On Tuesday, 22 June 2021 2:16:50 AM AEST Peter Xu wrote:

On Mon, Jun 21, 2021 at 10:09:00PM +1000, Alistair Popple wrote:

quoted

On Friday, 28 May 2021 6:21:30 AM AEST 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        | 31 ++++++++-----------------------
 2 files changed, 26 insertions(+), 24 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index db155be8e66c..52d3ef2ed753 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -1721,13 +1721,30 @@ static inline bool can_do_mlock(void) { return false; }
 extern int user_shm_lock(size_t, struct user_struct *);
 extern void user_shm_unlock(size_t, struct user_struct *);
 
+/* Whether to check page->mapping when zapping */
+#define  ZAP_FLAG_CHECK_MAPPING             BIT(0)
+
 /*
  * 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;
+	unsigned long zap_flags;
 };
 
+/* Return true if skip zapping this page, false otherwise */
+static inline bool
+zap_check_mapping_skip(struct zap_details *details, struct page *page)
+{
+	if (!details || !page)
+		return false;
+
+	if (!(details->zap_flags & ZAP_FLAG_CHECK_MAPPING))
+		return false;

[1]

quoted

quoted

+
+	return details->zap_mapping != page_rmapping(page);

I doubt this matters in practice, but there is a slight behaviour change
here that might be worth checking. Previously this check was equivalent
to:

details->zap_mapping && details->zap_mapping != page_rmapping(page)

Yes; IMHO "details->zap_mapping" is just replaced by the check at [1].

Yes, but what I meant is that this check is slightly different in behaviour
from the old code which would never skip if check/zap_mapping == NULL where as
the new code will skip if
details->zap_mapping == NULL && page_rmapping(page) != NULL because the check
has effectively become:

				if ((details->zap_flags & ZAP_FLAG_CHECK_MAPPING) &&
				    details->zap_mapping != page_rmapping(page))
					continue;

instead of:

				if (details->zap_mapping &&
				    details->zap_mapping != page_rmapping(page))
					continue;

As I said though I only looked at this superficially from the perspective of
whether this patch changes existing code behaviour. I doubt this is a real
problem because I assume
details->check_mapping == NULL && page_rmapping(page) != NULL can never
actually happen in practice.

For example, there's only one real user of this mapping check, which is
unmap_mapping_pages() below [2].

With the old code, we have:

    details.check_mapping = even_cows ? NULL : mapping;

So "details->zap_mapping" is only true if "!even_cows".

With the new code, we'll have:

    if (!even_cows)
        details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;

So ZAP_FLAG_CHECK_MAPPING is only set if "!even_cows", while that's what we
check exactly at [1].

quoted

Otherwise I think this looks good.

quoted

+}
+
 struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 			     pte_t pte);
 struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,

diff --git a/mm/memory.c b/mm/memory.c
index 27cf8a6375c6..c9dc4e9e05b5 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -1330,16 +1330,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 			struct page *page;
 
 			page = vm_normal_page(vma, addr, ptent);
-			if (unlikely(details) && page) {
-				/*
-				 * unmap_shared_mapping_pages() wants to
-				 * invalidate cache without truncating:
-				 * unmap shared but keep private pages.
-				 */
-				if (details->check_mapping &&
-				    details->check_mapping != page_rmapping(page))
-					continue;
-			}
+			if (unlikely(zap_check_mapping_skip(details, page)))
+				continue;
 			ptent = ptep_get_and_clear_full(mm, addr, pte,
 							tlb->fullmm);
 			tlb_remove_tlb_entry(tlb, pte, addr);

@@ -1372,17 +1364,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 		    is_device_exclusive_entry(entry)) {
 			struct page *page = pfn_swap_entry_to_page(entry);
 
-			if (unlikely(details && details->check_mapping)) {
-				/*
-				 * unmap_shared_mapping_pages() wants to
-				 * invalidate cache without truncating:
-				 * unmap shared but keep private pages.
-				 */
-				if (details->check_mapping !=
-				    page_rmapping(page))
-					continue;
-			}
-
+			if (unlikely(zap_check_mapping_skip(details, page)))
+				continue;
 			pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
 			rss[mm_counter(page)]--;
 

@@ -3345,9 +3328,11 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
 		pgoff_t nr, bool even_cows)
 {
 	pgoff_t	first_index = start, last_index = start + nr - 1;
-	struct zap_details details = { };
+	struct zap_details details = { .zap_mapping = mapping };
+
+	if (!even_cows)
+		details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;
 
-	details.check_mapping = even_cows ? NULL : mapping;

[2]

quoted

quoted

 	if (last_index < first_index)
 		last_index = ULONG_MAX;

Thanks,