64eda3e("mm: avoid early COW write protect games during fork()") Signed-off-by: Jason Gunthorpe <jgg@nvidia.com> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Reviewed-by: John Hubbard <jhubbard@nvidia.com> Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Peter Xu <peterx@redhat.com> Acked-by: "Ahmed S. Darwish" <a.darwish@linutronix.de> [seqcount_t parts] Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> Cc: Christoph Hellwig <hch@lst.de> Cc: Hugh Dickins <hughd@google.com> Cc: Jann Horn <jannh@google.com> Cc: Kirill Shutemov <kirill@shutemov.name> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Leon Romanovsky <leonro@nvidia.com> Cc: Michal Hocko <mhocko@suse.com> Cc: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
43 lines
1.3 KiB
C
43 lines
1.3 KiB
C
// SPDX-License-Identifier: GPL-2.0
|
|
#include <linux/mm_types.h>
|
|
#include <linux/rbtree.h>
|
|
#include <linux/rwsem.h>
|
|
#include <linux/spinlock.h>
|
|
#include <linux/list.h>
|
|
#include <linux/cpumask.h>
|
|
#include <linux/mman.h>
|
|
#include <linux/pgtable.h>
|
|
|
|
#include <linux/atomic.h>
|
|
#include <linux/user_namespace.h>
|
|
#include <asm/mmu.h>
|
|
|
|
#ifndef INIT_MM_CONTEXT
|
|
#define INIT_MM_CONTEXT(name)
|
|
#endif
|
|
|
|
/*
|
|
* For dynamically allocated mm_structs, there is a dynamically sized cpumask
|
|
* at the end of the structure, the size of which depends on the maximum CPU
|
|
* number the system can see. That way we allocate only as much memory for
|
|
* mm_cpumask() as needed for the hundreds, or thousands of processes that
|
|
* a system typically runs.
|
|
*
|
|
* Since there is only one init_mm in the entire system, keep it simple
|
|
* and size this cpu_bitmask to NR_CPUS.
|
|
*/
|
|
struct mm_struct init_mm = {
|
|
.mm_rb = RB_ROOT,
|
|
.pgd = swapper_pg_dir,
|
|
.mm_users = ATOMIC_INIT(2),
|
|
.mm_count = ATOMIC_INIT(1),
|
|
.write_protect_seq = SEQCNT_ZERO(init_mm.write_protect_seq),
|
|
MMAP_LOCK_INITIALIZER(init_mm)
|
|
.page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
|
|
.arg_lock = __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
|
|
.mmlist = LIST_HEAD_INIT(init_mm.mmlist),
|
|
.user_ns = &init_user_ns,
|
|
.cpu_bitmap = CPU_BITS_NONE,
|
|
INIT_MM_CONTEXT(init_mm)
|
|
};
|