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mm/huge_memory: add two new (not yet used) functions for folio_split()

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This is a preparation patch, both added functions are not used yet.

The added __split_unmapped_folio() is able to split a folio with its
mapping removed in two manners: 1) uniform split (the existing way), and
2) buddy allocator like (or non-uniform) split.

The added __split_folio_to_order() can split a folio into any lower order.
For uniform split, __split_unmapped_folio() calls it once to split the
given folio to the new order.  For buddy allocator like (non-uniform)
split, __split_unmapped_folio() calls it (folio_order - new_order) times
and each time splits the folio containing the given page to one lower
order.

[ziy@nvidia.com: unfreeze head folio after page cache entries are updated]
  Link: https://lkml.kernel.org/r/0F15DA7F-1977-412F-9A3E-F06B515D4BD2@nvidia.com
[ziy@nvidia.com: use NULL instead of 0 for folio->private assignment]
  Link: https://lkml.kernel.org/r/1E11B9DD-3A87-4C9C-8FB4-E1324FB6A21A@nvidia.com
Link: https://lkml.kernel.org/r/20250307174001.242794-3-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Zi Yan
2025-03-07 12:39:55 -05:00
committed by Andrew Morton
parent 3fec86f8aa
commit 00527733d0
1 changed files with 353 additions and 1 deletions
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354
mm/huge_memory.c
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@@ -3265,7 +3265,6 @@ static void remap_page(struct folio *folio, unsigned long nr, int flags)
static void lru_add_page_tail(struct folio *folio, struct page *tail,
struct lruvec *lruvec, struct list_head *list)
{
VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
VM_BUG_ON_FOLIO(PageLRU(tail), folio);
lockdep_assert_held(&lruvec->lru_lock);
@@ -3517,6 +3516,359 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
caller_pins;
}
/*
* It splits @folio into @new_order folios and copies the @folio metadata to
* all the resulting folios.
*/
static void __split_folio_to_order(struct folio *folio, int old_order,
int new_order)
{
long new_nr_pages = 1 << new_order;
long nr_pages = 1 << old_order;
long i;
/*
* Skip the first new_nr_pages, since the new folio from them have all
* the flags from the original folio.
*/
for (i = new_nr_pages; i < nr_pages; i += new_nr_pages) {
struct page *new_head = &folio->page + i;
/*
* Careful: new_folio is not a "real" folio before we cleared PageTail.
* Don't pass it around before clear_compound_head().
*/
struct folio *new_folio = (struct folio *)new_head;
VM_BUG_ON_PAGE(atomic_read(&new_folio->_mapcount) != -1, new_head);
/*
* Clone page flags before unfreezing refcount.
*
* After successful get_page_unless_zero() might follow flags change,
* for example lock_page() which set PG_waiters.
*
* Note that for mapped sub-pages of an anonymous THP,
* PG_anon_exclusive has been cleared in unmap_folio() and is stored in
* the migration entry instead from where remap_page() will restore it.
* We can still have PG_anon_exclusive set on effectively unmapped and
* unreferenced sub-pages of an anonymous THP: we can simply drop
* PG_anon_exclusive (-> PG_mappedtodisk) for these here.
*/
new_folio->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
new_folio->flags |= (folio->flags &
((1L << PG_referenced) |
(1L << PG_swapbacked) |
(1L << PG_swapcache) |
(1L << PG_mlocked) |
(1L << PG_uptodate) |
(1L << PG_active) |
(1L << PG_workingset) |
(1L << PG_locked) |
(1L << PG_unevictable) |
#ifdef CONFIG_ARCH_USES_PG_ARCH_2
(1L << PG_arch_2) |
#endif
#ifdef CONFIG_ARCH_USES_PG_ARCH_3
(1L << PG_arch_3) |
#endif
(1L << PG_dirty) |
LRU_GEN_MASK | LRU_REFS_MASK));
new_folio->mapping = folio->mapping;
new_folio->index = folio->index + i;
/*
* page->private should not be set in tail pages. Fix up and warn once
* if private is unexpectedly set.
*/
if (unlikely(new_folio->private)) {
VM_WARN_ON_ONCE_PAGE(true, new_head);
new_folio->private = NULL;
}
if (folio_test_swapcache(folio))
new_folio->swap.val = folio->swap.val + i;
/* Page flags must be visible before we make the page non-compound. */
smp_wmb();
/*
* Clear PageTail before unfreezing page refcount.
*
* After successful get_page_unless_zero() might follow put_page()
* which needs correct compound_head().
*/
clear_compound_head(new_head);
if (new_order) {
prep_compound_page(new_head, new_order);
folio_set_large_rmappable(new_folio);
}
if (folio_test_young(folio))
folio_set_young(new_folio);
if (folio_test_idle(folio))
folio_set_idle(new_folio);
folio_xchg_last_cpupid(new_folio, folio_last_cpupid(folio));
}
if (new_order)
folio_set_order(folio, new_order);
else
ClearPageCompound(&folio->page);
}
/*
* It splits an unmapped @folio to lower order smaller folios in two ways.
* @folio: the to-be-split folio
* @new_order: the smallest order of the after split folios (since buddy
* allocator like split generates folios with orders from @folio's
* order - 1 to new_order).
* @split_at: in buddy allocator like split, the folio containing @split_at
* will be split until its order becomes @new_order.
* @lock_at: the folio containing @lock_at is left locked for caller.
* @list: the after split folios will be added to @list if it is not NULL,
* otherwise to LRU lists.
* @end: the end of the file @folio maps to. -1 if @folio is anonymous memory.
* @xas: xa_state pointing to folio->mapping->i_pages and locked by caller
* @mapping: @folio->mapping
* @uniform_split: if the split is uniform or not (buddy allocator like split)
*
*
* 1. uniform split: the given @folio into multiple @new_order small folios,
* where all small folios have the same order. This is done when
* uniform_split is true.
* 2. buddy allocator like (non-uniform) split: the given @folio is split into
* half and one of the half (containing the given page) is split into half
* until the given @page's order becomes @new_order. This is done when
* uniform_split is false.
*
* The high level flow for these two methods are:
* 1. uniform split: a single __split_folio_to_order() is called to split the
* @folio into @new_order, then we traverse all the resulting folios one by
* one in PFN ascending order and perform stats, unfreeze, adding to list,
* and file mapping index operations.
* 2. non-uniform split: in general, folio_order - @new_order calls to
* __split_folio_to_order() are made in a for loop to split the @folio
* to one lower order at a time. The resulting small folios are processed
* like what is done during the traversal in 1, except the one containing
* @page, which is split in next for loop.
*
* After splitting, the caller's folio reference will be transferred to the
* folio containing @page. The other folios may be freed if they are not mapped.
*
* In terms of locking, after splitting,
* 1. uniform split leaves @page (or the folio contains it) locked;
* 2. buddy allocator like (non-uniform) split leaves @folio locked.
*
*
* For !uniform_split, when -ENOMEM is returned, the original folio might be
* split. The caller needs to check the input folio.
*/
static int __split_unmapped_folio(struct folio *folio, int new_order,
struct page *split_at, struct page *lock_at,
struct list_head *list, pgoff_t end,
struct xa_state *xas, struct address_space *mapping,
bool uniform_split)
{
struct lruvec *lruvec;
struct address_space *swap_cache = NULL;
struct folio *origin_folio = folio;
struct folio *next_folio = folio_next(folio);
struct folio *new_folio;
struct folio *next;
int order = folio_order(folio);
int split_order;
int start_order = uniform_split ? new_order : order - 1;
int nr_dropped = 0;
int ret = 0;
bool stop_split = false;
if (folio_test_swapcache(folio)) {
VM_BUG_ON(mapping);
/* a swapcache folio can only be uniformly split to order-0 */
if (!uniform_split || new_order != 0)
return -EINVAL;
swap_cache = swap_address_space(folio->swap);
xa_lock(&swap_cache->i_pages);
}
if (folio_test_anon(folio))
mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
/* lock lru list/PageCompound, ref frozen by page_ref_freeze */
lruvec = folio_lruvec_lock(folio);
folio_clear_has_hwpoisoned(folio);
/*
* split to new_order one order at a time. For uniform split,
* folio is split to new_order directly.
*/
for (split_order = start_order;
split_order >= new_order && !stop_split;
split_order--) {
int old_order = folio_order(folio);
struct folio *release;
struct folio *end_folio = folio_next(folio);
/* order-1 anonymous folio is not supported */
if (folio_test_anon(folio) && split_order == 1)
continue;
if (uniform_split && split_order != new_order)
continue;
if (mapping) {
/*
* uniform split has xas_split_alloc() called before
* irq is disabled to allocate enough memory, whereas
* non-uniform split can handle ENOMEM.
*/
if (uniform_split)
xas_split(xas, folio, old_order);
else {
xas_set_order(xas, folio->index, split_order);
xas_try_split(xas, folio, old_order);
if (xas_error(xas)) {
ret = xas_error(xas);
stop_split = true;
goto after_split;
}
}
}
/*
* Reset any memcg data overlay in the tail pages.
* folio_nr_pages() is unreliable until prep_compound_page()
* was called again.
*/
#ifdef NR_PAGES_IN_LARGE_FOLIO
folio->_nr_pages = 0;
#endif
/* complete memcg works before add pages to LRU */
split_page_memcg(&folio->page, old_order, split_order);
split_page_owner(&folio->page, old_order, split_order);
pgalloc_tag_split(folio, old_order, split_order);
__split_folio_to_order(folio, old_order, split_order);
after_split:
/*
* Iterate through after-split folios and perform related
* operations. But in buddy allocator like split, the folio
* containing the specified page is skipped until its order
* is new_order, since the folio will be worked on in next
* iteration.
*/
for (release = folio; release != end_folio; release = next) {
next = folio_next(release);
/*
* for buddy allocator like split, the folio containing
* page will be split next and should not be released,
* until the folio's order is new_order or stop_split
* is set to true by the above xas_split() failure.
*/
if (release == page_folio(split_at)) {
folio = release;
if (split_order != new_order && !stop_split)
continue;
}
if (folio_test_anon(release)) {
mod_mthp_stat(folio_order(release),
MTHP_STAT_NR_ANON, 1);
}
/*
* origin_folio should be kept frozon until page cache
* entries are updated with all the other after-split
* folios to prevent others seeing stale page cache
* entries.
*/
if (release == origin_folio)
continue;
folio_ref_unfreeze(release, 1 +
((mapping || swap_cache) ?
folio_nr_pages(release) : 0));
lru_add_page_tail(origin_folio, &release->page,
lruvec, list);
/* Some pages can be beyond EOF: drop them from cache */
if (release->index >= end) {
if (shmem_mapping(mapping))
nr_dropped += folio_nr_pages(release);
else if (folio_test_clear_dirty(release))
folio_account_cleaned(release,
inode_to_wb(mapping->host));
__filemap_remove_folio(release, NULL);
folio_put_refs(release, folio_nr_pages(release));
} else if (mapping) {
__xa_store(&mapping->i_pages,
release->index, release, 0);
} else if (swap_cache) {
__xa_store(&swap_cache->i_pages,
swap_cache_index(release->swap),
release, 0);
}
}
}
/*
* Unfreeze origin_folio only after all page cache entries, which used
* to point to it, have been updated with new folios. Otherwise,
* a parallel folio_try_get() can grab origin_folio and its caller can
* see stale page cache entries.
*/
folio_ref_unfreeze(origin_folio, 1 +
((mapping || swap_cache) ? folio_nr_pages(origin_folio) : 0));
unlock_page_lruvec(lruvec);
if (swap_cache)
xa_unlock(&swap_cache->i_pages);
if (mapping)
xa_unlock(&mapping->i_pages);
/* Caller disabled irqs, so they are still disabled here */
local_irq_enable();
if (nr_dropped)
shmem_uncharge(mapping->host, nr_dropped);
remap_page(origin_folio, 1 << order,
folio_test_anon(origin_folio) ?
RMP_USE_SHARED_ZEROPAGE : 0);
/*
* At this point, folio should contain the specified page.
* For uniform split, it is left for caller to unlock.
* For buddy allocator like split, the first after-split folio is left
* for caller to unlock.
*/
for (new_folio = origin_folio; new_folio != next_folio; new_folio = next) {
next = folio_next(new_folio);
if (new_folio == page_folio(lock_at))
continue;
folio_unlock(new_folio);
/*
* Subpages may be freed if there wasn't any mapping
* like if add_to_swap() is running on a lru page that
* had its mapping zapped. And freeing these pages
* requires taking the lru_lock so we do the put_page
* of the tail pages after the split is complete.
*/
free_page_and_swap_cache(&new_folio->page);
}
return ret;
}
/*
* This function splits a large folio into smaller folios of order @new_order.
* @page can point to any page of the large folio to split. The split operation