From 1b46884a372f362e5edb1bb406bf0c5ca98bade7 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Tue, 19 Oct 2004 18:17:12 -0700 Subject: [PATCH] lighten mmlist_lock Let's lighten the global spinlock mmlist_lock. What's it for? 1. Its original role is to guard mmlist. 2. It later got a second role, to prevent get_task_mm from raising mm_users from the dead, just after it went down to 0. Firstly consider the second: __exit_mm sets tsk->mm NULL while holding task_lock before calling mmput; so mmlist_lock only guards against the exceptional case, of get_task_mm on a kernel workthread which did AIO's use_mm (which transiently sets its tsk->mm without raising mm_users) on an mm now exiting. Well, I don't think get_task_mm should succeed at all on use_mm tasks. It's mainly used by /proc/pid and ptrace, seems at best confusing for those to present the kernel thread as having a user mm, which it won't have a moment later. Define PF_BORROWED_MM, set in use_mm, clear in unuse_mm (though we could just leave it), get_task_mm give NULL if set. Secondly consider the first: and what's mmlist for? 1. Its original role was for swap_out to scan: rmap ended that in 2.5.27. 2. In 2.4.10 it got a second role, for try_to_unuse to scan for swapoff. So, make mmlist a list of mms which maybe have pages on swap: add mm to mmlist when first swap entry is assigned in try_to_unmap_one (pageout), or in copy_page_range (fork); and mmput remove it from mmlist as before, except usually list_empty and there's no need to lock. drain_mmlist added to swapoff, to empty out the mmlist if no swap is then in use. mmput leave mm on mmlist until after its exit_mmap, so try_to_unmap_one can still add mm to mmlist without worrying about the mm_users 0 case; but try_to_unuse must avoid the mm_users 0 case (when an mm might be removed from mmlist, and freed, while it's down in unuse_process): use atomic_inc_return now all architectures support that. Some of the detailed comments in try_to_unuse have grown out of date: updated and trimmed some, but leave SWAP_MAP_MAX for another occasion. Signed-off-by: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/sched.h | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) (limited to 'include') diff --git a/include/linux/sched.h b/include/linux/sched.h index c8f981f108d4..1c556975b2dd 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -218,7 +218,7 @@ struct mm_struct { struct rw_semaphore mmap_sem; spinlock_t page_table_lock; /* Protects task page tables and mm->rss */ - struct list_head mmlist; /* List of all active mm's. These are globally strung + struct list_head mmlist; /* List of maybe swapped mm's. These are globally strung * together off init_mm.mmlist, and are protected * by mmlist_lock */ @@ -252,8 +252,6 @@ struct mm_struct { struct kioctx default_kioctx; }; -extern int mmlist_nr; - struct sighand_struct { atomic_t count; struct k_sigaction action[_NSIG]; @@ -722,6 +720,7 @@ do { if (atomic_dec_and_test(&(tsk)->usage)) __put_task_struct(tsk); } while(0) #define PF_SWAPOFF 0x00080000 /* I am in swapoff */ #define PF_LESS_THROTTLE 0x00100000 /* Throttle me less: I clean memory */ #define PF_SYNCWRITE 0x00200000 /* I am doing a sync write */ +#define PF_BORROWED_MM 0x00400000 /* I am a kthread doing use_mm */ #ifdef CONFIG_SMP extern int set_cpus_allowed(task_t *p, cpumask_t new_mask); -- cgit v1.2.3