[PATCH v8 02/14] mm: shmem: shmem_getattr(): set blksize to highest supported THP order

[Luiz Capitulino <luizcap@redhat.com>]

Today, shmem_getattr() sets stat->blksize to PMD size whenever
shmem_huge_global_enabled() returns non-zero. While this works fine
for the normal THP-enabled case as explained by Baolin in [1], this
has two problems:

 1. Theoretically, when shmem is configured for within_size, this
    could set blksize to PMD size even though the allocation may
    be a smaller mTHP order

 2. A future commit will allow shmem THP support to be enabled
    even when the CPU doesn't support PMD-sized pages. We should
    not allow blksize to be set to PMD size in this case

In order to fix #1 and prepare for #2, this commit sets blksize
to the size of the highest supported order returned by
shmem_huge_global_enabled().

  [1] https://lore.kernel.org/linux-mm/6591a74c-7ef9-4614-9ae9-cb2fbed86ebf@linux.alibaba.com/

Suggested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 mm/shmem.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index b572c60f2af8..776dff8a848e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1506,6 +1506,7 @@ static int shmem_getattr(struct mnt_idmap *idmap,
 {
 	struct inode *inode = path->dentry->d_inode;
 	struct shmem_inode_info *info = SHMEM_I(inode);
+	unsigned int orders;
 
 	/* Fast-path hint; recalc under info->lock corrects any stale read. */
 	if (data_race(info->alloced - info->swapped != inode->i_mapping->nrpages))
@@ -1522,8 +1523,9 @@ static int shmem_getattr(struct mnt_idmap *idmap,
 			STATX_ATTR_NODUMP);
 	generic_fillattr(idmap, request_mask, inode, stat);
 
-	if (shmem_huge_global_enabled(inode, 0, 0, false, NULL, 0))
-		stat->blksize = HPAGE_PMD_SIZE;
+	orders = shmem_huge_global_enabled(inode, 0, 0, false, NULL, 0);
+	if (orders)
+		stat->blksize = PAGE_SIZE << highest_order(orders);
 
 	if (request_mask & STATX_BTIME) {
 		stat->result_mask |= STATX_BTIME;
-- 
2.55.0