diff options
authorLinus Torvalds <>2010-10-15 11:09:28 -0700
committerLinus Torvalds <>2010-10-15 11:09:28 -0700
commit799c10559d60f159ab2232203f222f18fa3c4a5f (patch)
parent5a2b3ef4559f3d0ef58cbfb723f528f1c6b2e601 (diff)
De-pessimize rds_page_copy_user
Don't try to "optimize" rds_page_copy_user() by using kmap_atomic() and the unsafe atomic user mode accessor functions. It's actually slower than the straightforward code on any reasonable modern CPU. Back when the code was written (although probably not by the time it was actually merged, though), 32-bit x86 may have been the dominant architecture. And there kmap_atomic() can be a lot faster than kmap() (unless you have very good locality, in which case the virtual address caching by kmap() can overcome all the downsides). But these days, x86-64 may not be more populous, but it's getting there (and if you care about performance, it's definitely already there - you'd have upgraded your CPU's already in the last few years). And on x86-64, the non-kmap_atomic() version is faster, simply because the code is simpler and doesn't have the "re-try page fault" case. People with old hardware are not likely to care about RDS anyway, and the optimization for the 32-bit case is simply buggy, since it doesn't verify the user addresses properly. Reported-by: Dan Rosenberg <> Acked-by: Andrew Morton <> Cc: Signed-off-by: Linus Torvalds <>
1 files changed, 7 insertions, 20 deletions
diff --git a/net/rds/page.c b/net/rds/page.c
index 595a952d4b17..1dfbfea12e9b 100644
--- a/net/rds/page.c
+++ b/net/rds/page.c
@@ -57,30 +57,17 @@ int rds_page_copy_user(struct page *page, unsigned long offset,
unsigned long ret;
void *addr;
- if (to_user)
+ addr = kmap(page);
+ if (to_user) {
rds_stats_add(s_copy_to_user, bytes);
- else
+ ret = copy_to_user(ptr, addr + offset, bytes);
+ } else {
rds_stats_add(s_copy_from_user, bytes);
- addr = kmap_atomic(page, KM_USER0);
- if (to_user)
- ret = __copy_to_user_inatomic(ptr, addr + offset, bytes);
- else
- ret = __copy_from_user_inatomic(addr + offset, ptr, bytes);
- kunmap_atomic(addr, KM_USER0);
- if (ret) {
- addr = kmap(page);
- if (to_user)
- ret = copy_to_user(ptr, addr + offset, bytes);
- else
- ret = copy_from_user(addr + offset, ptr, bytes);
- kunmap(page);
- if (ret)
- return -EFAULT;
+ ret = copy_from_user(addr + offset, ptr, bytes);
+ kunmap(page);
- return 0;
+ return ret ? -EFAULT : 0;