[PATCH v10 03/10] mm/rmap: Split try_to_munlock from try_to_unmap

[Alistair Popple]

The behaviour of try_to_unmap_one() is difficult to follow because it
performs different operations based on a fairly large set of flags used
in different combinations.

TTU_MUNLOCK is one such flag. However it is exclusively used by
try_to_munlock() which specifies no other flags. Therefore rather than
overload try_to_unmap_one() with unrelated behaviour split this out into
it's own function and remove the flag.

Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>

---

v10:
* More comment fixes
* Restored the check of VM_LOCKED under that ptl. This closess a race in
  unmap path.

v9:
* Improved comments

v8:
* Renamed try_to_munlock to page_mlock to better reflect what the
  function actually does.
* Removed the TODO from the documentation that this patch addresses.

v7:
* Added Christoph's Reviewed-by

v4:
* Removed redundant check for VM_LOCKED
---
 Documentation/vm/unevictable-lru.rst | 33 ++++++--------
 include/linux/rmap.h                 |  3 +-
 mm/mlock.c                           | 12 ++---
 mm/rmap.c                            | 66 +++++++++++++++++++++-------
 4 files changed, 69 insertions(+), 45 deletions(-)

diff --git a/Documentation/vm/unevictable-lru.rst b/Documentation/vm/unevictable-lru.rst
index 0e1490524f53..eae3af17f2d9 100644
--- a/Documentation/vm/unevictable-lru.rst
+++ b/Documentation/vm/unevictable-lru.rst
@@ -389,14 +389,14 @@ mlocked, munlock_vma_page() updates that zone statistics for the number of
 mlocked pages.  Note, however, that at this point we haven't checked whether
 the page is mapped by other VM_LOCKED VMAs.
 
-We can't call try_to_munlock(), the function that walks the reverse map to
+We can't call page_mlock(), the function that walks the reverse map to
 check for other VM_LOCKED VMAs, without first isolating the page from the LRU.
-try_to_munlock() is a variant of try_to_unmap() and thus requires that the page
+page_mlock() is a variant of try_to_unmap() and thus requires that the page
 not be on an LRU list [more on these below].  However, the call to
-isolate_lru_page() could fail, in which case we couldn't try_to_munlock().  So,
+isolate_lru_page() could fail, in which case we can't call page_mlock().  So,
 we go ahead and clear PG_mlocked up front, as this might be the only chance we
-have.  If we can successfully isolate the page, we go ahead and
-try_to_munlock(), which will restore the PG_mlocked flag and update the zone
+have.  If we can successfully isolate the page, we go ahead and call
+page_mlock(), which will restore the PG_mlocked flag and update the zone
 page statistics if it finds another VMA holding the page mlocked.  If we fail
 to isolate the page, we'll have left a potentially mlocked page on the LRU.
 This is fine, because we'll catch it later if and if vmscan tries to reclaim
@@ -545,31 +545,24 @@ munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim,
 holepunching, and truncation of file pages and their anonymous COWed pages.
 
 
-try_to_munlock() Reverse Map Scan
+page_mlock() Reverse Map Scan
 ---------------------------------
 
-.. warning::
-   [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
-   page_referenced() reverse map walker.
-
 When munlock_vma_page() [see section :ref:`munlock()/munlockall() System Call
 Handling <munlock_munlockall_handling>` above] tries to munlock a
 page, it needs to determine whether or not the page is mapped by any
 VM_LOCKED VMA without actually attempting to unmap all PTEs from the
 page.  For this purpose, the unevictable/mlock infrastructure
-introduced a variant of try_to_unmap() called try_to_munlock().
+introduced a variant of try_to_unmap() called page_mlock().
 
-try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
-mapped file and KSM pages with a flag argument specifying unlock versus unmap
-processing.  Again, these functions walk the respective reverse maps looking
-for VM_LOCKED VMAs.  When such a VMA is found, as in the try_to_unmap() case,
-the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK.  This
-undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page.
+page_mlock() walks the respective reverse maps looking for VM_LOCKED VMAs. When
+such a VMA is found the page is mlocked via mlock_vma_page(). This undoes the
+pre-clearing of the page's PG_mlocked done by munlock_vma_page.
 
-Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's
+Note that page_mlock()'s reverse map walk must visit every VMA in a page's
 reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA.
 However, the scan can terminate when it encounters a VM_LOCKED VMA.
-Although try_to_munlock() might be called a great many times when munlocking a
+Although page_mlock() might be called a great many times when munlocking a
 large region or tearing down a large address space that has been mlocked via
 mlockall(), overall this is a fairly rare event.
 
@@ -602,7 +595,7 @@ inactive lists to the appropriate node's unevictable list.
 shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd
 after shrink_active_list() had moved them to the inactive list, or pages mapped
 into VM_LOCKED VMAs that munlock_vma_page() couldn't isolate from the LRU to
-recheck via try_to_munlock().  shrink_inactive_list() won't notice the latter,
+recheck via page_mlock().  shrink_inactive_list() won't notice the latter,
 but will pass on to shrink_page_list().
 
 shrink_page_list() again culls obviously unevictable pages that it could
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index def5c62c93b3..38a746787c2f 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -87,7 +87,6 @@ struct anon_vma_chain {
 
 enum ttu_flags {
 	TTU_MIGRATION		= 0x1,	/* migration mode */
-	TTU_MUNLOCK		= 0x2,	/* munlock mode */
 
 	TTU_SPLIT_HUGE_PMD	= 0x4,	/* split huge PMD if any */
 	TTU_IGNORE_MLOCK	= 0x8,	/* ignore mlock */
@@ -239,7 +238,7 @@ int page_mkclean(struct page *);
  * called in munlock()/munmap() path to check for other vmas holding
  * the page mlocked.
  */
-void try_to_munlock(struct page *);
+void page_mlock(struct page *page);
 
 void remove_migration_ptes(struct page *old, struct page *new, bool locked);
 
diff --git a/mm/mlock.c b/mm/mlock.c
index df590fda5688..4ab757ab6fe8 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -108,7 +108,7 @@ void mlock_vma_page(struct page *page)
 /*
  * Finish munlock after successful page isolation
  *
- * Page must be locked. This is a wrapper for try_to_munlock()
+ * Page must be locked. This is a wrapper for page_mlock()
  * and putback_lru_page() with munlock accounting.
  */
 static void __munlock_isolated_page(struct page *page)
@@ -118,7 +118,7 @@ static void __munlock_isolated_page(struct page *page)
 	 * and we don't need to check all the other vmas.
 	 */
 	if (page_mapcount(page) > 1)
-		try_to_munlock(page);
+		page_mlock(page);
 
 	/* Did try_to_unlock() succeed or punt? */
 	if (!PageMlocked(page))
@@ -158,7 +158,7 @@ static void __munlock_isolation_failed(struct page *page)
  * munlock()ed or munmap()ed, we want to check whether other vmas hold the
  * page locked so that we can leave it on the unevictable lru list and not
  * bother vmscan with it.  However, to walk the page's rmap list in
- * try_to_munlock() we must isolate the page from the LRU.  If some other
+ * page_mlock() we must isolate the page from the LRU.  If some other
  * task has removed the page from the LRU, we won't be able to do that.
  * So we clear the PageMlocked as we might not get another chance.  If we
  * can't isolate the page, we leave it for putback_lru_page() and vmscan
@@ -168,7 +168,7 @@ unsigned int munlock_vma_page(struct page *page)
 {
 	int nr_pages;
 
-	/* For try_to_munlock() and to serialize with page migration */
+	/* For page_mlock() and to serialize with page migration */
 	BUG_ON(!PageLocked(page));
 	VM_BUG_ON_PAGE(PageTail(page), page);
 
@@ -205,7 +205,7 @@ static int __mlock_posix_error_return(long retval)
  *
  * The fast path is available only for evictable pages with single mapping.
  * Then we can bypass the per-cpu pvec and get better performance.
- * when mapcount > 1 we need try_to_munlock() which can fail.
+ * when mapcount > 1 we need page_mlock() which can fail.
  * when !page_evictable(), we need the full redo logic of putback_lru_page to
  * avoid leaving evictable page in unevictable list.
  *
@@ -414,7 +414,7 @@ static unsigned long __munlock_pagevec_fill(struct pagevec *pvec,
  *
  * We don't save and restore VM_LOCKED here because pages are
  * still on lru.  In unmap path, pages might be scanned by reclaim
- * and re-mlocked by try_to_{munlock|unmap} before we unmap and
+ * and re-mlocked by page_mlock/try_to_unmap before we unmap and
  * free them.  This will result in freeing mlocked pages.
  */
 void munlock_vma_pages_range(struct vm_area_struct *vma,
diff --git a/mm/rmap.c b/mm/rmap.c
index bc08c4d4b58a..b6c50df08b3b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1405,10 +1405,6 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 	struct mmu_notifier_range range;
 	enum ttu_flags flags = (enum ttu_flags)(long)arg;
 
-	/* munlock has nothing to gain from examining un-locked vmas */
-	if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
-		return true;
-
 	if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
 	    is_zone_device_page(page) && !is_device_private_page(page))
 		return true;
@@ -1469,8 +1465,6 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 				page_vma_mapped_walk_done(&pvmw);
 				break;
 			}
-			if (flags & TTU_MUNLOCK)
-				continue;
 		}
 
 		/* Unexpected PMD-mapped THP? */
@@ -1784,20 +1778,58 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
 	return !page_mapcount(page) ? true : false;
 }
 
+/*
+ * Walks the vma's mapping a page and mlocks the page if any locked vma's are
+ * found. Once one is found the page is locked and the scan can be terminated.
+ */
+static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
+				 unsigned long address, void *unused)
+{
+	struct page_vma_mapped_walk pvmw = {
+		.page = page,
+		.vma = vma,
+		.address = address,
+	};
+
+	/* An un-locked vma doesn't have any pages to lock, continue the scan */
+	if (!(vma->vm_flags & VM_LOCKED))
+		return true;
+
+	while (page_vma_mapped_walk(&pvmw)) {
+		/*
+		 * Need to recheck under the ptl to serialise with
+		 * __munlock_pagevec_fill() after VM_LOCKED is cleared in
+		 * munlock_vma_pages_range().
+		 */
+		if (vma->vm_flags & VM_LOCKED) {
+			/* PTE-mapped THP are never mlocked */
+			if (!PageTransCompound(page))
+				mlock_vma_page(page);
+			page_vma_mapped_walk_done(&pvmw);
+		}
+
+		/*
+		 * no need to continue scanning other vma's if the page has
+		 * been locked.
+		 */
+		return false;
+	}
+
+	return true;
+}
+
 /**
- * try_to_munlock - try to munlock a page
- * @page: the page to be munlocked
+ * page_mlock - try to mlock a page
+ * @page: the page to be mlocked
  *
- * Called from munlock code.  Checks all of the VMAs mapping the page
- * to make sure nobody else has this page mlocked. The page will be
- * returned with PG_mlocked cleared if no other vmas have it mlocked.
+ * Called from munlock code. Checks all of the VMAs mapping the page and mlocks
+ * the page if any are found. The page will be returned with PG_mlocked cleared
+ * if it is not mapped by any locked vmas.
  */
-
-void try_to_munlock(struct page *page)
+void page_mlock(struct page *page)
 {
 	struct rmap_walk_control rwc = {
-		.rmap_one = try_to_unmap_one,
-		.arg = (void *)TTU_MUNLOCK,
+		.rmap_one = page_mlock_one,
 		.done = page_not_mapped,
 		.anon_lock = page_lock_anon_vma_read,
 
@@ -1849,7 +1881,7 @@ static struct anon_vma *rmap_walk_anon_lock(struct page *page,
  * Find all the mappings of a page using the mapping pointer and the vma chains
  * contained in the anon_vma struct it points to.
  *
- * When called from try_to_munlock(), the mmap_lock of the mm containing the vma
+ * When called from page_mlock(), the mmap_lock of the mm containing the vma
  * where the page was found will be held for write.  So, we won't recheck
  * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
  * LOCKED.
@@ -1901,7 +1933,7 @@ static void rmap_walk_anon(struct page *page, struct rmap_walk_control *rwc,
  * Find all the mappings of a page using the mapping pointer and the vma chains
  * contained in the address_space struct it points to.
  *
- * When called from try_to_munlock(), the mmap_lock of the mm containing the vma
+ * When called from page_mlock(), the mmap_lock of the mm containing the vma
  * where the page was found will be held for write.  So, we won't recheck
  * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
  * LOCKED.
-- 
2.20.1