[RFC PATCH 7/9] shmem: check if a block is uptodate before splice into pipe

[Daniel Gomez]

The splice_read() path assumes folios are always uptodate. Make sure
all blocks in the given range are uptodate or else, splice zeropage into
the pipe. Maximize the number of blocks that can be spliced into pipe at
once by increasing the 'part' to the latest uptodate block found.

Signed-off-by: Daniel Gomez <da.gomez@xxxxxxxxxxx>
---
 mm/shmem.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 9fa86cb82da9..2d2eeb40f19b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3196,8 +3196,30 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
 		if (unlikely(*ppos >= isize))
 			break;
 		part = min_t(loff_t, isize - *ppos, len);
+		if (folio && folio_test_large(folio) &&
+		    folio_test_private(folio)) {
+			unsigned long from = offset_in_folio(folio, *ppos);
+			unsigned int bfirst = from >> inode->i_blkbits;
+			unsigned int blast, blast_upd;
+
+			len = min(folio_size(folio) - from, len);
+			blast = (from + len - 1) >> inode->i_blkbits;
+
+			blast_upd = sfs_get_last_block_uptodate(folio, bfirst,
+								blast);
+			if (blast_upd <= blast) {
+				unsigned int bsize = 1 << inode->i_blkbits;
+				unsigned int blks = blast_upd - bfirst + 1;
+				unsigned int bbytes = blks << inode->i_blkbits;
+				unsigned int boff = (*ppos % bsize);
+
+				part = min_t(loff_t, bbytes - boff, len);
+			}
+		}
 
-		if (folio) {
+		if (folio && shmem_is_block_uptodate(
+				     folio, offset_in_folio(folio, *ppos) >>
+						    inode->i_blkbits)) {
 			/*
 			 * If users can be writing to this page using arbitrary
 			 * virtual addresses, take care about potential aliasing
-- 
2.43.0