On Thu, Sep 02, 2021 at 09:28:42AM +0200, David Hildenbrand wrote:

On 01.09.21 22:57, Peter Xu wrote:

quoted

Instead of trying to introduce one variable for every new zap_details fields,
let's introduce a flag so that it can start to encode true/false informations.

Let's start to use this flag first to clean up the only check_mapping variable.
Firstly, the name "check_mapping" implies this is a "boolean", but actually it
stores the mapping inside, just in a way that it won't be set if we don't want
to check the mapping.

To make things clearer, introduce the 1st zap flag ZAP_FLAG_CHECK_MAPPING, so
that we only check against the mapping if this bit set.  At the same time, we
can rename check_mapping into zap_mapping and set it always.

Since at it, introduce another helper zap_check_mapping_skip() and use it in
zap_pte_range() properly.

Some old comments have been removed in zap_pte_range() because they're
duplicated, and since now we're with ZAP_FLAG_CHECK_MAPPING flag, it'll be very
easy to grep this information by simply grepping the flag.

It'll also make life easier when we want to e.g. pass in zap_flags into the
callers like unmap_mapping_pages() (instead of adding new booleans besides the
even_cows parameter).

Signed-off-by: Peter Xu <peterx@redhat.com>
---
  include/linux/mm.h | 19 ++++++++++++++++++-
  mm/memory.c        | 34 ++++++++++------------------------
  2 files changed, 28 insertions(+), 25 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 69259229f090..fcbc1c4f8e8e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -1716,14 +1716,31 @@ static inline bool can_do_mlock(void) { return false; }
  extern int user_shm_lock(size_t, struct ucounts *);
  extern void user_shm_unlock(size_t, struct ucounts *);
+/* Whether to check page->mapping when zapping */
+#define  ZAP_FLAG_CHECK_MAPPING             BIT(0)

So we want to go full way, like:

typedef int __bitwise zap_flags_t;

#define  ZAP_FLAG_CHECK_MAPPING		((__force zap_flags_t)BIT(0))

Sure.

quoted

+
  /*
   * Parameter block passed down to zap_pte_range in exceptional cases.
   */
  struct zap_details {
-	struct address_space *check_mapping;	/* Check page->mapping if set */
+	struct address_space *zap_mapping;
  	struct page *single_page;		/* Locked page to be unmapped */
+	unsigned long zap_flags;

Why call it "zap_*" if everything in the structure is related to zapping?
IOW, simply "mapping", "flags" would be good enough.

Not sure if it's a good habit or bad - it's just for tagging system to be able
to identify other "mapping" variables, or a simple grep with the name.  So I
normally prefix fields with some special wording to avoid collisions.

quoted

  };
+/* Return true if skip zapping this page, false otherwise */
+static inline bool
+zap_skip_check_mapping(struct zap_details *details, struct page *page)
+{
+	if (!details || !page)
+		return false;
+
+	if (!(details->zap_flags & ZAP_FLAG_CHECK_MAPPING))
+		return false;
+
+	return details->zap_mapping != page_rmapping(page);
+}

I'm confused, why isn't "!details->zap_mapping" vs. "details->zap_mapping"
sufficient? I can see that you may need flags for other purposes (next
patch), but why do we need it here?

Factoring it out into this helper is a nice cleanup, though. But I'd just
not introduce ZAP_FLAG_CHECK_MAPPING.

Yes I think it's okay. I wanted to separate them as they're fundamentall two
things to me.  Example: what if the mapping we want to check is NULL itself
(remove private pages only; though it may not have a real user at least so
far)?  In that case one variable won't be able to cover it.

But indeed Matthew raised similar comment, so it seems to be a common
preference.  No strong opinion on my side, let me coordinate with it.

Thanks for looking,

-- 
Peter Xu