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:
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.
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. :)
- 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.
- 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 :)
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(¤t->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(¤t->mm->mmap_sem); - ret = videobuf_dma_init_user_locked(dma, direction, data, size); - up_read(¤t->mm->mmap_sem); + ret = videobuf_dma_init_user_locked(dma, direction, data, size, &locked); + if (locked) + up_read(¤t->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; }