Re: [PATCH v8 01/10] fs: Allow fine-grained control of folio sizes

["Pankaj Raghav (Samsung)" <kernel@xxxxxxxxxxxxxxxx>]

For now, this is the only patch that is blocking for the next version.

Based on the discussion, is the following logical @ryan, @dave and
@willy?

- We give explicit VM_WARN_ONCE if we try to set folio order range if
  the THP is disabled, min and max is greater than MAX_PAGECACHE_ORDER.

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 14e1415f7dcf4..313c9fad61859 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -394,13 +394,24 @@ static inline void mapping_set_folio_order_range(struct address_space *mapping,
                                                 unsigned int min,
                                                 unsigned int max)
 {
-       if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
+       if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
+               VM_WARN_ONCE(1, 
+       "THP needs to be enabled to support mapping folio order range");
                return;
+       }
 
-       if (min > MAX_PAGECACHE_ORDER)
+       if (min > MAX_PAGECACHE_ORDER) {
+               VM_WARN_ONCE(1, 
+       "min order > MAX_PAGECACHE_ORDER. Setting min_order to MAX_PAGECACHE_ORDER");
                min = MAX_PAGECACHE_ORDER;
-       if (max > MAX_PAGECACHE_ORDER)
+       }
+
+       if (max > MAX_PAGECACHE_ORDER) {
+               VM_WARN_ONCE(1, 
+       "max order > MAX_PAGECACHE_ORDER. Setting max_order to MAX_PAGECACHE_ORDER");
                max = MAX_PAGECACHE_ORDER;
+       }
+
        if (max < min)
                max = min;

- We make THP an explicit dependency for XFS:

diff --git a/fs/xfs/Kconfig b/fs/xfs/Kconfig
index d41edd30388b7..be2c1c0e9fe8b 100644
--- a/fs/xfs/Kconfig
+++ b/fs/xfs/Kconfig
@@ -5,6 +5,7 @@ config XFS_FS
        select EXPORTFS
        select LIBCRC32C
        select FS_IOMAP
+       select TRANSPARENT_HUGEPAGE
        help
          XFS is a high performance journaling filesystem which originated
          on the SGI IRIX platform.  It is completely multi-threaded, can

OR

We create a helper in page cache that FSs can use to check if a specific
order can be supported at mount time:

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 14e1415f7dcf..9be775ef11a5 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -374,6 +374,14 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
 #define MAX_XAS_ORDER          (XA_CHUNK_SHIFT * 2 - 1)
 #define MAX_PAGECACHE_ORDER    min(MAX_XAS_ORDER, PREFERRED_MAX_PAGECACHE_ORDER)
 
+
+static inline unsigned int mapping_max_folio_order_supported()
+{
+    if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
+      return 0;
+    return MAX_PAGECACHE_ORDER;
+}
+


diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index b8a93a8f35cac..e2be8743c2c20 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1647,6 +1647,15 @@ xfs_fs_fill_super(
                        goto out_free_sb;
                }
 
+               if (mp->m_sb.sb_blocklog - PAGE_SHIFT >
+                   mapping_max_folio_order_supported()) {
+                       xfs_warn(mp,
+"Block Size (%d bytes) is not supported. Check MAX_PAGECACHE_ORDER",
+                       mp->m_sb.sb_blocksize);
+                       error = -ENOSYS;
+                       goto out_free_sb;
+               }
+
                xfs_warn(mp,
 "EXPERIMENTAL: V5 Filesystem with Large Block Size (%d bytes) enabled.",
                        mp->m_sb.sb_blocksize);


--
Pankaj