Date: Mon, 08 Sep 2014 12:59:43 -0500 From: Alan Cox <alc@rice.edu> To: Konstantin Belousov <kostikbel@gmail.com> Cc: alc@freebsd.org, Pieter de Goeje <pieter@degoeje.nl>, hackers@freebsd.org Subject: Re: mmap MAP_NOSYNC regression in 10.x Message-ID: <540DEE8F.5080005@rice.edu> In-Reply-To: <20140908084126.GX2737@kib.kiev.ua> References: <540903FF.6010602@degoeje.nl> <CAJUyCcNiLwLuL9crpQBjSdg4ED5kR53fPjyJG3HNmP5Roor8RQ@mail.gmail.com> <20140905080633.GM2737@kib.kiev.ua> <5409A4F8.6020204@degoeje.nl> <20140905123826.GP2737@kib.kiev.ua> <540BA416.7010106@rice.edu> <20140908084126.GX2737@kib.kiev.ua>
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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
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