[PATCH v9 14/15] filemap: Add support for authoritative mappings

An authoritative mapping knows about all the folios in the mapping.
If read() finds a missing folio, there's no reason to allocate one and
try to read it because we know it's a zero region of the file.  We can
just call iov_iter_zero() instead.

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Jane Chu <[email protected]>
---
 include/linux/pagemap.h | 11 +++++++++++
 mm/filemap.c            | 18 ++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 968b791cfd14..22d48935ffda 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -210,6 +210,7 @@ enum mapping_flags {
 	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
 	AS_KERNEL_FILE = 10,	/* mapping for a fake kernel file that shouldn't
 				   account usage to user cgroups */
+	AS_AUTHORITATIVE = 11,	/* If we miss in the page cache, it's a hole */
 	/* Bits 16-25 are used for FOLIO_ORDER */
 	AS_FOLIO_ORDER_BITS = 5,
 	AS_FOLIO_ORDER_MIN = 16,
@@ -345,6 +346,16 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(const struct addres
 	return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
 }
 
+static inline void mapping_set_authoritative(struct address_space *mapping)
+{
+	set_bit(AS_AUTHORITATIVE, &mapping->flags);
+}
+
+static inline bool mapping_is_authoritative(const struct address_space *mapping)
+{
+	return test_bit(AS_AUTHORITATIVE, &mapping->flags);
+}
+
 static inline gfp_t mapping_gfp_mask(const struct address_space *mapping)
 {
 	return mapping->gfp_mask;
diff --git a/mm/filemap.c b/mm/filemap.c
index 26a5f18121f9..5a8cc20e624e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2697,6 +2697,8 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
 	if (!folio_batch_count(fbatch)) {
 		DEFINE_READAHEAD(ractl, filp, &filp->f_ra, mapping, index);
 
+		if (mapping_is_authoritative(mapping))
+			return 0;
 		if (iocb->ki_flags & IOCB_NOIO)
 			return -EAGAIN;
 		if (iocb->ki_flags & IOCB_NOWAIT)
@@ -2853,6 +2855,22 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 			goto put_folios;
 		end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count);
 
+		if (!folio_batch_count(&fbatch)) {
+			size_t fsize = mapping_min_folio_nrbytes(mapping);
+			size_t offset = iocb->ki_pos & (fsize - 1);
+			size_t bytes = min_t(loff_t, end_offset - iocb->ki_pos,
+					     fsize - offset);
+			size_t copied = iov_iter_zero(bytes, iter);
+
+			already_read += copied;
+			iocb->ki_pos += copied;
+			last_pos = iocb->ki_pos;
+
+			if (copied < bytes)
+				error = -EFAULT;
+			continue;
+		}
+
 		/*
 		 * Once we start copying data, we don't want to be touching any
 		 * cachelines that might be contended:
-- 
2.47.3