[PATCH v4 02/11] fs: Allow fine-grained control of folio sizes

Pankaj Raghav (Samsung)
2024-04-25T11:46:57Z

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

Some filesystems want to be able to ensure that folios that are added to
the page cache are at least a certain size.
Add mapping_set_folio_min_order() to allow this level of control.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Co-developed-by: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
include/linux/pagemap.h | 116 +++++++++++++++++++++++++++++++++-------
1 file changed, 96 insertions(+), 20 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2df35e65557d..2e5612de1749 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -202,13 +202,18 @@ enum mapping_flags {
AS_EXITING = 4, /* final truncate in progress */
/* writeback related tags are not used */
AS_NO_WRITEBACK_TAGS = 5,
- AS_LARGE_FOLIO_SUPPORT = 6,
- AS_RELEASE_ALWAYS, /* Call ->release_folio(), even if no private data */
- AS_STABLE_WRITES, /* must wait for writeback before modifying
+ AS_RELEASE_ALWAYS = 6, /* Call ->release_folio(), even if no private data */
+ AS_STABLE_WRITES = 7, /* must wait for writeback before modifying
folio contents */
- AS_UNMOVABLE, /* The mapping cannot be moved, ever */
+ AS_FOLIO_ORDER_MIN = 8,
+ AS_FOLIO_ORDER_MAX = 13, /* Bit 8-17 are used for FOLIO_ORDER */
+ AS_UNMOVABLE = 18, /* The mapping cannot be moved, ever */
};

+#define AS_FOLIO_ORDER_MIN_MASK 0x00001f00
+#define AS_FOLIO_ORDER_MAX_MASK 0x0003e000
+#define AS_FOLIO_ORDER_MASK (AS_FOLIO_ORDER_MIN_MASK | AS_FOLIO_ORDER_MAX_MASK)
+
/**
* mapping_set_error - record a writeback error in the address_space
* @mapping: the mapping in which an error should be set
@@ -344,9 +349,63 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
m->gfp_mask = mask;
}

+/*
+ * There are some parts of the kernel which assume that PMD entries
+ * are exactly HPAGE_PMD_ORDER. Those should be fixed, but until then,
+ * limit the maximum allocation order to PMD size. I'm not aware of any
+ * assumptions about maximum order if THP are disabled, but 8 seems like
+ * a good order (that's 1MB if you're using 4kB pages)
+ */
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#define MAX_PAGECACHE_ORDER HPAGE_PMD_ORDER
+#else
+#define MAX_PAGECACHE_ORDER 8
+#endif
+
+/*
+ * mapping_set_folio_order_range() - Set the folio order range
+ * @mapping: The address_space.
+ * @min: Minimum folio order (between 0-MAX_PAGECACHE_ORDER inclusive).
+ * @max: Maximum folio order (between @min-MAX_PAGECACHE_ORDER inclusive).
+ *
+ * The filesystem should call this function in its inode constructor to
+ * indicate which base size (min) and maximum size (max) of folio the VFS
+ * can use to cache the contents of the file. This should only be used
+ * if the filesystem needs special handling of folio sizes (ie there is
+ * something the core cannot know).
+ * Do not tune it based on, eg, i_size.
+ *
+ * Context: This should not be called while the inode is active as it
+ * is non-atomic.
+ */
+static inline void mapping_set_folio_order_range(struct address_space *mapping,
+ unsigned int min_order,
+ unsigned int max_order)
+{
+ if (min_order > MAX_PAGECACHE_ORDER)
+ min_order = MAX_PAGECACHE_ORDER;
+
+ if (max_order > MAX_PAGECACHE_ORDER)
+ max_order = MAX_PAGECACHE_ORDER;
+
+ max_order = max(max_order, min_order);
+ /*
+ * TODO: max_order is not yet supported in filemap.
+ */
+ mapping->flags = (mapping->flags & ~AS_FOLIO_ORDER_MASK) |
+ (min_order << AS_FOLIO_ORDER_MIN) |
+ (max_order << AS_FOLIO_ORDER_MAX);
+}
+
+static inline void mapping_set_folio_min_order(struct address_space *mapping,
+ unsigned int min)
+{
+ mapping_set_folio_order_range(mapping, min, MAX_PAGECACHE_ORDER);
+}
+
/**
* mapping_set_large_folios() - Indicate the file supports large folios.
- * @mapping: The file.
+ * @mapping: The address_space.
*
* The filesystem should call this function in its inode constructor to
* indicate that the VFS can use large folios to cache the contents of
@@ -357,7 +416,37 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
*/
static inline void mapping_set_large_folios(struct address_space *mapping)
{
- __set_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
+ mapping_set_folio_order_range(mapping, 0, MAX_PAGECACHE_ORDER);
+}
+
+static inline unsigned int mapping_max_folio_order(struct address_space *mapping)
+{
+ return (mapping->flags & AS_FOLIO_ORDER_MAX_MASK) >> AS_FOLIO_ORDER_MAX;
+}
+
+static inline unsigned int mapping_min_folio_order(struct address_space *mapping)
+{
+ return (mapping->flags & AS_FOLIO_ORDER_MIN_MASK) >> AS_FOLIO_ORDER_MIN;
+}
+
+static inline unsigned long mapping_min_folio_nrpages(struct address_space *mapping)
+{
+ return 1UL << mapping_min_folio_order(mapping);
+}
+
+/**
+ * mapping_align_start_index() - Align starting index based on the min
+ * folio order of the page cache.
+ * @mapping: The address_space.
+ *
+ * Ensure the index used is aligned to the minimum folio order when adding
+ * new folios to the page cache by rounding down to the nearest minimum
+ * folio number of pages.
+ */
+static inline pgoff_t mapping_align_start_index(struct address_space *mapping,
+ pgoff_t index)
+{
+ return round_down(index, mapping_min_folio_nrpages(mapping));
}

/*
@@ -367,7 +456,7 @@ static inline void mapping_set_large_folios(struct address_space *mapping)
static inline bool mapping_large_folio_support(struct address_space *mapping)
{
return IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
- test_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
+ (mapping_max_folio_order(mapping) > 0);
}

static inline int filemap_nr_thps(struct address_space *mapping)
@@ -528,19 +617,6 @@ static inline void *detach_page_private(struct page *page)
return folio_detach_private(page_folio(page));
}

-/*
- * There are some parts of the kernel which assume that PMD entries
- * are exactly HPAGE_PMD_ORDER. Those should be fixed, but until then,
- * limit the maximum allocation order to PMD size. I'm not aware of any
- * assumptions about maximum order if THP are disabled, but 8 seems like
- * a good order (that's 1MB if you're using 4kB pages)
- */
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-#define MAX_PAGECACHE_ORDER HPAGE_PMD_ORDER
-#else
-#define MAX_PAGECACHE_ORDER 8
-#endif
-
#ifdef CONFIG_NUMA
struct folio *filemap_alloc_folio(gfp_t gfp, unsigned int order);
#else
--
2.34.1