[PATCH 2/2] xfs: count folio alloc'd xfs_buf items in NR_KERNEL_MISC_RECLAIMABLE

xfs_buf allocations via xfs_buf_alloc_folio are actually reclaimable,
but are not accounted for as such. NR_KERNEL_MISC_RECLAIMABLE seems
made for this purpose, although AFAICT there are no users today.

To achieve this, add a new flag _XBF_PAGES to track what we have
allocated this way, increment the NR_KERNEL_MISC_RECLAIMABLE
count when we do, and then do the reverse when they are freed.

These allocations now show up under nr_kernel_misc_reclaimable in
/proc/vmstat when allocations are active, which in turn makes
MemAvailable more accurate in /proc/meminfo.

Signed-off-by: Eric Sandeen <[email protected]>
---
 fs/xfs/xfs_buf.c | 16 +++++++++++++---
 fs/xfs/xfs_buf.h |  2 ++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 2e00a33ebb62..0a853ec361d0 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -114,8 +114,15 @@ xfs_buf_free(
 		vfree(bp->b_addr);
 	else if (bp->b_flags & _XBF_KMEM)
 		kfree(bp->b_addr);
-	else if (bp->b_addr)
-		folio_put(virt_to_folio(bp->b_addr));
+	else if (bp->b_addr) {
+		struct folio *folio = virt_to_folio(bp->b_addr);
+
+		if (bp->b_flags & _XBF_PAGES)
+			mod_node_page_state(folio_pgdat(folio),
+					NR_KERNEL_MISC_RECLAIMABLE,
+					-folio_nr_pages(folio));
+		folio_put(folio);
+	}
 
 	call_rcu(&bp->b_rcu, xfs_buf_free_callback);
 }
@@ -132,6 +139,9 @@ xfs_buf_alloc_folio(
 	if (!folio)
 		return -ENOMEM;
 	bp->b_addr = folio_address(folio);
+	bp->b_flags |= _XBF_PAGES;
+	mod_node_page_state(folio_pgdat(folio), NR_KERNEL_MISC_RECLAIMABLE,
+			folio_nr_pages(folio));
 	trace_xfs_buf_backing_folio(bp, _RET_IP_);
 	return 0;
 }
@@ -427,7 +437,7 @@ xfs_buf_find_lock(
 			return -ENOENT;
 		}
 		ASSERT((bp->b_flags & _XBF_DELWRI_Q) == 0);
-		bp->b_flags &= _XBF_KMEM;
+		bp->b_flags &= (_XBF_PAGES | _XBF_KMEM);
 		bp->b_ops = NULL;
 	}
 	return 0;
diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index 79cc9c3f0254..ba7bfafe67b6 100644
--- a/fs/xfs/xfs_buf.h
+++ b/fs/xfs/xfs_buf.h
@@ -38,6 +38,7 @@ struct xfs_buf;
 #define _XBF_LOGRECOVERY (1u << 18)/* log recovery buffer */
 
 /* flags used only internally */
+#define _XBF_PAGES	 (1u << 20)/* backed by folio/page allocator */
 #define _XBF_KMEM	 (1u << 21)/* backed by heap memory */
 #define _XBF_DELWRI_Q	 (1u << 22)/* buffer on a delwri queue */
 
@@ -62,6 +63,7 @@ typedef unsigned int xfs_buf_flags_t;
 	{ XBF_STALE,		"STALE" }, \
 	{ XBF_WRITE_FAIL,	"WRITE_FAIL" }, \
 	{ _XBF_LOGRECOVERY,	"LOG_RECOVERY" }, \
+	{ _XBF_PAGES,		"PAGES" }, \
 	{ _XBF_KMEM,		"KMEM" }, \
 	{ _XBF_DELWRI_Q,	"DELWRI_Q" }, \
 	/* The following interface flags should never be set */ \
-- 
2.55.0