2018-12-08 13:38:41

by Eric Dumazet

Subject: [PATCH] iov_iter: optimize page_copy_sane()

Avoid cache line miss dereferencing struct page if we can.

page_copy_sane() mostly deals with order-0 pages.

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Al Viro <[email protected]>
---
lib/iov_iter.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 54c248526b55fc498c996f2a5ea651262fcc7f61..2a1e282c3d38e2d03b075f54b7f9313e48b90289 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -854,8 +854,18 @@ EXPORT_SYMBOL(_copy_from_iter_full_nocache);

static inline bool page_copy_sane(struct page *page, size_t offset, size_t n)
{
- struct page *head = compound_head(page);
- size_t v = n + offset + page_address(page) - page_address(head);
+ struct page *head;
+ size_t v = n + offset;
+
+ /*
+ * Fast path if this is an order-0 page,
+ * or if the copied area fits the first page of a compound one.
+ */
+ if (likely(n <= v && v <= PAGE_SIZE))
+ return true;
+
+ head = compound_head(page);
+ v += (page - head) << PAGE_SHIFT;

if (likely(n <= v && v <= (PAGE_SIZE << compound_order(head))))
return true;
--
2.20.0.rc2.403.gdbc3b29805-goog

2018-12-17 18:43:52

by Eric Dumazet

Subject: Re: [PATCH] iov_iter: optimize page_copy_sane()

On 12/08/2018 05:37 AM, Eric Dumazet wrote:
> Avoid cache line miss dereferencing struct page if we can.
>
> page_copy_sane() mostly deals with order-0 pages.
>
> Signed-off-by: Eric Dumazet <[email protected]>
> Cc: Al Viro <[email protected]>
> ---
> lib/iov_iter.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 54c248526b55fc498c996f2a5ea651262fcc7f61..2a1e282c3d38e2d03b075f54b7f9313e48b90289 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -854,8 +854,18 @@ EXPORT_SYMBOL(_copy_from_iter_full_nocache);
>
> static inline bool page_copy_sane(struct page *page, size_t offset, size_t n)
> {
> - struct page *head = compound_head(page);
> - size_t v = n + offset + page_address(page) - page_address(head);
> + struct page *head;
> + size_t v = n + offset;
> +
> + /*
> + * Fast path if this is an order-0 page,
> + * or if the copied area fits the first page of a compound one.
> + */
> + if (likely(n <= v && v <= PAGE_SIZE))
> + return true;
> +
> + head = compound_head(page);
> + v += (page - head) << PAGE_SHIFT;
>
> if (likely(n <= v && v <= (PAGE_SIZE << compound_order(head))))
> return true;
>

Hi Alexander !

Any news on this patch ?

Thanks !