[PATCH 4/4] mvolatile: Add page purging logic & SIGBUS trap

[John Stultz <john.stultz@linaro.org>]

This patch adds the hooks in the vmscan logic to purge volatile pages
and mark their pte as purged. With this, volatile pages will be purged
under pressure, and their ptes swap entry's marked. If the purged pages
are accessed before being marked non-volatile, we catch this and send a
SIGBUS.

This is a simplified implementation that uses logic from Minchan's earlier
efforts, so credit to Minchan for his work.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Android Kernel Team <kernel-team@android.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Robert Love <rlove@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Rik van Riel <riel@redhat.com>
Cc: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Hommey <mh@glandium.org>
Cc: Taras Glek <tglek@mozilla.com>
Cc: Jan Kara <jack@suse.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Keith Packard <keithp@keithp.com>
Cc: linux-mm@kvack.org <linux-mm@kvack.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 include/linux/mvolatile.h |   2 +
 mm/internal.h             |   2 -
 mm/memory.c               |   8 ++++
 mm/mvolatile.c            | 120 ++++++++++++++++++++++++++++++++++++++++++++++
 mm/rmap.c                 |   5 ++
 mm/vmscan.c               |  12 +++++
 6 files changed, 147 insertions(+), 2 deletions(-)

diff --git a/include/linux/mvolatile.h b/include/linux/mvolatile.h
index 973bb3b..8cfe6e0 100644
--- a/include/linux/mvolatile.h
+++ b/include/linux/mvolatile.h
@@ -5,4 +5,6 @@
 
 #define MVOLATILE_VALID_FLAGS (0) /* Don't yet support any flags */
 
+extern int purge_volatile_page(struct page *page);
+
 #endif /* _LINUX_MVOLATILE_H */
diff --git a/mm/internal.h b/mm/internal.h
index 29e1e76..ea66bf9 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -225,10 +225,8 @@ static inline void mlock_migrate_page(struct page *newpage, struct page *page)
 
 extern pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma);
 
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
 extern unsigned long vma_address(struct page *page,
 				 struct vm_area_struct *vma);
-#endif
 #else /* !CONFIG_MMU */
 static inline int mlocked_vma_newpage(struct vm_area_struct *v, struct page *p)
 {
diff --git a/mm/memory.c b/mm/memory.c
index 22dfa61..9043e4c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -60,6 +60,7 @@
 #include <linux/migrate.h>
 #include <linux/string.h>
 #include <linux/dma-debug.h>
+#include <linux/mvolatile.h>
 
 #include <asm/io.h>
 #include <asm/pgalloc.h>
@@ -3643,6 +3644,8 @@ static int handle_pte_fault(struct mm_struct *mm,
 
 	entry = *pte;
 	if (!pte_present(entry)) {
+		swp_entry_t mvolatile_entry;
+
 		if (pte_none(entry)) {
 			if (vma->vm_ops) {
 				if (likely(vma->vm_ops->fault))
@@ -3652,6 +3655,11 @@ static int handle_pte_fault(struct mm_struct *mm,
 			return do_anonymous_page(mm, vma, address,
 						 pte, pmd, flags);
 		}
+
+		mvolatile_entry = pte_to_swp_entry(entry);
+		if (unlikely(is_purged_entry(mvolatile_entry)))
+			return VM_FAULT_SIGBUS;
+
 		if (pte_file(entry))
 			return do_nonlinear_fault(mm, vma, address,
 					pte, pmd, flags, entry);
diff --git a/mm/mvolatile.c b/mm/mvolatile.c
index 38c8315..16dccee 100644
--- a/mm/mvolatile.c
+++ b/mm/mvolatile.c
@@ -279,3 +279,123 @@ SYSCALL_DEFINE5(mvolatile, unsigned long, start, size_t, len,
 out:
 	return ret;
 }
+
+
+/**
+ * try_to_purge_one - Purge a volatile page from a vma
+ * @page: page to purge
+ * @vma: vma to purge page from
+ *
+ * Finds the pte for a page in a vma, marks the pte as purged
+ * and release the page.
+ */
+static void try_to_purge_one(struct page *page, struct vm_area_struct *vma)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	pte_t *pte;
+	pte_t pteval;
+	spinlock_t *ptl;
+	unsigned long addr;
+
+	VM_BUG_ON(!PageLocked(page));
+
+	addr = vma_address(page, vma);
+	pte = page_check_address(page, mm, addr, &ptl, 0);
+	if (!pte)
+		return;
+
+	BUG_ON(vma->vm_flags & (VM_SPECIAL|VM_LOCKED|VM_MIXEDMAP|VM_HUGETLB));
+
+	flush_cache_page(vma, addr, page_to_pfn(page));
+	pteval = ptep_clear_flush(vma, addr, pte);
+
+	update_hiwater_rss(mm);
+	if (PageAnon(page))
+		dec_mm_counter(mm, MM_ANONPAGES);
+	else
+		dec_mm_counter(mm, MM_FILEPAGES);
+
+	page_remove_rmap(page);
+	page_cache_release(page);
+
+	set_pte_at(mm, addr, pte, swp_entry_to_pte(make_purged_entry()));
+
+	pte_unmap_unlock(pte, ptl);
+	mmu_notifier_invalidate_page(mm, addr);
+
+}
+
+/**
+ * try_to_purge_vpage - check vma chain and purge from vmas marked volatile
+ * @page: page to purge
+ *
+ * Goes over all the vmas that hold a page, and where the vmas are volatile,
+ * purge the page from the vma.
+ *
+ * Returns 0 on success, -1 on error.
+ */
+static int try_to_purge_vpage(struct page *page)
+{
+	struct anon_vma *anon_vma;
+	struct anon_vma_chain *avc;
+	pgoff_t pgoff;
+	int ret = 0;
+
+	anon_vma = page_lock_anon_vma_read(page);
+	if (!anon_vma)
+		return -1;
+
+	pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+	/*
+	 * During interating the loop, some processes could see a page as
+	 * purged while others could see a page as not-purged because we have
+	 * no global lock between parent and child for protecting mvolatile
+	 * system call during this loop. But it's not a problem because the
+	 * page is  not *SHARED* page but *COW* page so parent and child can
+	 * see other data anytime. The worst case by this race is a page was
+	 * purged but couldn't be discarded so it makes unnecessary pagefault
+	 * but it wouldn't be severe.
+	 */
+	anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
+		struct vm_area_struct *vma = avc->vma;
+
+		if (!(vma->vm_flags & VM_VOLATILE)) {
+			ret = -1;
+			break;
+		}
+		try_to_purge_one(page, vma);
+	}
+	page_unlock_anon_vma_read(anon_vma);
+	return ret;
+}
+
+
+/**
+ * purge_volatile_page - If possible, purge the specified volatile page
+ * @page: page to purge
+ *
+ * Attempts to purge a volatile page, and if needed frees the swap page
+ *
+ * Returns 0 on success, -1 on error.
+ */
+int purge_volatile_page(struct page *page)
+{
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(PageLRU(page));
+
+	/* XXX - for now we only support anonymous volatile pages */
+	if (!PageAnon(page))
+		return -1;
+
+	if (!try_to_purge_vpage(page)) {
+		if (PageSwapCache(page))
+			try_to_free_swap(page);
+
+		if (page_freeze_refs(page, 1)) {
+			unlock_page(page);
+			return 0;
+		}
+	}
+
+	return -1;
+}
diff --git a/mm/rmap.c b/mm/rmap.c
index 8fc049f..2c2aa7d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -728,6 +728,11 @@ int page_referenced_one(struct page *page, struct vm_area_struct *vma,
 				referenced++;
 		}
 		pte_unmap_unlock(pte, ptl);
+		if (vma->vm_flags & VM_VOLATILE) {
+			pra->mapcount = 0;
+			pra->vm_flags |= VM_VOLATILE;
+			return SWAP_FAIL;
+		}
 	}
 
 	if (referenced) {
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a9c74b4..0cbfbf6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -43,6 +43,7 @@
 #include <linux/sysctl.h>
 #include <linux/oom.h>
 #include <linux/prefetch.h>
+#include <linux/mvolatile.h>
 
 #include <asm/tlbflush.h>
 #include <asm/div64.h>
@@ -683,6 +684,7 @@ enum page_references {
 	PAGEREF_RECLAIM,
 	PAGEREF_RECLAIM_CLEAN,
 	PAGEREF_KEEP,
+	PAGEREF_PURGE,
 	PAGEREF_ACTIVATE,
 };
 
@@ -703,6 +705,13 @@ static enum page_references page_check_references(struct page *page,
 	if (vm_flags & VM_LOCKED)
 		return PAGEREF_RECLAIM;
 
+	/*
+	 * If volatile page is reached on LRU's tail, we discard the
+	 * page without considering recycle the page.
+	 */
+	if (vm_flags & VM_VOLATILE)
+		return PAGEREF_PURGE;
+
 	if (referenced_ptes) {
 		if (PageSwapBacked(page))
 			return PAGEREF_ACTIVATE;
@@ -930,6 +939,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		switch (references) {
 		case PAGEREF_ACTIVATE:
 			goto activate_locked;
+		case PAGEREF_PURGE:
+			if (!purge_volatile_page(page))
+				goto free_it;
 		case PAGEREF_KEEP:
 			goto keep_locked;
 		case PAGEREF_RECLAIM:
-- 
1.8.3.2

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