mm: add FGP_DONTCACHE folio creation flag

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.

Link: https://lkml.kernel.org/r/20241220154831.1086649-13-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Cc: Brian Foster <bfoster@redhat.com>
Cc: Chris Mason <clm@meta.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: axboe

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)

mm/filemap.c

@@ -1973,6 +1973,8 @@ 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)
@@ -1995,6 +1997,9 @@ 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);