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Message-ID: <20101204065530.GA27895@google.com>
Date: Fri, 3 Dec 2010 22:55:30 -0800
From: Michel Lespinasse <walken@...gle.com>
To: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Rik van Riel <riel@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Nick Piggin <npiggin@...nel.dk>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc: linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2/6] mm: add FOLL_MLOCK follow_page flag.
I did not realize that previously, but it turns out to be possible to
manipulate LRU lists under control of the pte lock, rather than having
to get/put the page. This allows the FOLL_MLOCK follow_page flag
implementation to look much nicer than it did in initial proposal.
Please consider the following as a replacement for the initial
patch 2/6 proposal...
--------------------------------- 8< --------------------------------------
Move the code to mlock pages from __mlock_vma_pages_range()
to follow_page().
This allows __mlock_vma_pages_range() to not have to break down work
into 16-page batches.
An additional motivation for doing this within the present patch series
is that it'll make it easier for a later chagne to drop mmap_sem when
blocking on disk (we'd like to be able to resume at the page that was
read from disk instead of at the start of a 16-page batch).
Signed-off-by: Michel Lespinasse <walken@...gle.com>
---
include/linux/mm.h | 1 +
mm/memory.c | 22 +++++++++++++++++
mm/mlock.c | 65 ++++------------------------------------------------
3 files changed, 28 insertions(+), 60 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 721f451..cebbb0d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1415,6 +1415,7 @@ struct page *follow_page(struct vm_area_struct *, unsigned long address,
#define FOLL_GET 0x04 /* do get_page on page */
#define FOLL_DUMP 0x08 /* give error on hole if it would be zero */
#define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */
+#define FOLL_MLOCK 0x20 /* mark page as mlocked */
typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
void *data);
diff --git a/mm/memory.c b/mm/memory.c
index b8f97b8..15e1f19 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1310,6 +1310,28 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
*/
mark_page_accessed(page);
}
+ if (flags & FOLL_MLOCK) {
+ /*
+ * The preliminary mapping check is mainly to avoid the
+ * pointless overhead of lock_page on the ZERO_PAGE
+ * which might bounce very badly if there is contention.
+ *
+ * If the page is already locked, we don't need to
+ * handle it now - vmscan will handle it later if and
+ * when it attempts to reclaim the page.
+ */
+ if (page->mapping && trylock_page(page)) {
+ lru_add_drain(); /* push cached pages to LRU */
+ /*
+ * Because we lock page here and migration is
+ * blocked by the pte's page reference, we need
+ * only check for file-cache page truncation.
+ */
+ if (page->mapping)
+ mlock_vma_page(page);
+ unlock_page(page);
+ }
+ }
unlock:
pte_unmap_unlock(ptep, ptl);
out:
diff --git a/mm/mlock.c b/mm/mlock.c
index 71db83e..573bd2c 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -159,10 +159,9 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
{
struct mm_struct *mm = vma->vm_mm;
unsigned long addr = start;
- struct page *pages[16]; /* 16 gives a reasonable batch */
int nr_pages = (end - start) / PAGE_SIZE;
- int ret = 0;
int gup_flags;
+ int ret;
VM_BUG_ON(start & ~PAGE_MASK);
VM_BUG_ON(end & ~PAGE_MASK);
@@ -170,7 +169,7 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
VM_BUG_ON(end > vma->vm_end);
VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
- gup_flags = FOLL_TOUCH | FOLL_GET;
+ gup_flags = FOLL_TOUCH | FOLL_MLOCK;
/*
* We want to touch writable mappings with a write fault in order
* to break COW, except for shared mappings because these don't COW
@@ -185,63 +184,9 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
nr_pages--;
}
- while (nr_pages > 0) {
- int i;
-
- cond_resched();
-
- /*
- * get_user_pages makes pages present if we are
- * setting mlock. and this extra reference count will
- * disable migration of this page. However, page may
- * still be truncated out from under us.
- */
- ret = __get_user_pages(current, mm, addr,
- min_t(int, nr_pages, ARRAY_SIZE(pages)),
- gup_flags, pages, NULL);
- /*
- * This can happen for, e.g., VM_NONLINEAR regions before
- * a page has been allocated and mapped at a given offset,
- * or for addresses that map beyond end of a file.
- * We'll mlock the pages if/when they get faulted in.
- */
- if (ret < 0)
- break;
-
- lru_add_drain(); /* push cached pages to LRU */
-
- for (i = 0; i < ret; i++) {
- struct page *page = pages[i];
-
- if (page->mapping) {
- /*
- * That preliminary check is mainly to avoid
- * the pointless overhead of lock_page on the
- * ZERO_PAGE: which might bounce very badly if
- * there is contention. However, we're still
- * dirtying its cacheline with get/put_page:
- * we'll add another __get_user_pages flag to
- * avoid it if that case turns out to matter.
- */
- lock_page(page);
- /*
- * Because we lock page here and migration is
- * blocked by the elevated reference, we need
- * only check for file-cache page truncation.
- */
- if (page->mapping)
- mlock_vma_page(page);
- unlock_page(page);
- }
- put_page(page); /* ref from get_user_pages() */
- }
-
- addr += ret * PAGE_SIZE;
- nr_pages -= ret;
- ret = 0;
- }
-
- return ret; /* 0 or negative error code */
+ ret = __get_user_pages(current, mm, addr, nr_pages, gup_flags,
+ NULL, NULL);
+ return max(ret, 0); /* 0 or negative error code */
}
/*
--
1.7.3.1
--
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
--
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