Re: [PATCH] ib/core: not to set page dirty bit if it's already set.

On 5/19/2017 6:05 AM, Christoph Hellwig wrote:

On Thu, May 18, 2017 at 04:33:53PM -0700, Qing Huang wrote:

quoted

This change will optimize kernel memory deregistration operations.
__ib_umem_release() used to call set_page_dirty_lock() against every
writable page in its memory region. Its purpose is to keep data
synced between CPU and DMA device when swapping happens after mem
deregistration ops. Now we choose not to set page dirty bit if it's
already set by kernel prior to calling __ib_umem_release(). This
reduces memory deregistration time by half or even more when we ran
application simulation test program.

As far as I can tell this code doesn't even need set_page_dirty_lock
and could just use set_page_dirty

It seems that set_page_dirty_lock has been used here for more than 10 
years. Don't know the original purpose. Maybe it was used to prevent 
races between setting dirty bits and swapping out pages?

Perhaps we can call set_page_dirty before calling ib_dma_unmap_sg?

quoted

Signed-off-by: Qing Huang<redacted>
---
  drivers/infiniband/core/umem.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
index 3dbf811..21e60b1 100644
--- a/drivers/infiniband/core/umem.c
+++ b/drivers/infiniband/core/umem.c

@@ -58,7 +58,7 @@ static void __ib_umem_release(struct ib_device *dev, struct ib_umem *umem, int d
  	for_each_sg(umem->sg_head.sgl, sg, umem->npages, i) {
  
  		page = sg_page(sg);
-		if (umem->writable && dirty)
+		if (!PageDirty(page) && umem->writable && dirty)
  			set_page_dirty_lock(page);
  		put_page(page);
  	}

-- 
2.9.3

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---end quoted text---