[PATCH 0/3] mm: Some rework on zap_details

[PATCH 0/3] mm: Some rework on zap_details

[Peter Xu]

Posted this small series out to rework zap_details a bit, before adding
something new in.  Hopefully it makes things slighly clearer.

Smoke test only.  Please have a look, thanks.

Peter Xu (3):
  mm: Drop first_index/last_index in zap_details
  mm: Introduce zap_details.zap_flags
  mm: Introduce ZAP_FLAG_SKIP_SWAP

 include/linux/mm.h | 33 ++++++++++++++++++++++++---
 mm/memory.c        | 57 ++++++++++++++++++----------------------------
 2 files changed, 52 insertions(+), 38 deletions(-)

-- 
2.26.2

[PATCH 1/3] mm: Drop first_index/last_index in zap_details

[Peter Xu]

The first_index/last_index parameters in zap_details are actually only used in
unmap_mapping_range_tree().  At the meantime, this function is only called by
unmap_mapping_pages() once.  Instead of passing these two variables through the
whole stack of page zapping code, remove them from zap_details and let them
simply be parameters of unmap_mapping_range_tree(), which is inlined.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/mm.h |  2 --
 mm/memory.c        | 20 ++++++++++----------
 2 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 16b799a0522c..5d977e484095 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1625,8 +1625,6 @@ extern void user_shm_unlock(size_t, struct user_struct *);
  */
 struct zap_details {
 	struct address_space *check_mapping;	/* Check page->mapping if set */
-	pgoff_t	first_index;			/* Lowest page->index to unmap */
-	pgoff_t last_index;			/* Highest page->index to unmap */
 };
 
 struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
diff --git a/mm/memory.c b/mm/memory.c
index eeae590e526a..70d57c39380d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3136,20 +3136,20 @@ static void unmap_mapping_range_vma(struct vm_area_struct *vma,
 }
 
 static inline void unmap_mapping_range_tree(struct rb_root_cached *root,
+					    pgoff_t first_index,
+					    pgoff_t last_index,
 					    struct zap_details *details)
 {
 	struct vm_area_struct *vma;
 	pgoff_t vba, vea, zba, zea;
 
-	vma_interval_tree_foreach(vma, root,
-			details->first_index, details->last_index) {
-
+	vma_interval_tree_foreach(vma, root, first_index, last_index) {
 		vba = vma->vm_pgoff;
 		vea = vba + vma_pages(vma) - 1;
-		zba = details->first_index;
+		zba = first_index;
 		if (zba < vba)
 			zba = vba;
-		zea = details->last_index;
+		zea = last_index;
 		if (zea > vea)
 			zea = vea;
 
@@ -3175,17 +3175,17 @@ static inline void unmap_mapping_range_tree(struct rb_root_cached *root,
 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 = { };
 
 	details.check_mapping = even_cows ? NULL : mapping;
-	details.first_index = start;
-	details.last_index = start + nr - 1;
-	if (details.last_index < details.first_index)
-		details.last_index = ULONG_MAX;
+	if (last_index < first_index)
+		last_index = ULONG_MAX;
 
 	i_mmap_lock_write(mapping);
 	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
-		unmap_mapping_range_tree(&mapping->i_mmap, &details);
+		unmap_mapping_range_tree(&mapping->i_mmap, first_index,
+					 last_index, &details);
 	i_mmap_unlock_write(mapping);
 }
 
-- 
2.26.2

[PATCH 2/3] mm: Introduce zap_details.zap_flags

[Peter Xu]

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 5d977e484095..7ed4352ec84f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1620,13 +1620,30 @@ 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;	/* Check page->mapping if set */
+	unsigned long zap_flags;		/* Special flags for zapping */
 };
 
+/* 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;
+
+	return details->zap_mapping != page_rmapping(page);
+}
+
 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 70d57c39380d..20a8ba05c334 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1220,16 +1220,8 @@ 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);
@@ -1261,17 +1253,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 		if (is_device_private_entry(entry)) {
 			struct page *page = device_private_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)]--;
 			page_remove_rmap(page, false);
@@ -3176,9 +3159,11 @@ 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;
 	if (last_index < first_index)
 		last_index = ULONG_MAX;
 
-- 
2.26.2

[PATCH 3/3] mm: Introduce ZAP_FLAG_SKIP_SWAP

[Peter Xu]

Firstly, the comment in zap_pte_range() is misleading because it checks against
details rather than check_mappings, so it's against what the code did.

Meanwhile, it's confusing too on not explaining why passing in the details
pointer would mean to skip all swap entries.  New user of zap_details could
very possibly miss this fact if they don't read deep until zap_pte_range()
because there's no comment at zap_details talking about it at all, so swap
entries could be errornously skipped without being noticed.

This partly reverts 3e8715fdc03e ("mm: drop zap_details::check_swap_entries"),
but introduce ZAP_FLAG_SKIP_SWAP flag, which means the opposite of previous
"details" parameter: the caller should explicitly set this to skip swap
entries, otherwise swap entries will always be considered (which is still the
major case here).

Cc: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/mm.h | 12 ++++++++++++
 mm/memory.c        |  8 +++++---
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7ed4352ec84f..16631ee5eb9d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1622,6 +1622,8 @@ extern void user_shm_unlock(size_t, struct user_struct *);
 
 /* Whether to check page->mapping when zapping */
 #define  ZAP_FLAG_CHECK_MAPPING             BIT(0)
+/* Whether to skip zapping swap entries */
+#define  ZAP_FLAG_SKIP_SWAP                 BIT(1)
 
 /*
  * Parameter block passed down to zap_pte_range in exceptional cases.
@@ -1644,6 +1646,16 @@ zap_check_mapping_skip(struct zap_details *details, struct page *page)
 	return details->zap_mapping != page_rmapping(page);
 }
 
+/* Return true if skip swap entries, false otherwise */
+static inline bool
+zap_skip_swap(struct zap_details *details)
+{
+	if (!details)
+		return false;
+
+	return details->zap_flags & ZAP_FLAG_SKIP_SWAP;
+}
+
 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 20a8ba05c334..c9945f3c374d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1262,8 +1262,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 			continue;
 		}
 
-		/* If details->check_mapping, we leave swap entries. */
-		if (unlikely(details))
+		if (unlikely(zap_skip_swap(details)))
 			continue;
 
 		if (!non_swap_entry(entry))
@@ -3159,7 +3158,10 @@ 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 = { .zap_mapping = mapping };
+	struct zap_details details = {
+		.zap_mapping = mapping,
+		.zap_flags = ZAP_FLAG_SKIP_SWAP,
+	};
 
 	if (!even_cows)
 		details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;
-- 
2.26.2

Re: [PATCH 0/3] mm: Some rework on zap_details

[Peter Xu]

On Mon, Dec 07, 2020 at 09:50:19PM -0500, Peter Xu wrote:
> Posted this small series out to rework zap_details a bit, before adding
> something new in.  Hopefully it makes things slighly clearer.
> 
> Smoke test only.  Please have a look, thanks.
> 
> Peter Xu (3):
>   mm: Drop first_index/last_index in zap_details
>   mm: Introduce zap_details.zap_flags
>   mm: Introduce ZAP_FLAG_SKIP_SWAP

Some more information on "before adding something new in" - the new flag as a
reference but not yet posted anywhere...

https://github.com/xzpeter/linux/commit/102790511a1a25b7be841fb2059a8c8d8f1a87b2

I still think this small series worths to be reviewed/merged even before that
new flag, because patch 1 should definitely be something good to have, patch 2
prepares for patch 3 and the new bit (which can be seen as optional), but patch
3 should help make things clearer rather than using non-null "details" pointer
to show "whether we should skip swap entries", which can be easily misused IMHO
when someone accidentally replaced one "details==NULL" with some valid pointer.

Of course I can repost these series with the larger one when time comes too,
but I'd still like to at least get a NO early if there is...

Thanks,

-- 
Peter Xu