[PATCH] mm/hugetlb: remove unnecessary memory fetch in PageHeadHuge()

[PATCH] mm/hugetlb: remove unnecessary memory fetch in PageHeadHuge()

[Vlastimil Babka]

Commit f1e61557f023 ("mm: pack compound_dtor and compound_order into one word
in struct page") changed compound_dtor from a pointer to an array index in
order to pack it. To check if page has the hugeltbfs compound_dtor, we can
just compare the index directly without fetching the function pointer.
Said commit did that with PageHuge() and we can do the same with PageHeadHuge()
to make the code a bit smaller and faster.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/hugetlb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index dd8737a94bec..ba1ca452aa7f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1313,7 +1313,7 @@ int PageHeadHuge(struct page *page_head)
 	if (!PageHead(page_head))
 		return 0;
 
-	return get_compound_page_dtor(page_head) == free_huge_page;
+	return page_head[1].compound_dtor == HUGETLB_PAGE_DTOR;
 }
 
 pgoff_t __basepage_index(struct page *page)
-- 
2.25.1

Re: [PATCH] mm/hugetlb: remove unnecessary memory fetch in PageHeadHuge()

[Mike Kravetz]

On 3/11/20 10:24 AM, Vlastimil Babka wrote:
> Commit f1e61557f023 ("mm: pack compound_dtor and compound_order into one word
> in struct page") changed compound_dtor from a pointer to an array index in
> order to pack it. To check if page has the hugeltbfs compound_dtor, we can
> just compare the index directly without fetching the function pointer.
> Said commit did that with PageHuge() and we can do the same with PageHeadHuge()
> to make the code a bit smaller and faster.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Thanks!

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
-- 
Mike Kravetz

Re: [PATCH] mm/hugetlb: remove unnecessary memory fetch in PageHeadHuge()

[David Rientjes]

On Wed, 11 Mar 2020, Vlastimil Babka wrote:

> Commit f1e61557f023 ("mm: pack compound_dtor and compound_order into one word
> in struct page") changed compound_dtor from a pointer to an array index in
> order to pack it. To check if page has the hugeltbfs compound_dtor, we can
> just compare the index directly without fetching the function pointer.
> Said commit did that with PageHuge() and we can do the same with PageHeadHuge()
> to make the code a bit smaller and faster.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Fixes: f1e61557f023 ("mm: pack compound_dtor and compound_order into one 
word in struct page")

Acked-by: David Rientjes <rientjes@google.com>

[+nehaagarwal]

We've been running with this patch for a few years and it works as 
intended.

> ---
>  mm/hugetlb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index dd8737a94bec..ba1ca452aa7f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1313,7 +1313,7 @@ int PageHeadHuge(struct page *page_head)
>  	if (!PageHead(page_head))
>  		return 0;
>  
> -	return get_compound_page_dtor(page_head) == free_huge_page;
> +	return page_head[1].compound_dtor == HUGETLB_PAGE_DTOR;
>  }
>  
>  pgoff_t __basepage_index(struct page *page)

Re: [PATCH] mm/hugetlb: remove unnecessary memory fetch in PageHeadHuge()

[Kirill A. Shutemov]

On Wed, Mar 11, 2020 at 06:24:40PM +0100, Vlastimil Babka wrote:
> Commit f1e61557f023 ("mm: pack compound_dtor and compound_order into one word
> in struct page") changed compound_dtor from a pointer to an array index in
> order to pack it. To check if page has the hugeltbfs compound_dtor, we can
> just compare the index directly without fetching the function pointer.
> Said commit did that with PageHuge() and we can do the same with PageHeadHuge()
> to make the code a bit smaller and faster.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

We use literaly the same check in the function next to this one --
PageHuge().


-- 
 Kirill A. Shutemov

Re: [PATCH] mm/hugetlb: remove unnecessary memory fetch in PageHeadHuge()

[Kirill A. Shutemov]

On Thu, Mar 12, 2020 at 03:41:19AM +0300, Kirill A. Shutemov wrote:
> On Wed, Mar 11, 2020 at 06:24:40PM +0100, Vlastimil Babka wrote:
> > Commit f1e61557f023 ("mm: pack compound_dtor and compound_order into one word
> > in struct page") changed compound_dtor from a pointer to an array index in
> > order to pack it. To check if page has the hugeltbfs compound_dtor, we can
> > just compare the index directly without fetching the function pointer.
> > Said commit did that with PageHuge() and we can do the same with PageHeadHuge()
> > to make the code a bit smaller and faster.
> > 
> > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Mike Kravetz <mike.kravetz@oracle.com>
> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> 
> We use literaly the same check in the function next to this one --
> PageHuge().

Ugh.. I have to read the commit message, not only the code :P

-- 
 Kirill A. Shutemov