Message-ID: <03F6CCF4-EE7D-4259-AF85-455DEA721098@nvidia.com>
Date: Fri, 14 Feb 2025 17:03:37 -0500
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Ryan Roberts <ryan.roberts@....com>, Hugh Dickins <hughd@...gle.com>,
 Yang Shi <yang@...amperecomputing.com>, Miaohe Lin <linmiaohe@...wei.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Yu Zhao <yuzhao@...gle.com>,
 John Hubbard <jhubbard@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/8] mm/huge_memory: add two new (not yet used)
 functions for folio_split()

On 14 Feb 2025, at 16:59, David Hildenbrand wrote:

> On 11.02.25 16:50, Zi Yan wrote:
>> 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 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 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.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>   mm/huge_memory.c | 349 ++++++++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 348 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index a0277f4154c2..12d3f515c408 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3262,7 +3262,6 @@ 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);
>>  @@ -3506,6 +3505,354 @@ 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 int __split_folio_to_order(struct folio *folio, int new_order)
>> +{
>> +	int curr_order = folio_order(folio);
>> +	long nr_pages = folio_nr_pages(folio);
>> +	long new_nr_pages = 1 << new_order;
>> +	long index;
>> +
>> +	if (curr_order <= new_order)
>> +		return -EINVAL;
>> +
>> +	/*
>> +	 * Skip the first new_nr_pages, since the new folio from them have all
>> +	 * the flags from the original folio.
>> +	 */
>> +	for (index = new_nr_pages; index < nr_pages; index += new_nr_pages) {
>> +		struct page *head = &folio->page;
>> +		struct page *new_head = head + index;
>> +
>> +		/*
>> +		 * 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_head->_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_head->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
>> +		new_head->flags |= (head->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));
>> +
>> +		/* ->mapping in first and second tail page is replaced by other uses */
>> +		VM_BUG_ON_PAGE(new_nr_pages > 2 && new_head->mapping != TAIL_MAPPING,
>> +			       new_head);
>> +		new_head->mapping = head->mapping;
>> +		new_head->index = head->index + index;
>> +
>> +		/*
>> +		 * page->private should not be set in tail pages. Fix up and warn once
>> +		 * if private is unexpectedly set.
>> +		 */
>> +		if (unlikely(new_head->private)) {
>> +			VM_WARN_ON_ONCE_PAGE(true, new_head);
>> +			new_head->private = 0;
>> +		}
>> +
>> +		if (folio_test_swapcache(folio))
>> +			new_folio->swap.val = folio->swap.val + index;
>> +
>> +		/* 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);
>> +
>> +			folio_set_order(folio, new_order);
>> +		}
>> +
>> +		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)
>> +		ClearPageCompound(&folio->page);
>> +
>> +	return 0;
>> +}
>> +
>> +/*
>> + * 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).
>> + * @page: in buddy allocator like split, the folio containing @page will be
>> + *        split until its order becomes @new_order.
>> + * @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 *page, 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_anon(folio) && folio_test_swapcache(folio)) {
>> +		/* 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);
>> +		int status;
>> +
>> +		/* 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,
>> +						GFP_NOWAIT);
>> +				if (xas_error(xas)) {
>> +					ret = xas_error(xas);
>> +					stop_split = true;
>> +					goto after_split;
>> +				}
>> +			}
>> +		}
>> +
>> +		/* 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);
>> +
>> +		status = __split_folio_to_order(folio, split_order);
>> +
>
> Stumbling over that code (sorry for the late reply ... ).
>
> That looks weird. We split memcg/owner/pgalloc ... and then figure out in __split_folio_to_order() that we don't want to ... split?
>
> Should that all be moved into __split_folio_to_order() and performed only when we really want to split?

Yes, or move it after the status check. In reality, __split_folio_to_order()
only fails split_order is bigger than folio’s order, which should not happen.
But still. I will fix it in the next version.


Best Regards,
Yan, Zi

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