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mm/huge_memory: add buddy allocator like (non-uniform) folio_split() 

folio_split() splits a large folio in the same way as buddy allocator
splits a large free page for allocation.  The purpose is to minimize the
number of folios after the split.  For example, if user wants to free the
3rd subpage in a order-9 folio, folio_split() will split the order-9 folio
as:

O-0, O-0, O-0, O-0, O-2, O-3, O-4, O-5, O-6, O-7, O-8 if it is anon,
since anon folio does not support order-1 yet.
-----------------------------------------------------------------
|   |   |   |   |     |   |       |                             |
|O-0|O-0|O-0|O-0| O-2 |...|  O-7  |             O-8             |
|   |   |   |   |     |   |       |                             |
-----------------------------------------------------------------

O-1,      O-0, O-0, O-2, O-3, O-4, O-5, O-6, O-7, O-9 if it is pagecache
---------------------------------------------------------------
|     |   |   |     |   |       |                             |
| O-1 |O-0|O-0| O-2 |...|  O-7  |             O-8             |
|     |   |   |     |   |       |                             |
---------------------------------------------------------------

It generates fewer folios (i.e., 11 or 10) than existing page split
approach, which splits the order-9 to 512 order-0 folios.  It also reduces
the number of new xa_node needed during a pagecache folio split from 8 to
1, potentially decreasing the folio split failure rate due to memory
constraints.

folio_split() and existing split_huge_page_to_list_to_order() share the
folio unmapping and remapping code in __folio_split() and the common
backend split code in __split_unmapped_folio() using uniform_split
variable to distinguish their operations.

uniform_split_supported() and non_uniform_split_supported() are added to
factor out check code and will be used outside __folio_split() in the
following commit.

Link: https://lkml.kernel.org/r/20250307174001.242794-5-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 12:39:57 -05:00 committed by Andrew Morton
parent 6384dd1d18
commit 58729c04cf
1 changed files with 128 additions and 42 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

160
mm/huge_memory.c
View File

@ -3869,12 +3869,85 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
return ret;
}
static bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
bool warns)
{
if (folio_test_anon(folio)) {
/* order-1 is not supported for anonymous THP. */
VM_WARN_ONCE(warns && new_order == 1,
"Cannot split to order-1 folio");
return new_order != 1;
} else if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
!mapping_large_folio_support(folio->mapping)) {
/*
* No split if the file system does not support large folio.
* Note that we might still have THPs in such mappings due to
* CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
* does not actually support large folios properly.
*/
VM_WARN_ONCE(warns,
"Cannot split file folio to non-0 order");
return false;
}
/* Only swapping a whole PMD-mapped folio is supported */
if (folio_test_swapcache(folio)) {
VM_WARN_ONCE(warns,
"Cannot split swapcache folio to non-0 order");
return false;
}
return true;
}
/* See comments in non_uniform_split_supported() */
static bool uniform_split_supported(struct folio *folio, unsigned int new_order,
bool warns)
{
if (folio_test_anon(folio)) {
VM_WARN_ONCE(warns && new_order == 1,
"Cannot split to order-1 folio");
return new_order != 1;
} else if (new_order) {
if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
!mapping_large_folio_support(folio->mapping)) {
VM_WARN_ONCE(warns,
"Cannot split file folio to non-0 order");
return false;
}
}
if (new_order && folio_test_swapcache(folio)) {
VM_WARN_ONCE(warns,
"Cannot split swapcache folio to non-0 order");
return false;
}
return true;
}
/*
* __folio_split: split a folio at @split_at to a @new_order folio
* @folio: folio to split
* @new_order: the order of the new folio
* @split_at: a page within the new folio
* @lock_at: a page within @folio to be left locked to caller
* @list: after-split folios will be put on it if non NULL
* @uniform_split: perform uniform split or not (non-uniform split)
*
* It calls __split_unmapped_folio() to perform uniform and non-uniform split.
* It is in charge of checking whether the split is supported or not and
* preparing @folio for __split_unmapped_folio().
*
* return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be
* split but not to @new_order, the caller needs to check)
*/
static int __folio_split(struct folio *folio, unsigned int new_order,
struct page *page, struct list_head *list)
struct page *split_at, struct page *lock_at,
struct list_head *list, bool uniform_split)
{
struct deferred_split *ds_queue = get_deferred_split_queue(folio);
/* reset xarray order to new order after split */
XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
XA_STATE(xas, &folio->mapping->i_pages, folio->index);
bool is_anon = folio_test_anon(folio);
struct address_space *mapping = NULL;
struct anon_vma *anon_vma = NULL;
@ -3886,32 +3959,17 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
if (folio != page_folio(split_at) || folio != page_folio(lock_at))
return -EINVAL;
if (new_order >= folio_order(folio))
return -EINVAL;
if (is_anon) {
/* order-1 is not supported for anonymous THP. */
if (new_order == 1) {
VM_WARN_ONCE(1, "Cannot split to order-1 folio");
if (uniform_split && !uniform_split_supported(folio, new_order, true))
return -EINVAL;
}
} else if (new_order) {
/*
* No split if the file system does not support large folio.
* Note that we might still have THPs in such mappings due to
* CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
* does not actually support large folios properly.
*/
if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
!mapping_large_folio_support(folio->mapping)) {
VM_WARN_ONCE(1,
"Cannot split file folio to non-0 order");
return -EINVAL;
}
}
/* Only swapping a whole PMD-mapped folio is supported */
if (folio_test_swapcache(folio) && new_order)
if (!uniform_split &&
!non_uniform_split_supported(folio, new_order, true))
return -EINVAL;
is_hzp = is_huge_zero_folio(folio);
@ -3973,21 +4031,24 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
goto out;
}
if (uniform_split) {
xas_set_order(&xas, folio->index, new_order);
xas_split_alloc(&xas, folio, folio_order(folio), gfp);
if (xas_error(&xas)) {
ret = xas_error(&xas);
goto out;
}
}
anon_vma = NULL;
i_mmap_lock_read(mapping);
/*
*__split_huge_page() may need to trim off pages beyond EOF:
* but on 32-bit, i_size_read() takes an irq-unsafe seqlock,
* which cannot be nested inside the page tree lock. So note
* end now: i_size itself may be changed at any moment, but
* folio lock is good enough to serialize the trimming.
*__split_unmapped_folio() may need to trim off pages beyond
* EOF: but on 32-bit, i_size_read() takes an irq-unsafe
* seqlock, which cannot be nested inside the page tree lock.
* So note end now: i_size itself may be changed at any moment,
* but folio lock is good enough to serialize the trimming.
*/
end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
if (shmem_mapping(mapping))
@ -4041,7 +4102,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
if (mapping) {
int nr = folio_nr_pages(folio);
xas_split(&xas, folio, folio_order(folio));
if (folio_test_pmd_mappable(folio) &&
new_order < HPAGE_PMD_ORDER) {
if (folio_test_swapbacked(folio)) {
@ -4055,12 +4115,9 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
}
}
if (is_anon) {
mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
mod_mthp_stat(new_order, MTHP_STAT_NR_ANON, 1 << (order - new_order));
}
__split_huge_page(page, list, end, new_order);
ret = 0;
ret = __split_unmapped_folio(folio, new_order,
split_at, lock_at, list, end, &xas, mapping,
uniform_split);
} else {
spin_unlock(&ds_queue->split_queue_lock);
fail:
@ -4138,7 +4195,36 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
{
struct folio *folio = page_folio(page);
return __folio_split(folio, new_order, page, list);
return __folio_split(folio, new_order, &folio->page, page, list, true);
}
/*
* folio_split: split a folio at @split_at to a @new_order folio
* @folio: folio to split
* @new_order: the order of the new folio
* @split_at: a page within the new folio
*
* return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be
* split but not to @new_order, the caller needs to check)
*
* It has the same prerequisites and returns as
* split_huge_page_to_list_to_order().
*
* Split a folio at @split_at to a new_order folio, leave the
* remaining subpages of the original folio as large as possible. For example,
* in the case of splitting an order-9 folio at its third order-3 subpages to
* an order-3 folio, there are 2^(9-3)=64 order-3 subpages in the order-9 folio.
* After the split, there will be a group of folios with different orders and
* the new folio containing @split_at is marked in bracket:
* [order-4, {order-3}, order-3, order-5, order-6, order-7, order-8].
*
* After split, folio is left locked for caller.
*/
static int folio_split(struct folio *folio, unsigned int new_order,
struct page *split_at, struct list_head *list)
{
return __folio_split(folio, new_order, split_at, &folio->page, list,
false);
}
int min_order_for_split(struct folio *folio)