mm: introduce and use mapping_empty()

Patch series "Remove nrexceptional tracking", v2.

We actually use nrexceptional for very little these days.  It's a minor
pain to keep in sync with nrpages, but the pain becomes much bigger with
the THP patches because we don't know how many indices a shadow entry
occupies.  It's easier to just remove it than keep it accurate.

Also, we save 8 bytes per inode which is nothing to sneeze at; on my
laptop, it would improve shmem_inode_cache from 22 to 23 objects per
16kB, and inode_cache from 26 to 27 objects.  Combined, that saves
a megabyte of memory from a combined usage of 25MB for both caches.
Unfortunately, ext4 doesn't cross a magic boundary, so it doesn't save
any memory for ext4.

This patch (of 4):

Instead of checking the two counters (nrpages and nrexceptional), we can
just check whether i_pages is empty.

Link: https://lkml.kernel.org/r/20201026151849.24232-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20201026151849.24232-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Vishal Verma <vishal.l.verma@intel.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/block_dev.c b/fs/block_dev.c
index a5244e08b6c8e3de6b9cc9e01469a0dfdc2e4bc9..9114e0a0e7b4a5a05b56d0ff6bb220f68da085bf 100644 (file)
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -79,7 +79,7 @@ static void kill_bdev(struct block_device *bdev)
 {
        struct address_space *mapping = bdev->bd_inode->i_mapping;
 
-       if (mapping->nrpages == 0 && mapping->nrexceptional == 0)
+       if (mapping_empty(mapping))
                return;
 
        invalidate_bh_lrus();

diff --git a/fs/dax.c b/fs/dax.c
index b3d27fdc67752df52637bccdbde008c0086aa312..999f3f22aea3f114a7f21dd83cd47af64492e0d6 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -965,7 +965,7 @@ int dax_writeback_mapping_range(struct address_space *mapping,
        if (WARN_ON_ONCE(inode->i_blkbits != PAGE_SHIFT))
                return -EIO;
 
-       if (!mapping->nrexceptional || wbc->sync_mode != WB_SYNC_ALL)
+       if (mapping_empty(mapping) || wbc->sync_mode != WB_SYNC_ALL)
                return 0;
 
        trace_dax_writeback_range(inode, xas.xa_index, end_index);

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index 84c38103aa0639e71bf2ac11b3ce84c6a766fdd9..ea7fc5c641c7e8e25d9e4a477075b25a4614a36e 100644 (file)
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -273,8 +273,7 @@ static void __gfs2_glock_put(struct gfs2_glock *gl)
        if (mapping) {
                truncate_inode_pages_final(mapping);
                if (!gfs2_withdrawn(sdp))
-                       GLOCK_BUG_ON(gl, mapping->nrpages ||
-                                    mapping->nrexceptional);
+                       GLOCK_BUG_ON(gl, !mapping_empty(mapping));
        }
        trace_gfs2_glock_put(gl);
        sdp->sd_lockstruct.ls_ops->lm_put_lock(gl);

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 469fa7ffcf9634c9c6030cd73bf4ac32d5618e53..a4bd41128bf318eb4ae536c56ccda93e6e29b436 100644 (file)
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -18,6 +18,11 @@
 
 struct pagevec;
 
+static inline bool mapping_empty(struct address_space *mapping)
+{
+       return xa_empty(&mapping->i_pages);
+}
+
 /*
  * Bits in mapping->flags.
  */

diff --git a/mm/truncate.c b/mm/truncate.c
index 455944264663ef30f90df7027f4766b84446cdee..adb8d4107988e257d25464b7a506e94fd476ec8c 100644 (file)
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -295,7 +295,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
        pgoff_t         index;
        int             i;
 
-       if (mapping->nrpages == 0 && mapping->nrexceptional == 0)
+       if (mapping_empty(mapping))
                goto out;
 
        /* Offsets within partial pages */
@@ -440,9 +440,6 @@ EXPORT_SYMBOL(truncate_inode_pages);
  */
 void truncate_inode_pages_final(struct address_space *mapping)
 {
-       unsigned long nrexceptional;
-       unsigned long nrpages;
-
        /*
         * Page reclaim can not participate in regular inode lifetime
         * management (can't call iput()) and thus can race with the
@@ -452,16 +449,7 @@ void truncate_inode_pages_final(struct address_space *mapping)
         */
        mapping_set_exiting(mapping);
 
-       /*
-        * When reclaim installs eviction entries, it increases
-        * nrexceptional first, then decreases nrpages.  Make sure we see
-        * this in the right order or we might miss an entry.
-        */
-       nrpages = mapping->nrpages;
-       smp_rmb();
-       nrexceptional = mapping->nrexceptional;
-
-       if (nrpages || nrexceptional) {
+       if (!mapping_empty(mapping)) {
                /*
                 * As truncation uses a lockless tree lookup, cycle
                 * the tree lock to make sure any ongoing tree
@@ -633,7 +621,7 @@ int invalidate_inode_pages2_range(struct address_space *mapping,
        int ret2 = 0;
        int did_range_unmap = 0;
 
-       if (mapping->nrpages == 0 && mapping->nrexceptional == 0)
+       if (mapping_empty(mapping))
                goto out;
 
        pagevec_init(&pvec);