From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Subject: mm: tighten up the fault path a little
The round_up() macro generates a couple of unnecessary instructions
in this usage:
48cd: 49 8b 47 50 mov 0x50(%r15),%rax
48d1: 48 83 e8 01 sub $0x1,%rax
48d5: 48 0d ff 0f 00 00 or $0xfff,%rax
48db: 48 83 c0 01 add $0x1,%rax
48df: 48 c1 f8 0c sar $0xc,%rax
48e3: 48 39 c3 cmp %rax,%rbx
48e6: 72 2e jb 4916 <filemap_fault+0x96>
If we change round_up() to ((x) + __round_mask(x, y)) & ~__round_mask(x, y)
then GCC can see through it and remove the mask (because that would be
dead code given the subsequent shift):
48cd: 49 8b 47 50 mov 0x50(%r15),%rax
48d1: 48 05 ff 0f 00 00 add $0xfff,%rax
48d7: 48 c1 e8 0c shr $0xc,%rax
48db: 48 39 c3 cmp %rax,%rbx
48de: 72 2e jb 490e <filemap_fault+0x8e>
But that's problematic because we'd evaluate 'y' twice. Converting
round_up into an inline function prevents it from being used in other
definitions. The easiest thing to do is just change these three usages of
round_up to use DIV_ROUND_UP. Also add an unlikely() because GCC's
heuristic is wrong in this case.
Link: http://lkml.kernel.org/r/20170207192812.5281-1-willy@xxxxxxxxxxxxx
Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)
diff -puN mm/filemap.c~mm-tighten-up-the-fault-path-a-little mm/filemap.c
--- a/mm/filemap.c~mm-tighten-up-the-fault-path-a-little
+++ a/mm/filemap.c
@@ -2204,12 +2204,12 @@ int filemap_fault(struct vm_fault *vmf)
struct file_ra_state *ra = &file->f_ra;
struct inode *inode = mapping->host;
pgoff_t offset = vmf->pgoff;
+ pgoff_t max_off;
struct page *page;
- loff_t size;
int ret = 0;
- size = round_up(i_size_read(inode), PAGE_SIZE);
- if (offset >= size >> PAGE_SHIFT)
+ max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
+ if (unlikely(offset >= max_off))
return VM_FAULT_SIGBUS;
/*
@@ -2258,8 +2258,8 @@ retry_find:
* Found the page and have a reference on it.
* We must recheck i_size under page lock.
*/
- size = round_up(i_size_read(inode), PAGE_SIZE);
- if (unlikely(offset >= size >> PAGE_SHIFT)) {
+ max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
+ if (unlikely(offset >= max_off)) {
unlock_page(page);
put_page(page);
return VM_FAULT_SIGBUS;
@@ -2325,7 +2325,7 @@ void filemap_map_pages(struct vm_fault *
struct file *file = vmf->vma->vm_file;
struct address_space *mapping = file->f_mapping;
pgoff_t last_pgoff = start_pgoff;
- loff_t size;
+ unsigned long max_idx;
struct page *head, *page;
rcu_read_lock();
@@ -2371,8 +2371,8 @@ repeat:
if (page->mapping != mapping || !PageUptodate(page))
goto unlock;
- size = round_up(i_size_read(mapping->host), PAGE_SIZE);
- if (page->index >= size >> PAGE_SHIFT)
+ max_idx = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
+ if (page->index >= max_idx)
goto unlock;
if (file->f_ra.mmap_miss > 0)
_
--
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