mmap MAP_NOSYNC regression in 10.x

[Alan Cox]

On 09/08/2014 03:41, Konstantin Belousov wrote:
> On Sat, Sep 06, 2014 at 07:17:26PM -0500, Alan Cox wrote:
>> On 09/05/2014 07:38, Konstantin Belousov wrote:
>>> On Fri, Sep 05, 2014 at 01:56:40PM +0200, Pieter de Goeje wrote:
>>>> Thanks, works for me!
>>> I realized that the patch contains yet another bug. The oflags page
>>> flags update is protected by the exclusive vm object lock, which is only
>>> held in shared mode on the fast path. Below is the fixed patch, where
>>> I take the page lock around setting VPO_NOSYNC (exclusive lock owners
>>> cannot race with fast path since we own shared lock, and two parallel
>>> fast path execution would be handled by the page lock).
>>
>> Suppose that the page is clean and two threads are executing this code
>> concurrently.  One's map entry has MAP_NOSYNC set, and the other's
>> doesn't.  Let's call these threads NOSYNC and SYNC, respectively.
>>
>> Suppose that the thread SYNC is slightly ahead.  It has already
>> performed "m->oflags &= ~VPO_NOSYNC;" and now it's about to perform
>> "vm_page_dirty(fs.m);".  However, just before the thread SYNC calls
>> vm_page_dirty(), the thread NOSYNC evaluates "m->dirty == 0", which is
>> still true, and it performs "m->oflags |= VPO_NOSYNC; "
>>
>> This can't happen on the slow path.  That is, a fault by a thread
>> without MAP_NOSYNC set on its map entry will reliably clear VPO_NOSYNC.
> As I understand things, it is indeed not possible on the slow path, due
> to PG_RW only set from pmap_enter(), am I right ?  I.e. this is another
> place where the rule 'no PG_RW without PG_M' is important.


Yes, it's not possible, but I'm a little confused by the rest of your
question, specifically, the statement "no PG_RW without PG_M".  Did you
actually mean "no PG_M without PG_RW"?


> Let me formulate my question another way:  what are the guarantees we
> provide to the applications when the same page is mapped with and
> without MAP_NOSYNC simultaneously ?  Is it contractually guaranteed that
> any write from !MAP_NOSYNC entry triggers write in the syncer activity
> period ?


Yes, that is the intent.  However, I can think of at least one case
where the existing code doesn't work as intended.  Suppose that the
first fault on a !MAP_NOSYNC entry is triggered by a read access.  Then,
vm_fault() won't call vm_page_dirty(), but it will nonetheless install a
mapping in the pmap that allows write access.  Now, suppose this same
process writes to the page.  Finally, suppose that the second fault
happens on a MAP_NOSYNC entry.  That fault will see a clean page, i.e.,
m->dirty == 0, and set VPO_NOSYNC on the page, even though the first
faulting process that wants the page sync'ed has dirtied the page.



>> The best course of action may be to fall back to the slow path if you
>> actually need to change VPO_NOSYNC's state.  Usually, you won't need to.
>>
> Let me first try to improve the original patch to handle
> MAP_ENTRY_NOSYNC on fast path as well. It seems to be one of the cases
> when the parallel faults are actually useful.


I think it may be time to take a step back, decide what semantics we
really want, and see if there is a better way of implementing those
semantics.  The current approach based on toggling VPO_NOSYNC only
really works for the simplest cases.


> One more note: the previous patch handled m->oflags inconsistency for
> setting VPO_NOSYNC operation, but missed the clear one line later.
> I think that increasing the page lock to cover also the vm_page_dirty()
> would fix the race you described, and the second manipulation with
> oflags.
>
> diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
> index 30b0456..944b479 100644
> --- a/sys/vm/vm_fault.c
> +++ b/sys/vm/vm_fault.c
> @@ -174,6 +174,70 @@ unlock_and_deallocate(struct faultstate *fs)
>  	}
>  }
>  
> +static void
> +vm_fault_dirty(vm_map_entry_t entry, vm_page_t m, vm_prot_t prot,
> +    vm_prot_t fault_type, int fault_flags, boolean_t set_wd)
> +{
> +	boolean_t need_dirty;
> +
> +	if (((prot & VM_PROT_WRITE) == 0 &&
> +	    (fault_flags & VM_FAULT_DIRTY) == 0) ||
> +	    (m->oflags & VPO_UNMANAGED) != 0)
> +		return;
> +
> +	VM_OBJECT_ASSERT_LOCKED(m->object);
> +
> +	need_dirty = ((fault_type & VM_PROT_WRITE) != 0 &&
> +	    (fault_flags & VM_FAULT_CHANGE_WIRING) == 0) ||
> +	    (fault_flags & VM_FAULT_DIRTY) != 0;
> +
> +	if (set_wd)
> +		vm_object_set_writeable_dirty(m->object);
> +	else
> +		/*
> +		 * If two callers of vm_fault_dirty() with set_wd ==
> +		 * FALSE, one for the map entry with MAP_ENTRY_NOSYNC
> +		 * flag set, other with flag clear, race, it is
> +		 * possible for the no-NOSYNC thread to see m->dirty
> +		 * != 0 and not clear VPO_NOSYNC.  Take vm_page lock
> +		 * around manipulation of VPO_NOSYNC and
> +		 * vm_page_dirty() call, to avoid the race and keep
> +		 * m->oflags consistent.
> +		 */
> +		vm_page_lock(m);
> +
> +	/*
> +	 * If this is a NOSYNC mmap we do not want to set VPO_NOSYNC
> +	 * if the page is already dirty to prevent data written with
> +	 * the expectation of being synced from not being synced.
> +	 * Likewise if this entry does not request NOSYNC then make
> +	 * sure the page isn't marked NOSYNC.  Applications sharing
> +	 * data should use the same flags to avoid ping ponging.
> +	 */
> +	if ((entry->eflags & MAP_ENTRY_NOSYNC) != 0) {
> +		if (m->dirty == 0) {
> +			m->oflags |= VPO_NOSYNC;
> +		}
> +	} else {
> +		m->oflags &= ~VPO_NOSYNC;
> +	}
> +
> +	/*
> +	 * If the fault is a write, we know that this page is being
> +	 * written NOW so dirty it explicitly to save on
> +	 * pmap_is_modified() calls later.
> +	 *
> +	 * Also tell the backing pager, if any, that it should remove
> +	 * any swap backing since the page is now dirty.
> +	 */
> +	if (need_dirty)
> +		vm_page_dirty(m);
> +	if (!set_wd)
> +		vm_page_unlock(m);
> +	if (need_dirty)
> +		vm_pager_page_unswapped(m);
> +}
> +
>  /*
>   * TRYPAGER - used by vm_fault to calculate whether the pager for the
>   *	      current object *might* contain the page.
> @@ -321,11 +385,8 @@ RetryFault:;
>  			vm_page_hold(m);
>  			vm_page_unlock(m);
>  		}
> -		if ((fault_type & VM_PROT_WRITE) != 0 &&
> -		    (m->oflags & VPO_UNMANAGED) == 0) {
> -			vm_page_dirty(m);
> -			vm_pager_page_unswapped(m);
> -		}
> +		vm_fault_dirty(fs.entry, m, prot, fault_type, fault_flags,
> +		    FALSE);
>  		VM_OBJECT_RUNLOCK(fs.first_object);
>  		if (!wired)
>  			vm_fault_prefault(&fs, vaddr, 0, 0);
> @@ -898,42 +959,7 @@ vnode_locked:
>  	if (hardfault)
>  		fs.entry->next_read = fs.pindex + faultcount - reqpage;
>  
> -	if (((prot & VM_PROT_WRITE) != 0 ||
> -	    (fault_flags & VM_FAULT_DIRTY) != 0) &&
> -	    (fs.m->oflags & VPO_UNMANAGED) == 0) {
> -		vm_object_set_writeable_dirty(fs.object);
> -
> -		/*
> -		 * If this is a NOSYNC mmap we do not want to set VPO_NOSYNC
> -		 * if the page is already dirty to prevent data written with
> -		 * the expectation of being synced from not being synced.
> -		 * Likewise if this entry does not request NOSYNC then make
> -		 * sure the page isn't marked NOSYNC.  Applications sharing
> -		 * data should use the same flags to avoid ping ponging.
> -		 */
> -		if (fs.entry->eflags & MAP_ENTRY_NOSYNC) {
> -			if (fs.m->dirty == 0)
> -				fs.m->oflags |= VPO_NOSYNC;
> -		} else {
> -			fs.m->oflags &= ~VPO_NOSYNC;
> -		}
> -
> -		/*
> -		 * If the fault is a write, we know that this page is being
> -		 * written NOW so dirty it explicitly to save on 
> -		 * pmap_is_modified() calls later.
> -		 *
> -		 * Also tell the backing pager, if any, that it should remove
> -		 * any swap backing since the page is now dirty.
> -		 */
> -		if (((fault_type & VM_PROT_WRITE) != 0 &&
> -		    (fault_flags & VM_FAULT_CHANGE_WIRING) == 0) ||
> -		    (fault_flags & VM_FAULT_DIRTY) != 0) {
> -			vm_page_dirty(fs.m);
> -			vm_pager_page_unswapped(fs.m);
> -		}
> -	}
> -
> +	vm_fault_dirty(fs.entry, fs.m, prot, fault_type, fault_flags, TRUE);
>  	vm_page_assert_xbusied(fs.m);
>  
>  	/*
> diff --git a/sys/vm/vm_page.h b/sys/vm/vm_page.h
> index f12b76c..a45648d 100644
> --- a/sys/vm/vm_page.h
> +++ b/sys/vm/vm_page.h
> @@ -147,7 +147,7 @@ struct vm_page {
>  	uint16_t hold_count;		/* page hold count (P) */
>  	uint16_t flags;			/* page PG_* flags (P) */
>  	uint8_t aflags;			/* access is atomic */
> -	uint8_t oflags;			/* page VPO_* flags (O) */
> +	uint8_t oflags;			/* page VPO_* flags (OM) */
>  	uint8_t	queue;			/* page queue index (P,Q) */
>  	int8_t psind;			/* pagesizes[] index (O) */
>  	int8_t segind;
> @@ -163,8 +163,9 @@ struct vm_page {
>  /*
>   * Page flags stored in oflags:
>   *
> - * Access to these page flags is synchronized by the lock on the object
> - * containing the page (O).
> + * Access to these page flags is synchronized by the exclusive lock on
> + * the object containing the page, or combination of shared object
> + * lock and the page lock (OM).
>   *
>   * Note: VPO_UNMANAGED (used by OBJT_DEVICE, OBJT_PHYS and OBJT_SG)
>   * 	 indicates that the page is not under PV management but