[patch 1/2] mm: fincore()

[Johannes Weiner]

On Mon, Feb 11, 2013 at 02:12:39PM -0800, Andrew Morton wrote:
> Also, having to mmap the file to be able to query pagecache state is a
> hack.  Whatever happened to the fincore() patch?

I don't know, but how about this one:

---
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: [patch 1/2] mm: fincore()

Provide a syscall to determine whether a given file's pages are cached
in memory.  This is more elegant than mmapping the file for the sole
purpose of using mincore(), and also works on NOMMU.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 include/linux/syscalls.h |   2 +
 mm/Makefile              |   2 +-
 mm/fincore.c             | 128 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 131 insertions(+), 1 deletion(-)
 create mode 100644 mm/fincore.c

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 313a8e0..3ceab2a 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -897,4 +897,6 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
 asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
 			 unsigned long idx1, unsigned long idx2);
 asmlinkage long sys_finit_module(int fd, const char __user *uargs, int flags);
+asmlinkage long sys_fincore(unsigned int fd, loff_t start, loff_t len,
+			    unsigned char __user * vec);
 #endif
diff --git a/mm/Makefile b/mm/Makefile
index 185a22b..221cdae 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -17,7 +17,7 @@ obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
 			   util.o mmzone.o vmstat.o backing-dev.o \
 			   mm_init.o mmu_context.o percpu.o slab_common.o \
 			   compaction.o balloon_compaction.o \
-			   interval_tree.o $(mmu-y)
+			   interval_tree.o fincore.o $(mmu-y)
 
 obj-y += init-mm.o
 
diff --git a/mm/fincore.c b/mm/fincore.c
new file mode 100644
index 0000000..d504611
--- /dev/null
+++ b/mm/fincore.c
@@ -0,0 +1,128 @@
+#include <linux/syscalls.h>
+#include <linux/pagemap.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+
+static long do_fincore(struct address_space *mapping, pgoff_t pgstart,
+		       unsigned long nr_pages, unsigned char *vec)
+{
+	pgoff_t pgend = pgstart + nr_pages;
+	struct radix_tree_iter iter;
+	void **slot;
+	long nr = 0;
+
+	rcu_read_lock();
+restart:
+	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, pgstart) {
+		unsigned char present;
+		struct page *page;
+
+		/* Handle holes */
+		if (iter.index != pgstart + nr) {
+			if (iter.index < pgend)
+				nr_pages = iter.index - pgstart;
+			break;
+		}
+repeat:
+		page = radix_tree_deref_slot(slot);
+		if (unlikely(!page))
+			continue;
+
+		if (radix_tree_exception(page)) {
+			if (radix_tree_deref_retry(page)) {
+				/*
+				 * Transient condition which can only trigger
+				 * when entry at index 0 moves out of or back
+				 * to root: none yet gotten, safe to restart.
+				 */
+				WARN_ON(iter.index);
+				goto restart;
+			}
+			present = 0;
+		} else {
+			if (!page_cache_get_speculative(page))
+				goto repeat;
+
+			/* Has the page moved? */
+			if (unlikely(page != *slot)) {
+				page_cache_release(page);
+				goto repeat;
+			}
+
+			present = PageUptodate(page);
+			page_cache_release(page);
+		}
+		vec[nr] = present;
+
+		if (++nr == nr_pages)
+			break;
+	}
+	rcu_read_unlock();
+
+	if (nr < nr_pages)
+		memset(vec + nr, 0, nr_pages - nr);
+
+	return nr_pages;
+}
+
+/*
+ * The fincore(2) system call.
+ *
+ * fincore() returns the memory residency status of the given file's
+ * pages, in the range [start, start + len].
+ * The status is returned in a vector of bytes.  The least significant
+ * bit of each byte is 1 if the referenced page is in memory, otherwise
+ * it is zero.
+ *
+ * Because the status of a page can change after fincore() checks it
+ * but before it returns to the application, the returned vector may
+ * contain stale information.
+ *
+ * return values:
+ *  zero    - success
+ *  -EBADF  - fd isn't a valid open file descriptor
+ *  -EFAULT - vec points to an illegal address
+ *  -EINVAL - start is not a multiple of PAGE_CACHE_SIZE
+ */
+SYSCALL_DEFINE4(fincore, unsigned int, fd, loff_t, start, loff_t, len,
+		unsigned char __user *, vec)
+{
+	unsigned long nr_pages;
+	pgoff_t pgstart;
+	struct fd f;
+	long ret;
+
+	if (start & ~PAGE_CACHE_MASK)
+		return -EINVAL;
+
+	f = fdget(fd);
+	if (!f.file)
+		return -EBADF;
+
+	pgstart = start >> PAGE_CACHE_SHIFT;
+	nr_pages = DIV_ROUND_UP(len, PAGE_CACHE_SIZE);
+
+	while (nr_pages) {
+		unsigned char tmp[64];
+
+		ret = do_fincore(f.file->f_mapping, pgstart,
+				 min(nr_pages, sizeof(tmp)), tmp);
+		if (ret <= 0)
+			break;
+
+		if (copy_to_user(vec, tmp, ret)) {
+			ret = -EFAULT;
+			break;
+		}
+
+		nr_pages -= ret;
+		pgstart += ret;
+		vec += ret;
+		ret = 0;
+	}
+
+	fdput(f);
+
+	return ret;
+}
-- 
1.7.11.7

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