[PATCH 12/12] mm: add FGP_DONTCACHE folio creation flag

[Jens Axboe <axboe@kernel.dk>]

Callers can pass this in for uncached folio creation, in which case if
a folio is newly created it gets marked as uncached. If a folio exists
for this index and lookup succeeds, then it will not get marked as
uncached. If an !uncached lookup finds a cached folio, clear the flag.
For that case, there are competeting uncached and cached users of the
folio, and it should not get pruned.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/pagemap.h | 2 ++
 mm/filemap.c            | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 5da4b6d42fae..64c6dada837e 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -710,6 +710,7 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
  * * %FGP_NOFS - __GFP_FS will get cleared in gfp.
  * * %FGP_NOWAIT - Don't block on the folio lock.
  * * %FGP_STABLE - Wait for the folio to be stable (finished writeback)
+ * * %FGP_DONTCACHE - Uncached buffered IO
  * * %FGP_WRITEBEGIN - The flags to use in a filesystem write_begin()
  *   implementation.
  */
@@ -723,6 +724,7 @@ typedef unsigned int __bitwise fgf_t;
 #define FGP_NOWAIT		((__force fgf_t)0x00000020)
 #define FGP_FOR_MMAP		((__force fgf_t)0x00000040)
 #define FGP_STABLE		((__force fgf_t)0x00000080)
+#define FGP_DONTCACHE		((__force fgf_t)0x00000100)
 #define FGF_GET_ORDER(fgf)	(((__force unsigned)fgf) >> 26)	/* top 6 bits */
 
 #define FGP_WRITEBEGIN		(FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE)
diff --git a/mm/filemap.c b/mm/filemap.c
index 9842258ba343..68bdfff4117e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2001,6 +2001,8 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 			/* Init accessed so avoid atomic mark_page_accessed later */
 			if (fgp_flags & FGP_ACCESSED)
 				__folio_set_referenced(folio);
+			if (fgp_flags & FGP_DONTCACHE)
+				__folio_set_dropbehind(folio);
 
 			err = filemap_add_folio(mapping, folio, index, gfp);
 			if (!err)
@@ -2023,6 +2025,9 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 
 	if (!folio)
 		return ERR_PTR(-ENOENT);
+	/* not an uncached lookup, clear uncached if set */
+	if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE))
+		folio_clear_dropbehind(folio);
 	return folio;
 }
 EXPORT_SYMBOL(__filemap_get_folio);
-- 
2.45.2