[2/2] mm: remove get_user_pages_locked()

Commit Message

Paolo Bonzini Oct. 31, 2016, 5:55 p.m. UTC

On 31/10/2016 14:48, Lorenzo Stoakes wrote:
> On Mon, Oct 31, 2016 at 12:45:36PM +0100, Paolo Bonzini wrote:
>>
>>
>> On 31/10/2016 11:02, Lorenzo Stoakes wrote:
>>> - *
>>> - * get_user_pages should be phased out in favor of
>>> - * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing
>>> - * should use get_user_pages because it cannot pass
>>> - * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.
>>
>> This comment should be preserved in some way.  In addition, removing
> 
> Could you clarify what you think should be retained?
> 
> The comment seems to me to be largely rendered redundant by this change -
> get_user_pages() now offers identical behaviour, and of course the latter part
> of the comment ('because it cannot pass FAULT_FLAG_ALLOW_RETRY') is rendered
> incorrect by this change.
> 
> It could be replaced with a recommendation to make use of VM_FAULT_RETRY logic
> if possible.

Yes, exactly.  locked == NULL should be avoided whenever mmap_sem can 
be dropped, and the comment indirectly said so.  Now most of those cases
actually are those where you can just call get_user_pages_unlocked.

>> get_user_pages_locked() makes it harder (compared to a simple "git grep
>> -w") to identify callers that lack allow-retry functionality).  So I'm
>> not sure about the benefits of these patches.
> 
> That's a fair point, and certainly this cleanup series is less obviously helpful
> than previous ones.
> 
> However, there are a few points in its favour:
> 
> 1. get_user_pages_remote() was the mirror of get_user_pages() prior to adding an
>    int *locked parameter to the former (necessary to allow for the unexport of
>    __get_user_pages_unlocked()), differing only in task/mm being specified and
>    FOLL_REMOTE being set. This patch series keeps these functions 'synchronised'
>    in this respect.
> 
> 2. There is currently only one caller of get_user_pages_locked() in
>    mm/frame_vector.c which seems to suggest this function isn't widely
>    used/known.

Or not widely necessary. :)

> 3. This change results in all slow-path get_user_pages*() functions having the
>    ability to use VM_FAULT_RETRY logic rather than 'defaulting' to using
>    get_user_pages() that doesn't let you do this even if you wanted to.

This is only true if someone does the work though.  From a quick look 
at your series, the following files can be trivially changed to use 
get_user_pages_unlocked:

- drivers/gpu/drm/via/via_dmablit.c
- drivers/platform/goldfish/goldfish_pipe.c
- drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
- drivers/rapidio/devices/rio_mport_cdev.c
- drivers/virt/fsl_hypervisor.c

Also, videobuf_dma_init_user could be changed to use retry by adding a 
*locked argument to videobuf_dma_init_user_locked, prototype patch 
after my signature.

Everything else is probably best kept using get_user_pages.

> 4. It's somewhat confusing/redundant having both get_user_pages_locked() and
>    get_user_pages() functions which both require mmap_sem to be held (i.e. both
>    are 'locked' versions.)
> 
>> If all callers were changed, then sure removing the _locked suffix would
>> be a good idea.
> 
> It seems many callers cannot release mmap_sem so VM_FAULT_RETRY logic couldn't
> happen anyway in this cases and in these cases we couldn't change the caller.

But then get_user_pages_locked remains a less-common case, so perhaps 
it's a good thing to give it a longer name!

> Overall, an alternative here might be to remove get_user_pages() and update
> get_user_pages_locked() to add a 'vmas' parameter, however doing this renders
> get_user_pages_unlocked() asymmetric as it would lack an vmas parameter (adding
> one there would make no sense as VM_FAULT_RETRY logic invalidates VMAs) though
> perhaps not such a big issue as it makes sense as to why this is the case.

Adding the 'vmas' parameter to get_user_pages_locked would make little 
sense.  Since VM_FAULT_RETRY invalidates it and g_u_p_locked can and 
does retry, it would generally not be useful.

So I think we have the right API now:

- do not have lock?  Use get_user_pages_unlocked, get retry for free,
no need to handle  mmap_sem and the locked argument; cannot get back vmas.

- have and cannot drop lock?  User get_user_pages, no need to pass NULL 
for the locked argument; can get back vmas.

- have but can drop lock (rare case)?  Use get_user_pages_locked, 
cannot get back vams.

Paolo

> get_user_pages_unlocked() definitely seems to be a useful helper and therefore
> makes sense to retain.

> Of course another alternative is to leave things be :)
> 

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Comments

Lorenzo Stoakes Oct. 31, 2016, 7:28 p.m. UTC | #1

On Mon, Oct 31, 2016 at 06:55:33PM +0100, Paolo Bonzini wrote:
> > 2. There is currently only one caller of get_user_pages_locked() in
> >    mm/frame_vector.c which seems to suggest this function isn't widely
> >    used/known.
>
> Or not widely necessary. :)

Well, quite :)
>
> > 3. This change results in all slow-path get_user_pages*() functions having the
> >    ability to use VM_FAULT_RETRY logic rather than 'defaulting' to using
> >    get_user_pages() that doesn't let you do this even if you wanted to.
>
> This is only true if someone does the work though.  From a quick look
> at your series, the following files can be trivially changed to use
> get_user_pages_unlocked:
>
> - drivers/gpu/drm/via/via_dmablit.c
> - drivers/platform/goldfish/goldfish_pipe.c
> - drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> - drivers/rapidio/devices/rio_mport_cdev.c
> - drivers/virt/fsl_hypervisor.c
>

Ah indeed, I rather glossed through the callers and noticed that at least some
held locks long enough or were called with a lock held sufficient that I wasn't
sure it could be released.

> Also, videobuf_dma_init_user could be changed to use retry by adding a
> *locked argument to videobuf_dma_init_user_locked, prototype patch
> after my signature.
>

Very nice!

> Everything else is probably best kept using get_user_pages.
>
> > 4. It's somewhat confusing/redundant having both get_user_pages_locked() and
> >    get_user_pages() functions which both require mmap_sem to be held (i.e. both
> >    are 'locked' versions.)
> >
> >> If all callers were changed, then sure removing the _locked suffix would
> >> be a good idea.
> >
> > It seems many callers cannot release mmap_sem so VM_FAULT_RETRY logic couldn't
> > happen anyway in this cases and in these cases we couldn't change the caller.
>
> But then get_user_pages_locked remains a less-common case, so perhaps
> it's a good thing to give it a longer name!

My (somewhat minor) concern was that there would be confusion due to the
existence of the triumvirate of g_u_p()/g_u_p_unlocked()/g_u_p_locked(), however
the comments do a decent enough job of explaining the situation.

>
> > Overall, an alternative here might be to remove get_user_pages() and update
> > get_user_pages_locked() to add a 'vmas' parameter, however doing this renders
> > get_user_pages_unlocked() asymmetric as it would lack an vmas parameter (adding
> > one there would make no sense as VM_FAULT_RETRY logic invalidates VMAs) though
> > perhaps not such a big issue as it makes sense as to why this is the case.
>
> Adding the 'vmas' parameter to get_user_pages_locked would make little
> sense.  Since VM_FAULT_RETRY invalidates it and g_u_p_locked can and
> does retry, it would generally not be useful.

I meant only in the case where we'd remove get_user_pages() and instead be left
with get_user_pages_[un]locked() only, meaning we'd have to add some means of
retrieving vmas if locked was set NULL, of course in cases where locked is not
NULL it makes no sense to add it.

>
> So I think we have the right API now:
>
> - do not have lock?  Use get_user_pages_unlocked, get retry for free,
> no need to handle  mmap_sem and the locked argument; cannot get back vmas.
>
> - have and cannot drop lock?  User get_user_pages, no need to pass NULL
> for the locked argument; can get back vmas.
>
> - have but can drop lock (rare case)?  Use get_user_pages_locked,
> cannot get back vams.

Yeah I think this is sane as it is actually, this patch set was a lot less
convincing of a cleanup than prior ones and overall it seems we are better off
with the existing API.

I wonder whether a better patch series to come out of this would be to find
cases where the lock could be dropped (i.e. the ones you mention above) and to
switch to using get_user_pages_[un]locked() where it makes sense to.

I am happy to look into these cases (though of course I must leave your
suggested patch here to you :)
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Patch

diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
index 1db0af6c7f94..dae4eb8e9d5b 100644
--- a/drivers/media/v4l2-core/videobuf-dma-sg.c
+++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
@@ -152,7 +152,8 @@  static void videobuf_dma_init(struct videobuf_dmabuf *dma)
 }
 
 static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
-			int direction, unsigned long data, unsigned long size)
+			int direction, unsigned long data, unsigned long size,
+			int *locked)
 {
 	unsigned long first, last;
 	int err, rw = 0;
@@ -185,8 +186,17 @@  static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
 	dprintk(1, "init user [0x%lx+0x%lx => %d pages]\n",
 		data, size, dma->nr_pages);
 
-	err = get_user_pages(data & PAGE_MASK, dma->nr_pages,
-			     flags, dma->pages, NULL);
+	if (locked && !*locked) {
+		down_read(&current->mm->mmap_sem);
+		*locked = 1;
+	}
+
+	/*
+	 * If the caller cannot have mmap_sem dropped, it passes locked == NULL
+	 * so get_user_pages_locked will not release it.
+	 */
+	err = get_user_pages_locked(data & PAGE_MASK, dma->nr_pages,
+				    flags, dma->pages, locked);
 
 	if (err != dma->nr_pages) {
 		dma->nr_pages = (err >= 0) ? err : 0;
@@ -200,10 +210,11 @@  static int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
 			   unsigned long data, unsigned long size)
 {
 	int ret;
+	int locked = 0;
 
-	down_read(&current->mm->mmap_sem);
-	ret = videobuf_dma_init_user_locked(dma, direction, data, size);
-	up_read(&current->mm->mmap_sem);
+	ret = videobuf_dma_init_user_locked(dma, direction, data, size, &locked);
+	if (locked)
+		up_read(&current->mm->mmap_sem);
 
 	return ret;
 }
@@ -540,7 +551,7 @@  static int __videobuf_iolock(struct videobuf_queue *q,
 
 			err = videobuf_dma_init_user_locked(&mem->dma,
 						      DMA_FROM_DEVICE,
-						      vb->baddr, vb->bsize);
+						      vb->baddr, vb->bsize, NULL);
 			if (0 != err)
 				return err;
 		}