On Wednesday, 3 March 2021 9:08:15 AM AEDT Zi Yan wrote:

On 26 Feb 2021, at 2:18, Alistair Popple wrote:

quoted

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 7f1ee411bd7b..77fa17de51d7 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h

@@ -86,8 +86,6 @@ struct anon_vma_chain {
 };

 enum ttu_flags {
-	TTU_MIGRATION		= 0x1,	/* migration mode */
-
 	TTU_SPLIT_HUGE_PMD	= 0x4,	/* split huge PMD if any */

It implies freeze in try_to_migrate() and no freeze in try_to_unmap(). I 

think

we need some comments here, above try_to_migrate(), and above try_to_unmap()
to clarify the implication.

Sure. This confused me for a bit and I was initially tempted to leave 
TTU_SPLIT_FREEZE as a separate mode flag but looking at what freeze actually 
does it made sense to remove it because try_to_migrate() is for installing 
migration entries (which is what freeze does) and try_to_unmap() just unmaps. 
So I'll add some comments to that effect.
 

quoted

 	TTU_IGNORE_MLOCK	= 0x8,	/* ignore mlock */
 	TTU_IGNORE_HWPOISON	= 0x20,	/* corrupted page is recoverable */

@@ -96,7 +94,6 @@ enum ttu_flags {
 					 * do a final flush if necessary */
 	TTU_RMAP_LOCKED		= 0x80,	/* do not grab rmap lock:
 					 * caller holds it */
-	TTU_SPLIT_FREEZE	= 0x100,		/* freeze pte under splitting thp */
 };

 #ifdef CONFIG_MMU

@@ -193,6 +190,7 @@ static inline void page_dup_rmap(struct page *page, 

bool compound)

quoted

 int page_referenced(struct page *, int is_locked,
 			struct mem_cgroup *memcg, unsigned long *vm_flags);

+bool try_to_migrate(struct page *page, enum ttu_flags flags);
 bool try_to_unmap(struct page *, enum ttu_flags flags);

 /* Avoid racy checks */

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d00b93dc2d9e..357052a4567b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -2351,16 +2351,16 @@ void vma_adjust_trans_huge(struct vm_area_struct 

*vma,

quoted

 static void unmap_page(struct page *page)
 {
-	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
-		TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
+	enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
 	bool unmap_success;

 	VM_BUG_ON_PAGE(!PageHead(page), page);

 	if (PageAnon(page))
-		ttu_flags |= TTU_SPLIT_FREEZE;
-
-	unmap_success = try_to_unmap(page, ttu_flags);
+		unmap_success = try_to_migrate(page, ttu_flags);
+	else
+		unmap_success = try_to_unmap(page, ttu_flags |
+						TTU_IGNORE_MLOCK);

I think we need a comment here about why anonymous pages need 

try_to_migrate()

and others need try_to_unmap().

Historically this comes from baa355fd3314 ("thp: file pages support for 
split_huge_page()") which says:

"We don't setup migration entries. Just unmap pages. It helps handling cases 
when i_size is in the middle of the page: no need handle unmap pages beyond 
i_size manually."

But I'll add a comment here, thanks.

 - Alistair

Thanks.

—
Best Regards,
Yan Zi