diff options
Diffstat (limited to 'mm/gup.c')
| -rw-r--r-- | mm/gup.c | 18 |
1 files changed, 15 insertions, 3 deletions
@@ -258,6 +258,11 @@ unmap: return ret; } +/* + * mmap_sem must be held on entry. If @nonblocking != NULL and + * *@flags does not include FOLL_NOWAIT, the mmap_sem may be released. + * If it is, *@nonblocking will be set to 0 and -EBUSY returned. + */ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma, unsigned long address, unsigned int *flags, int *nonblocking) { @@ -373,7 +378,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) * with a put_page() call when it is finished with. vmas will only * remain valid while mmap_sem is held. * - * Must be called with mmap_sem held for read or write. + * Must be called with mmap_sem held. It may be released. See below. * * __get_user_pages walks a process's page tables and takes a reference to * each struct page that each user address corresponds to at a given @@ -396,7 +401,14 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) * * If @nonblocking != NULL, __get_user_pages will not wait for disk IO * or mmap_sem contention, and if waiting is needed to pin all pages, - * *@nonblocking will be set to 0. + * *@nonblocking will be set to 0. Further, if @gup_flags does not + * include FOLL_NOWAIT, the mmap_sem will be released via up_read() in + * this case. + * + * A caller using such a combination of @nonblocking and @gup_flags + * must therefore hold the mmap_sem for reading only, and recognize + * when it's been released. Otherwise, it must be held for either + * reading or writing and will not be released. * * In most cases, get_user_pages or get_user_pages_fast should be used * instead of __get_user_pages. __get_user_pages should be used only if @@ -528,7 +540,7 @@ EXPORT_SYMBOL(__get_user_pages); * such architectures, gup() will not be enough to make a subsequent access * succeed. * - * This should be called with the mm_sem held for read. + * This has the same semantics wrt the @mm->mmap_sem as does filemap_fault(). */ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm, unsigned long address, unsigned int fault_flags) |