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Subject[RFC][PATCH 2/6] pagewalk: only split huge pages when necessary
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Right now, if a mm_walk has either ->pte_entry or ->pmd_entry
set, it will unconditionally split and transparent huge pages
it runs in to. In practice, that means that anyone doing a

cat /proc/$pid/smaps

will unconditionally break down every huge page in the process
and depend on khugepaged to re-collapse it later. This is
fairly suboptimal.

This patch changes that behavior. It teaches each ->pmd_entry
handler (there are three) that they must break down the THPs
themselves. Also, the _generic_ code will never break down
a THP unless a ->pte_entry handler is actually set.

This means that the ->pmd_entry handlers can now choose to
deal with THPs without breaking them down.


---

linux-2.6.git-dave/fs/proc/task_mmu.c | 6 ++++++
linux-2.6.git-dave/mm/pagewalk.c | 24 ++++++++++++++++++++----
2 files changed, 26 insertions(+), 4 deletions(-)

diff -puN mm/pagewalk.c~pagewalk-dont-always-split-thp mm/pagewalk.c
--- linux-2.6.git/mm/pagewalk.c~pagewalk-dont-always-split-thp 2011-01-27 10:57:02.309914973 -0800
+++ linux-2.6.git-dave/mm/pagewalk.c 2011-01-27 10:57:02.317914965 -0800
@@ -33,19 +33,35 @@ static int walk_pmd_range(pud_t *pud, un

pmd = pmd_offset(pud, addr);
do {
+ again:
next = pmd_addr_end(addr, end);
- split_huge_page_pmd(walk->mm, pmd);
- if (pmd_none_or_clear_bad(pmd)) {
+ if (pmd_none(*pmd)) {
if (walk->pte_hole)
err = walk->pte_hole(addr, next, walk);
if (err)
break;
continue;
}
+ /*
+ * This implies that each ->pmd_entry() handler
+ * needs to know about pmd_trans_huge() pmds
+ */
if (walk->pmd_entry)
err = walk->pmd_entry(pmd, addr, next, walk);
- if (!err && walk->pte_entry)
- err = walk_pte_range(pmd, addr, next, walk);
+ if (err)
+ break;
+
+ /*
+ * Check this here so we only break down trans_huge
+ * pages when we _need_ to
+ */
+ if (!walk->pte_entry)
+ continue;
+
+ split_huge_page_pmd(walk->mm, pmd);
+ if (pmd_none_or_clear_bad(pmd))
+ goto again;
+ err = walk_pte_range(pmd, addr, next, walk);
if (err)
break;
} while (pmd++, addr = next, addr != end);
diff -puN fs/proc/task_mmu.c~pagewalk-dont-always-split-thp fs/proc/task_mmu.c
--- linux-2.6.git/fs/proc/task_mmu.c~pagewalk-dont-always-split-thp 2011-01-27 10:57:02.313914969 -0800
+++ linux-2.6.git-dave/fs/proc/task_mmu.c 2011-01-27 10:57:02.321914961 -0800
@@ -343,6 +343,8 @@ static int smaps_pte_range(pmd_t *pmd, u
struct page *page;
int mapcount;

+ split_huge_page_pmd(walk->mm, pmd);
+
pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
for (; addr != end; pte++, addr += PAGE_SIZE) {
ptent = *pte;
@@ -467,6 +469,8 @@ static int clear_refs_pte_range(pmd_t *p
spinlock_t *ptl;
struct page *page;

+ split_huge_page_pmd(walk->mm, pmd);
+
pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
for (; addr != end; pte++, addr += PAGE_SIZE) {
ptent = *pte;
@@ -623,6 +627,8 @@ static int pagemap_pte_range(pmd_t *pmd,
pte_t *pte;
int err = 0;

+ split_huge_page_pmd(walk->mm, pmd);
+
/* find the first VMA at or above 'addr' */
vma = find_vma(walk->mm, addr);
for (; addr != end; addr += PAGE_SIZE) {
_

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