Re: [PATCH v2 03/19] fs: Add aops->migrate_folio

Matthew Wilcox
2022-06-09T14:35:35Z

On Thu, Jun 09, 2022 at 02:50:20PM +0200, David Hildenbrand wrote:
> On 08.06.22 17:02, Matthew Wilcox (Oracle) wrote:
> > diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> > index c0fe711f14d3..3d28b23676bd 100644
> > --- a/Documentation/filesystems/locking.rst
> > +++ b/Documentation/filesystems/locking.rst
> > @@ -253,7 +253,8 @@ prototypes::
> > void (*free_folio)(struct folio *);
> > int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
> > bool (*isolate_page) (struct page *, isolate_mode_t);
> > - int (*migratepage)(struct address_space *, struct page *, struct page *);
> > + int (*migrate_folio)(struct address_space *, struct folio *dst,
> > + struct folio *src, enum migrate_mode);
> > void (*putback_page) (struct page *);
>
> isolate_page/putback_page are leftovers from the previous patch, no?

Argh, right, I completely forgot I needed to update the documentation in
that patch.

> > +++ b/Documentation/vm/page_migration.rst
> > @@ -181,22 +181,23 @@ which are function pointers of struct address_space_operations.
> > Once page is successfully isolated, VM uses page.lru fields so driver
> > shouldn't expect to preserve values in those fields.
> >
> > -2. ``int (*migratepage) (struct address_space *mapping,``
> > -| ``struct page *newpage, struct page *oldpage, enum migrate_mode);``
> > -
> > - After isolation, VM calls migratepage() of driver with the isolated page.
> > - The function of migratepage() is to move the contents of the old page to the
> > - new page
> > - and set up fields of struct page newpage. Keep in mind that you should
> > - indicate to the VM the oldpage is no longer movable via __ClearPageMovable()
> > - under page_lock if you migrated the oldpage successfully and returned
> > - MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver
> > - can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time
> > - because VM interprets -EAGAIN as "temporary migration failure". On returning
> > - any error except -EAGAIN, VM will give up the page migration without
> > - retrying.
> > -
> > - Driver shouldn't touch the page.lru field while in the migratepage() function.
> > +2. ``int (*migrate_folio) (struct address_space *mapping,``
> > +| ``struct folio *dst, struct folio *src, enum migrate_mode);``
> > +
> > + After isolation, VM calls the driver's migrate_folio() with the
> > + isolated folio. The purpose of migrate_folio() is to move the contents
> > + of the source folio to the destination folio and set up the fields
> > + of destination folio. Keep in mind that you should indicate to the
> > + VM the source folio is no longer movable via __ClearPageMovable()
> > + under folio if you migrated the source successfully and returned
> > + MIGRATEPAGE_SUCCESS. If driver cannot migrate the folio at the
> > + moment, driver can return -EAGAIN. On -EAGAIN, VM will retry folio
> > + migration in a short time because VM interprets -EAGAIN as "temporary
> > + migration failure". On returning any error except -EAGAIN, VM will
> > + give up the folio migration without retrying.
> > +
> > + Driver shouldn't touch the folio.lru field while in the migrate_folio()
> > + function.
> >
> > 3. ``void (*putback_page)(struct page *);``
>
> Hmm, here it's a bit more complicated now, because we essentially have
> two paths: LRU+migrate_folio or !LRU+movable_ops
> (isolate/migrate/putback page)

Oh ... actually, this is just documenting the driver side of things.
I don't really like how it's written. Here, have some rewritten
documentation (which is now part of the previous patch):

+++ b/Documentation/vm/page_migration.rst
@@ -152,110 +152,15 @@ Steps:
Non-LRU page migration
======================

-Although migration originally aimed for reducing the latency of memory accesses
-for NUMA, compaction also uses migration to create high-order pages.
+Although migration originally aimed for reducing the latency of memory
+accesses for NUMA, compaction also uses migration to create high-order
+pages. For compaction purposes, it is also useful to be able to move
+non-LRU pages, such as zsmalloc and virtio-balloon pages.

-Current problem of the implementation is that it is designed to migrate only
-*LRU* pages. However, there are potential non-LRU pages which can be migrated
-in drivers, for example, zsmalloc, virtio-balloon pages.
-
-For virtio-balloon pages, some parts of migration code path have been hooked
-up and added virtio-balloon specific functions to intercept migration logics.
-It's too specific to a driver so other drivers who want to make their pages
-movable would have to add their own specific hooks in the migration path.
-
-To overcome the problem, VM supports non-LRU page migration which provides
-generic functions for non-LRU movable pages without driver specific hooks
-in the migration path.
-
-If a driver wants to make its pages movable, it should define three functions
-which are function pointers of struct address_space_operations.
-
-1. ``bool (*isolate_page) (struct page *page, isolate_mode_t mode);``
-
- What VM expects from isolate_page() function of driver is to return *true*
- if driver isolates the page successfully. On returning true, VM marks the page
- as PG_isolated so concurrent isolation in several CPUs skip the page
- for isolation. If a driver cannot isolate the page, it should return *false*.
-
- Once page is successfully isolated, VM uses page.lru fields so driver
- shouldn't expect to preserve values in those fields.
-
-2. ``int (*migratepage) (struct address_space *mapping,``
-| ``struct page *newpage, struct page *oldpage, enum migrate_mode);``
-
- After isolation, VM calls migratepage() of driver with the isolated page.
- The function of migratepage() is to move the contents of the old page to the
- new page
- and set up fields of struct page newpage. Keep in mind that you should
- indicate to the VM the oldpage is no longer movable via __ClearPageMovable()
- under page_lock if you migrated the oldpage successfully and returned
- MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver
- can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time
- because VM interprets -EAGAIN as "temporary migration failure". On returning
- any error except -EAGAIN, VM will give up the page migration without
- retrying.
-
- Driver shouldn't touch the page.lru field while in the migratepage() function.
-
-3. ``void (*putback_page)(struct page *);``
-
- If migration fails on the isolated page, VM should return the isolated page
- to the driver so VM calls the driver's putback_page() with the isolated page.
- In this function, the driver should put the isolated page back into its own data
- structure.
-
-Non-LRU movable page flags
-
- There are two page flags for supporting non-LRU movable page.
-
- * PG_movable
-
- Driver should use the function below to make page movable under page_lock::
-
- void __SetPageMovable(struct page *page, struct address_space *mapping)
-
- It needs argument of address_space for registering migration
- family functions which will be called by VM. Exactly speaking,
- PG_movable is not a real flag of struct page. Rather, VM
- reuses the page->mapping's lower bits to represent it::
-
- #define PAGE_MAPPING_MOVABLE 0x2
- page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
-
- so driver shouldn't access page->mapping directly. Instead, driver should
- use page_mapping() which masks off the low two bits of page->mapping under
- page lock so it can get the right struct address_space.
-
- For testing of non-LRU movable pages, VM supports __PageMovable() function.
- However, it doesn't guarantee to identify non-LRU movable pages because
- the page->mapping field is unified with other variables in struct page.
- If the driver releases the page after isolation by VM, page->mapping
- doesn't have a stable value although it has PAGE_MAPPING_MOVABLE set
- (look at __ClearPageMovable). But __PageMovable() is cheap to call whether
- page is LRU or non-LRU movable once the page has been isolated because LRU
- pages can never have PAGE_MAPPING_MOVABLE set in page->mapping. It is also
- good for just peeking to test non-LRU movable pages before more expensive
- checking with lock_page() in pfn scanning to select a victim.
-
- For guaranteeing non-LRU movable page, VM provides PageMovable() function.
- Unlike __PageMovable(), PageMovable() validates page->mapping and
- mapping->a_ops->isolate_page under lock_page(). The lock_page() prevents
- sudden destroying of page->mapping.
-
- Drivers using __SetPageMovable() should clear the flag via
- __ClearMovablePage() under page_lock() before the releasing the page.
-
- * PG_isolated
-
- To prevent concurrent isolation among several CPUs, VM marks isolated page
- as PG_isolated under lock_page(). So if a CPU encounters PG_isolated
- non-LRU movable page, it can skip it. Driver doesn't need to manipulate the
- flag because VM will set/clear it automatically. Keep in mind that if the
- driver sees a PG_isolated page, it means the page has been isolated by the
- VM so it shouldn't touch the page.lru field.
- The PG_isolated flag is aliased with the PG_reclaim flag so drivers
- shouldn't use PG_isolated for its own purposes.
+If a driver wants to make its pages movable, it should define a struct
+movable_operations. It then needs to call __SetPageMovable() on each
+page that it may be able to move. This uses the ``page->mapping`` field,
+so this field is not available for the driver to use for other purposes.

Monitoring Migration
=====================
@@ -286,3 +191,5 @@ THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase.

Christoph Lameter, May 8, 2006.
Minchan Kim, Mar 28, 2016.
+
+.. kernel-doc:: include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -19,6 +19,43 @@ struct migration_target_control;
*/
#define MIGRATEPAGE_SUCCESS 0

+/**
+ * struct movable_operations - Driver page migration
+ * @isolate_page:
+ * The VM calls this function to prepare the page to be moved. The page
+ * is locked and the driver should not unlock it. The driver should
+ * return ``true`` if the page is movable and ``false`` if it is not
+ * currently movable. After this function returns, the VM uses the
+ * page->lru field, so the driver must preserve any information which
+ * is usually stored here.
+ *
+ * @migrate_page:
+ * After isolation, the VM calls this function with the isolated
+ * @src page. The driver should copy the contents of the
+ * @src page to the @dst page and set up the fields of @dst page.
+ * Both pages are locked.
+ * If page migration is successful, the driver should call
+ * __ClearPageMovable(@src) and return MIGRATEPAGE_SUCCESS.
+ * If the driver cannot migrate the page at the moment, it can return
+ * -EAGAIN. The VM interprets this as a temporary migration failure and
+ * will retry it later. Any other error value is a permanent migration
+ * failure and migration will not be retried.
+ * The driver shouldn't touch the @src->lru field while in the
+ * migrate_page() function. It may write to @dst->lru.
+ *
+ * @putback_page:
+ * If migration fails on the isolated page, the VM informs the driver
+ * that the page is no longer a candidate for migration by calling
+ * this function. The driver should put the isolated page back into
+ * its own data structure.
+ */
+struct movable_operations {
+ bool (*isolate_page)(struct page *, isolate_mode_t);
+ int (*migrate_page)(struct page *dst, struct page *src,
+ enum migrate_mode);
+ void (*putback_page)(struct page *);
+};
+
/* Defined in mm/debug.c: */
extern const char *migrate_reason_names[MR_TYPES];