[PATCH 4/7] shmem: remove shmem_get_partial_folio

[Christoph Hellwig <hch@xxxxxx>]

Add a new SGP_FIND mode for shmem_get_partial_folio that works like
SGP_READ, but does not check i_size.  Use that instead of open coding
the page cache lookup in shmem_get_partial_folio.  Note that this is
a behavior change in that it reads in swap cache entries for offsets
outside i_size, possibly causing a little bit of extra work.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 include/linux/shmem_fs.h |  1 +
 mm/shmem.c               | 46 ++++++++++++----------------------------
 2 files changed, 15 insertions(+), 32 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index d09d54be4ffd99..7ba160ac066e5e 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -105,6 +105,7 @@ enum sgp_type {
 	SGP_CACHE,	/* don't exceed i_size, may allocate page */
 	SGP_WRITE,	/* may exceed i_size, may allocate !Uptodate page */
 	SGP_FALLOC,	/* like SGP_WRITE, but make existing page Uptodate */
+	SGP_FIND,	/* like SGP_READ, but also read outside i_size */
 };
 
 int shmem_get_folio(struct inode *inode, pgoff_t index, struct folio **foliop,
diff --git a/mm/shmem.c b/mm/shmem.c
index de6fa3980c7d6b..a8371ffeeee54a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -877,27 +877,6 @@ void shmem_unlock_mapping(struct address_space *mapping)
 	}
 }
 
-static struct folio *shmem_get_partial_folio(struct inode *inode, pgoff_t index)
-{
-	struct folio *folio;
-
-	/*
-	 * At first avoid shmem_get_folio(,,,SGP_READ): that fails
-	 * beyond i_size, and reports fallocated pages as holes.
-	 */
-	folio = __filemap_get_folio(inode->i_mapping, index,
-					FGP_ENTRY | FGP_LOCK, 0);
-	if (!xa_is_value(folio))
-		return folio;
-	/*
-	 * But read a page back from swap if any of it is within i_size
-	 * (although in some cases this is just a waste of time).
-	 */
-	folio = NULL;
-	shmem_get_folio(inode, index, &folio, SGP_READ);
-	return folio;
-}
-
 /*
  * Remove range of pages and swap entries from page cache, and free them.
  * If !unfalloc, truncate or punch hole; if unfalloc, undo failed fallocate.
@@ -957,7 +936,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
 		goto whole_folios;
 
 	same_folio = (lstart >> PAGE_SHIFT) == (lend >> PAGE_SHIFT);
-	folio = shmem_get_partial_folio(inode, lstart >> PAGE_SHIFT);
+	folio = NULL;
+	shmem_get_folio(inode, lstart >> PAGE_SHIFT, &folio, SGP_FIND);
 	if (folio) {
 		same_folio = lend < folio_pos(folio) + folio_size(folio);
 		folio_mark_dirty(folio);
@@ -971,14 +951,16 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
 		folio = NULL;
 	}
 
-	if (!same_folio)
-		folio = shmem_get_partial_folio(inode, lend >> PAGE_SHIFT);
-	if (folio) {
-		folio_mark_dirty(folio);
-		if (!truncate_inode_partial_folio(folio, lstart, lend))
-			end = folio->index;
-		folio_unlock(folio);
-		folio_put(folio);
+	if (!same_folio) {
+		folio = NULL;
+		shmem_get_folio(inode, lend >> PAGE_SHIFT, &folio, SGP_FIND);
+		if (folio) {
+			folio_mark_dirty(folio);
+			if (!truncate_inode_partial_folio(folio, lstart, lend))
+				end = folio->index;
+			folio_unlock(folio);
+			folio_put(folio);
+		}
 	}
 
 whole_folios:
@@ -1900,7 +1882,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 		if (folio_test_uptodate(folio))
 			goto out;
 		/* fallocated folio */
-		if (sgp != SGP_READ)
+		if (sgp != SGP_READ && sgp != SGP_FIND)
 			goto clear;
 		folio_unlock(folio);
 		folio_put(folio);
@@ -1911,7 +1893,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	 * SGP_NOALLOC: fail on hole, with NULL folio, letting caller fail.
 	 */
 	*foliop = NULL;
-	if (sgp == SGP_READ)
+	if (sgp == SGP_READ || sgp == SGP_FIND)
 		return 0;
 	if (sgp == SGP_NOALLOC)
 		return -ENOENT;
-- 
2.39.0