On 12/12/19 2:15 PM, Steven Price wrote:

On 12/12/2019 11:33, Thomas Hellström (VMware) wrote:

quoted

On 12/12/19 12:23 PM, Thomas Hellström (VMware) wrote:

quoted

On 12/6/19 2:53 PM, Steven Price wrote:

quoted

pgd_entry() and pud_entry() were removed by commit 0b1fbfe50006c410
("mm/pagewalk: remove pgd_entry() and pud_entry()") because there were
no users. We're about to add users so reintroduce them, along with
p4d_entry() as we now have 5 levels of tables.

Note that commit a00cc7d9dd93d66a ("mm, x86: add support for
PUD-sized transparent hugepages") already re-added pud_entry() but 
with
different semantics to the other callbacks. Since there have never
been upstream users of this, revert the semantics back to match the
other callbacks. This means pud_entry() is called for all entries, not
just transparent huge pages.

When I wrote that there were no upstream users, which sadly shows how
long ago that was :(

quoted

quoted

Actually, there are two users of pud_entry(), in hmm.c and since 
5.5rc1 also mapping_dirty_helpers.c. The latter one is unproblematic 
and requires no attention but the one in hmm.c is probably largely 
untested, and seems to assume it was called outside of the spinlock.

The problem with the current patch is that the hmm pud_entry will 
traverse also pmds, so that will be done twice now.

In another thread we were discussing a means of rerunning the level 
(in case of a race), or continuing after a level, based on the 
return value after the callback. The change was fairly invasive,

Hmm. Forgot to remove the above text that appears twice. :(. The 
correct one is inline below.

quoted

quoted

Tested-by: Zong Li <redacted>
Signed-off-by: Steven Price <steven.price@arm.com>
---
  include/linux/pagewalk.h | 19 +++++++++++++------
  mm/pagewalk.c            | 27 ++++++++++++++++-----------
  2 files changed, 29 insertions(+), 17 deletions(-)

diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 6ec82e92c87f..06790f23957f 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h

@@ -8,15 +8,15 @@ struct mm_walk;

    /**
   * mm_walk_ops - callbacks for walk_page_range
- * @pud_entry:        if set, called for each non-empty PUD 
(2nd-level) entry
- *            this handler should only handle pud_trans_huge() puds.
- *            the pmd_entry or pte_entry callbacks will be used for
- *            regular PUDs.
- * @pmd_entry:        if set, called for each non-empty PMD 
(3rd-level) entry
+ * @pgd_entry:        if set, called for each non-empty PGD 
(top-level) entry
+ * @p4d_entry:        if set, called for each non-empty P4D entry
+ * @pud_entry:        if set, called for each non-empty PUD entry
+ * @pmd_entry:        if set, called for each non-empty PMD entry
   *            this handler is required to be able to handle
   *            pmd_trans_huge() pmds.  They may simply choose to
   *            split_huge_page() instead of handling it explicitly.
- * @pte_entry:        if set, called for each non-empty PTE 
(4th-level) entry
+ * @pte_entry:        if set, called for each non-empty PTE 
(lowest-level)
+ *            entry
   * @pte_hole:        if set, called for each hole at all levels
   * @hugetlb_entry:    if set, called for each hugetlb entry
   * @test_walk:        caller specific callback function to 
determine whether

@@ -27,8 +27,15 @@ struct mm_walk;

   * @pre_vma:            if set, called before starting walk on a 
non-null vma.
   * @post_vma:           if set, called after a walk on a non-null 
vma, provided
   *                      that @pre_vma and the vma walk succeeded.
+ *
+ * p?d_entry callbacks are called even if those levels are folded 
on a
+ * particular architecture/configuration.
   */
  struct mm_walk_ops {
+    int (*pgd_entry)(pgd_t *pgd, unsigned long addr,
+             unsigned long next, struct mm_walk *walk);
+    int (*p4d_entry)(p4d_t *p4d, unsigned long addr,
+             unsigned long next, struct mm_walk *walk);
      int (*pud_entry)(pud_t *pud, unsigned long addr,
               unsigned long next, struct mm_walk *walk);
      int (*pmd_entry)(pmd_t *pmd, unsigned long addr,

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index ea0b9e606ad1..c089786e7a7f 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c

@@ -94,15 +94,9 @@ static int walk_pud_range(p4d_t *p4d, unsigned 

long addr, unsigned long end,
          }
            if (ops->pud_entry) {
-            spinlock_t *ptl = pud_trans_huge_lock(pud, walk->vma);
-
-            if (ptl) {
-                err = ops->pud_entry(pud, addr, next, walk);
-                spin_unlock(ptl);
-                if (err)
-                    break;
-                continue;
-            }
+            err = ops->pud_entry(pud, addr, next, walk);
+            if (err)
+                break;

Actually, there are two current users of pud_entry(), in hmm.c and 
since 5.5rc1 also mapping_dirty_helpers.c. The latter one is 
unproblematic and requires no attention but the one in hmm.c is 
probably largely untested, and seems to assume it was called outside 
of the spinlock.

Thanks for pointing that out, I guess the simplest fix would be to
squash in something like the below which should restore the old
behaviour for hmm.c without affecting others.

Steve 

I'm not fully sure that the old behaviour is the correct one, but definitely hmm's pud_entry needs some fixing.
I'm more concerned with the pagewalk code. With your patch it actually splits all huge puds present in the page-table
on each page walk which is not what we want.

One idea would be to add a new member to struct_mm_walk:

enum page_walk_ret_action {
	ACTION_SUBTREE = 0,
	ACTION_CONTINUE = 1,
	ACTION_AGAIN = 2 /* Only for levels that thave p?d_unstable */
};

struct mm_walk {
	...
	enum page_walk_ret_action action; /* or perhaps as an enum */
};


if (ops->pud_entry) {
	walk->action = ACTION_SUBTREE;
	...
	...
	...
	if (walk->action == ACTION_AGAIN)  /* Callback tried to split huge entry, but failed */
		goto again;
	else if (walk->action == ACTION_CONTINUE) /* Done with this subtree. Probably huge entry handled. */
		continue;
	/* ACTION_SUBTREE falls through */
}

we discussed something similar before on linux-mm, but the idea then was to redefine
the positive return value of the callback to the action, but that meant changing those existing callbacks that relied on
a positive return value. The above would be helpful also for pmd_entry.

/Thomas





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