Re: [PATCH 2/3] Retry page fault when blocking on disk transfer.

[Michel Lespinasse <walken@google.com>]

On Tue, Oct 05, 2010 at 03:44:22PM -0700, Michel Lespinasse wrote:
> On Tue, Oct 5, 2010 at 10:38 AM, Rik van Riel <riel@redhat.com> wrote:
> > Looks like it should be relatively easy to do something
> > similar in do_swap_page also.
> 
> Good idea. We don't make use of swap too much, which is probably why
> we didn't have that in our kernel, but it seems like a good idea just
> for uniformity. I'll add this in a follow-on patch.

So here's the patch. Sorry for the delay - it did not take long to write,
but I couldn't test it before today.

Please have a look - I'd like to add this to the series I sent earlier.

----------------------------------- 8< ---------------------------------

Retry page fault when blocking on swap in

This change is the cousin of 'Retry page fault when blocking
on disk transfer'. The idea here is to reduce mmap_sem hold times
that are caused by disk transfers when swapping in pages. We drop
mmap_sem while waiting for the page lock, and return the VM_FAULT_RETRY
flag. do_page_fault will then re-acquire mmap_sem and retry the
page fault. It is expected that upon retry the page will now be cached,
and thus the retry will complete with a low mmap_sem hold time.

Signed-off-by: Michel Lespinasse <walken@google.com>

diff --git a/mm/memory.c b/mm/memory.c
index b068c68..0ec70b4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2613,6 +2613,21 @@ int vmtruncate_range(struct inode *inode, loff_t offset, loff_t end)
 	return 0;
 }
 
+static inline int lock_page_or_retry(struct page *page, struct mm_struct *mm,
+				     unsigned int flags)
+{
+	if (trylock_page(page))
+		return 1;
+	if (!(flags & FAULT_FLAG_ALLOW_RETRY)) {
+		__lock_page(page);
+		return 1;
+	}
+
+	up_read(&mm->mmap_sem);
+	wait_on_page_locked(page);
+	return 0;
+}
+
 /*
  * We enter with non-exclusive mmap_sem (to exclude vma changes,
  * but allow concurrent faults), and pte mapped but not yet locked.
@@ -2626,6 +2641,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	struct page *page, *swapcache = NULL;
 	swp_entry_t entry;
 	pte_t pte;
+	int locked;
 	struct mem_cgroup *ptr = NULL;
 	int exclusive = 0;
 	int ret = 0;
@@ -2676,8 +2692,12 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		goto out_release;
 	}
 
-	lock_page(page);
+	locked = lock_page_or_retry(page, mm, flags);
 	delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
+	if (!locked) {
+		ret |= VM_FAULT_RETRY;
+		goto out_release;
+	}
 
 	/*
 	 * Make sure try_to_free_swap or reuse_swap_page or swapoff did not


-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.