On 04/17/2013 07:38 AM, Kirill A. Shutemov wrote:
>> > Ugh. This is essentially a copy-n-paste of code in __do_fault(),
>> > including the comments. Is there no way to consolidate the code so that
>> > there's less duplication here?
> I've looked into it once again and it seems there's not much space for
> consolidation. Code structure looks very similar, but there are many
> special cases for thp: fallback path, pte vs. pmd, etc. I don't see how we
> can consolidate them in them in sane way.
> I think copy is more maintainable :(
I took the two copies, put them each in a file, changed some of the
_very_ trivial stuff to match (foo=1 vs foo=true) and diffed them.
They're very similar lengths (in lines):
185 __do_fault
197 do_huge_linear_fault
If you diff them:
1 file changed, 68 insertions(+), 56 deletions(-)
That means that of 185 lines in __do_fault(), 129 (70%) of them were
copied *VERBATIM*. Not similar in structure or appearance. Bit-for-bit
the same.
I took a stab at consolidating them. I think we could add a
VM_FAULT_FALLBACK flag to explicitly indicate that we need to do a
huge->small fallback, as well as a FAULT_FLAG_TRANSHUGE to indicate that
a given fault has not attempted to be handled by a huge page. If we
call __do_fault() with FAULT_FLAG_TRANSHUGE and we get back
VM_FAULT_FALLBACK or VM_FAULT_OOM, then we clear FAULT_FLAG_TRANSHUGE
and retry in handle_mm_fault().
I only went about 1/4 of the way in to __do_fault(). If went and spent
another hour or two, I'm pretty convinced I could push this even further.
Are you still sure you can't do _any_ better than a verbatim copy of 129
lines?
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7acc9dc..d408b5b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -879,6 +879,7 @@ static inline int page_mapped(struct page *page)
#define VM_FAULT_NOPAGE 0x0100 /* ->fault installed the pte, not return page */
#define VM_FAULT_LOCKED 0x0200 /* ->fault locked the returned page */
#define VM_FAULT_RETRY 0x0400 /* ->fault blocked, must retry */
+#define VM_FAULT_FALLBACK 0x0800 /* large page fault failed, fall back to small */
#define VM_FAULT_HWPOISON_LARGE_MASK 0xf000 /* encodes hpage index for large hwpoison */
diff --git a/mm/memory.c b/mm/memory.c
index 494526a..9aced3a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3229,6 +3229,40 @@ oom:
return VM_FAULT_OOM;
}
+static inline bool transhuge_vma_suitable(struct vm_area_struct *vma, unsigned long addr)
+{
+ unsigned long haddr = address & HPAGE_PMD_MASK;
+
+ if (((vma->vm_start >> PAGE_SHIFT) & HPAGE_CACHE_INDEX_MASK) !=
+ (vma->vm_pgoff & HPAGE_CACHE_INDEX_MASK)) {
+ return false;
+ }
+ if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end) {
+ return false;
+ }
+ return true;
+}
+
+static struct page *alloc_fault_page_vma(gfp_t flags, vm_area_struct *vma,
+ unsigned long address, unsigned int flags)
+{
+ int try_huge_pages = flags & FAULT_FLAG_TRANSHUGE;
+ unsigned long haddr = address & HPAGE_PMD_MASK;
+
+ if (try_huge_pages) {
+ return alloc_hugepage_vma(transparent_hugepage_defrag(vma),
+ vma, haddr, numa_node_id(), 0);
+ }
+ return alloc_page_vma(flags, vma, address);
+}
+
+static inline void __user *align_fault_address(unsigned long address, unsigned int flags)
+{
+ if (flags & FAULT_FLAG_TRANSHUGE)
+ return (void __user *)address & HPAGE_PMD_MASK;
+ return (void __user *)(address & PAGE_MASK);
+}
+
/*
* __do_fault() tries to create a new page mapping. It aggressively
* tries to share with existing pages, but makes a separate copy if
@@ -3256,17 +3290,21 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
struct vm_fault vmf;
int ret;
int page_mkwrite = 0;
+ int try_huge_pages = !!(flags & FAULT_FLAG_TRANSHUGE);
+
+ if (try_huge_pages && !transhuge_vma_suitable(vma)) {
+ return VM_FAULT_FALLBACK;
+ }
/*
* If we do COW later, allocate page befor taking lock_page()
* on the file cache page. This will reduce lock holding time.
*/
if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
-
if (unlikely(anon_vma_prepare(vma)))
return VM_FAULT_OOM;
- cow_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
+ cow_page = alloc_fault_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
if (!cow_page)
return VM_FAULT_OOM;
@@ -3277,7 +3315,7 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
} else
cow_page = NULL;
- vmf.virtual_address = (void __user *)(address & PAGE_MASK);
+ vmf.virtual_address = align_fault_address(address, flags);
vmf.pgoff = pgoff;
vmf.flags = flags;
vmf.page = NULL;
@@ -3714,7 +3752,6 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
pud_t *pud;
pmd_t *pmd;
pte_t *pte;
-
__set_current_state(TASK_RUNNING);
count_vm_event(PGFAULT);
@@ -3726,6 +3763,9 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
if (unlikely(is_vm_hugetlb_page(vma)))
return hugetlb_fault(mm, vma, address, flags);
+ /* We will try a single shot (only if enabled an possible)
+ * to do a transparent huge page */
+ flags |= FAULT_FLAG_TRANSHUGE;
retry:
pgd = pgd_offset(mm, address);
pud = pud_alloc(mm, pgd, address);
@@ -3738,6 +3778,11 @@ retry:
if (!vma->vm_ops)
return do_huge_pmd_anonymous_page(mm, vma, address,
pmd, flags);
+ ret = __do_fault(mm, vma, address, pmd, ...)
+ if (ret & (VM_FAULT_OOM | VM_FAULT_FALLBACK)) {
+ flags &= ~FAULT_FLAG_TRANSHUGE;
+ goto retry;
+ }
} else {
pmd_t orig_pmd = *pmd;
int ret;
