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mm: Add AS_UNMOVABLE to mark mapping as completely unmovable

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Add an "unmovable" flag for mappings that cannot be migrated under any
circumstance.  KVM will use the flag for its upcoming GUEST_MEMFD support,
which will not support compaction/migration, at least not in the
foreseeable future.

Test AS_UNMOVABLE under folio lock as already done for the async
compaction/dirty folio case, as the mapping can be removed by truncation
while compaction is running.  To avoid having to lock every folio with a
mapping, assume/require that unmovable mappings are also unevictable, and
have mapping_set_unmovable() also set AS_UNEVICTABLE.

Cc: Matthew Wilcox <willy@infradead.org>
Co-developed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20231027182217.3615211-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2023-10-27 11:21:56 -07:00 committed by Paolo Bonzini
parent 5a475554db
commit 0003e2a414
3 changed files with 51 additions and 13 deletions
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19
include/linux/pagemap.h
View File

@ -203,7 +203,8 @@ enum mapping_flags {
/* 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_RELEASE_ALWAYS = 7, /* Call ->release_folio(), even if no private data */
AS_UNMOVABLE = 8, /* The mapping cannot be moved, ever */
};
/**
@ -289,6 +290,22 @@ static inline void mapping_clear_release_always(struct address_space *mapping)
clear_bit(AS_RELEASE_ALWAYS, &mapping->flags);
}
static inline void mapping_set_unmovable(struct address_space *mapping)
{
/*
* It's expected unmovable mappings are also unevictable. Compaction
* migrate scanner (isolate_migratepages_block()) relies on this to
* reduce page locking.
*/
set_bit(AS_UNEVICTABLE, &mapping->flags);
set_bit(AS_UNMOVABLE, &mapping->flags);
}
static inline bool mapping_unmovable(struct address_space *mapping)
{
return test_bit(AS_UNMOVABLE, &mapping->flags);
}
static inline gfp_t mapping_gfp_mask(struct address_space * mapping)
{
return mapping->gfp_mask;

43
mm/compaction.c
View File

@ -883,6 +883,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
/* Time to isolate some pages for migration */
for (; low_pfn < end_pfn; low_pfn++) {
bool is_dirty, is_unevictable;
if (skip_on_failure && low_pfn >= next_skip_pfn) {
/*
@ -1080,8 +1081,10 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
if (!folio_test_lru(folio))
goto isolate_fail_put;
is_unevictable = folio_test_unevictable(folio);
/* Compaction might skip unevictable pages but CMA takes them */
if (!(mode & ISOLATE_UNEVICTABLE) && folio_test_unevictable(folio))
if (!(mode & ISOLATE_UNEVICTABLE) && is_unevictable)
goto isolate_fail_put;
/*
@ -1093,26 +1096,42 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
if ((mode & ISOLATE_ASYNC_MIGRATE) && folio_test_writeback(folio))
goto isolate_fail_put;
if ((mode & ISOLATE_ASYNC_MIGRATE) && folio_test_dirty(folio)) {
bool migrate_dirty;
is_dirty = folio_test_dirty(folio);
if (((mode & ISOLATE_ASYNC_MIGRATE) && is_dirty) ||
(mapping && is_unevictable)) {
bool migrate_dirty = true;
bool is_unmovable;
/*
* Only folios without mappings or that have
* a ->migrate_folio callback are possible to
* migrate without blocking. However, we may
* be racing with truncation, which can free
* the mapping. Truncation holds the folio lock
* until after the folio is removed from the page
* cache so holding it ourselves is sufficient.
* a ->migrate_folio callback are possible to migrate
* without blocking.
*
* Folios from unmovable mappings are not migratable.
*
* However, we can be racing with truncation, which can
* free the mapping that we need to check. Truncation
* holds the folio lock until after the folio is removed
* from the page so holding it ourselves is sufficient.
*
* To avoid locking the folio just to check unmovable,
* assume every unmovable folio is also unevictable,
* which is a cheaper test. If our assumption goes
* wrong, it's not a correctness bug, just potentially
* wasted cycles.
*/
if (!folio_trylock(folio))
goto isolate_fail_put;
mapping = folio_mapping(folio);
migrate_dirty = !mapping ||
mapping->a_ops->migrate_folio;
if ((mode & ISOLATE_ASYNC_MIGRATE) && is_dirty) {
migrate_dirty = !mapping ||
mapping->a_ops->migrate_folio;
}
is_unmovable = mapping && mapping_unmovable(mapping);
folio_unlock(folio);
if (!migrate_dirty)
if (!migrate_dirty || is_unmovable)
goto isolate_fail_put;
}

2
mm/migrate.c
View File

@ -956,6 +956,8 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
if (!mapping)
rc = migrate_folio(mapping, dst, src, mode);
else if (mapping_unmovable(mapping))
rc = -EOPNOTSUPP;
else if (mapping->a_ops->migrate_folio)
/*
* Most folios have a mapping and most filesystems