From: Jason Gunthorpe jgg@nvidia.com
[ Upstream commit c28b1fc70390df32e29991eedd52bd86e7aba080 ]
Patch series "Add a seqcount between gup_fast and copy_page_range()", v4.
As discussed and suggested by Linus use a seqcount to close the small race between gup_fast and copy_page_range().
Ahmed confirms that raw_write_seqcount_begin() is the correct API to use in this case and it doesn't trigger any lockdeps.
I was able to test it using two threads, one forking and the other using ibv_reg_mr() to trigger GUP fast. Modifying copy_page_range() to sleep made the window large enough to reliably hit to test the logic.
This patch (of 2):
The next patch in this series makes the lockless flow a little more complex, so move the entire block into a new function and remove a level of indention. Tidy a bit of cruft:
- addr is always the same as start, so use start
- Use the modern check_add_overflow() for computing end = start + len
- nr_pinned/pages << PAGE_SHIFT needs the LHS to be unsigned long to avoid shift overflow, make the variables unsigned long to avoid coding casts in both places. nr_pinned was missing its cast
- The handling of ret and nr_pinned can be streamlined a bit
No functional change.
Link: https://lkml.kernel.org/r/0-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com Link: https://lkml.kernel.org/r/1-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com Signed-off-by: Jason Gunthorpe jgg@nvidia.com Reviewed-by: Jan Kara jack@suse.cz Reviewed-by: John Hubbard jhubbard@nvidia.com Reviewed-by: Peter Xu peterx@redhat.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- mm/gup.c | 99 ++++++++++++++++++++++++++++++-------------------------- 1 file changed, 54 insertions(+), 45 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c index 98eb8e6d2609c..c7e24301860ab 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2677,13 +2677,43 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages, return ret; }
-static int internal_get_user_pages_fast(unsigned long start, int nr_pages, +static unsigned long lockless_pages_from_mm(unsigned long start, + unsigned long end, + unsigned int gup_flags, + struct page **pages) +{ + unsigned long flags; + int nr_pinned = 0; + + if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) || + !gup_fast_permitted(start, end)) + return 0; + + /* + * Disable interrupts. The nested form is used, in order to allow full, + * general purpose use of this routine. + * + * With interrupts disabled, we block page table pages from being freed + * from under us. See struct mmu_table_batch comments in + * include/asm-generic/tlb.h for more details. + * + * We do not adopt an rcu_read_lock() here as we also want to block IPIs + * that come from THPs splitting. + */ + local_irq_save(flags); + gup_pgd_range(start, end, gup_flags, pages, &nr_pinned); + local_irq_restore(flags); + return nr_pinned; +} + +static int internal_get_user_pages_fast(unsigned long start, + unsigned long nr_pages, unsigned int gup_flags, struct page **pages) { - unsigned long addr, len, end; - unsigned long flags; - int nr_pinned = 0, ret = 0; + unsigned long len, end; + unsigned long nr_pinned; + int ret;
if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM | FOLL_FORCE | FOLL_PIN | FOLL_GET | @@ -2697,54 +2727,33 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages, might_lock_read(¤t->mm->mmap_lock);
start = untagged_addr(start) & PAGE_MASK; - addr = start; - len = (unsigned long) nr_pages << PAGE_SHIFT; - end = start + len; - - if (end <= start) + len = nr_pages << PAGE_SHIFT; + if (check_add_overflow(start, len, &end)) return 0; if (unlikely(!access_ok((void __user *)start, len))) return -EFAULT;
- /* - * Disable interrupts. The nested form is used, in order to allow - * full, general purpose use of this routine. - * - * With interrupts disabled, we block page table pages from being - * freed from under us. See struct mmu_table_batch comments in - * include/asm-generic/tlb.h for more details. - * - * We do not adopt an rcu_read_lock(.) here as we also want to - * block IPIs that come from THPs splitting. - */ - if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) { - unsigned long fast_flags = gup_flags; - - local_irq_save(flags); - gup_pgd_range(addr, end, fast_flags, pages, &nr_pinned); - local_irq_restore(flags); - ret = nr_pinned; - } - - if (nr_pinned < nr_pages && !(gup_flags & FOLL_FAST_ONLY)) { - /* Try to get the remaining pages with get_user_pages */ - start += nr_pinned << PAGE_SHIFT; - pages += nr_pinned; + nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages); + if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY) + return nr_pinned;
- ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, - gup_flags, pages); - - /* Have to be a bit careful with return values */ - if (nr_pinned > 0) { - if (ret < 0) - ret = nr_pinned; - else - ret += nr_pinned; - } + /* Slow path: try to get the remaining pages with get_user_pages */ + start += nr_pinned << PAGE_SHIFT; + pages += nr_pinned; + ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags, + pages); + if (ret < 0) { + /* + * The caller has to unpin the pages we already pinned so + * returning -errno is not an option + */ + if (nr_pinned) + return nr_pinned; + return ret; } - - return ret; + return ret + nr_pinned; } + /** * get_user_pages_fast_only() - pin user pages in memory * @start: starting user address