Message-ID: <604a1db2-bd64-455e-9cf7-968cacef0122@redhat.com>
Date: Fri, 25 Apr 2025 09:47:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Shivank Garg <shivankg@....com>, Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, shaggy@...nel.org,
 wangkefeng.wang@...wei.com, jane.chu@...cle.com, ziy@...dia.com,
 donettom@...ux.ibm.com, apopple@...dia.com,
 jfs-discussion@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, syzbot+8bb6fd945af4e0ad9299@...kaller.appspotmail.com
Subject: Re: [PATCH V4 1/2] mm: add folio_migration_expected_refs() as inline
 function

On 24.04.25 13:57, Shivank Garg wrote:
> Hi All,
> 
> Thank you for reviewing my patch and providing feedback.
> 
> On 4/24/2025 8:49 AM, Matthew Wilcox wrote:
>> On Wed, Apr 23, 2025 at 09:25:05AM +0200, David Hildenbrand wrote:
>>> On 23.04.25 09:22, David Hildenbrand wrote:
>>>> On 23.04.25 02:36, Matthew Wilcox wrote:
>>>>> On Tue, Apr 22, 2025 at 04:41:11PM -0700, Andrew Morton wrote:
>>>>>>> +/**
>>>>>>> + * folio_migrate_expected_refs - Count expected references for an unmapped folio.
>>>>>>
>>>>>> "folio_migration_expected_refs"
> 
> Thank you for catching this, I'll fix it.
> 
> I wasn't previously aware of using make W=1 to build kernel-docs and
> check for warnings - this is very useful information for me.
> 
> I'll add to changelog to better explain why this is needed for JFS.
> 
>>>>>
>>>>> What I do wonder is whether we want to have such a specialised
>>>>> function existing.  We have can_split_folio() in huge_memory.c
>>>>> which is somewhat more comprehensive and doesn't require the folio to be
>>>>> unmapped first.
>>>>
>>>> I was debating with myself whether we should do the usual "refs from
>>>> ->private, refs from page table mappings" .. dance, and look up the
>>>> mapping from the folio instead of passing it in.
>>>>
>>>> I concluded that for this (migration) purpose the function is good
>>>> enough as it is: if abused in wrong context (e.g., still ->private,
>>>> still page table mappings), it would not fake that there are no
>>>> unexpected references.
>>>
>>> Sorry, I forgot that we still care about the reference from ->private here.
>>> We expect the folio to be unmapped.
>>
>> Right, so just adding in folio_mapcount() will be a no-op for migration,
>> but enable its reuse by can_split_folio().  Maybe.  Anyway, the way I
>> explain page refocunts to people (and I need to put this in a document
>> somewhere):
>>
>> There are three types of contribution to the refcount:
>>
>>   - Expected.  These are deducible from the folio itself, and they're all
>>     findable.  You need to figure out what the expected number of
>>     references are to a folio if you're going to try to freeze it.
>>     These can be references from the mapcount, the page cache, the swap
>>     cache, the private data, your call chain.
>>   - Temporary.  Someone else has found the folio somehow; perhaps through
>>     the page cache, or by calling GUP or something.  They mean you can't
>>     freeze the folio because you don't know who has the reference or how
>>     long they might hold it for.
>>   - Spurious.  This is like a temporary reference, but worse because if
>>     you read the code, there should be no way for there to be any temporary
>>     references to the folio.  Someone's found a stale pointer to this
>>     folio and has bumped the reference count while they check that the
>>     folio they have is the one they expected to find.  They're going
>>     to find out that the pointer they followed is stale and put their
>>     refcount soon, but in the meantime you still can't freeze the folio.
>>
>> So I don't love the idea of having a function with the word "expected"
>> in the name that returns a value which doesn't take into account all
>> the potential contributors to the expected value.  And sure we can keep
>> adding qualifiers to the function name to indicate how it is to be used,
>> but at some point I think we should say "It's OK for this to be a little
>> less efficient so we can understand what it means".
> 
> Thank you, Willy, for the detailed explanation about page reference counting.
> This has helped me understand the concept much better.
> 
> Based on your explanation and the discussion, I'm summarizing the 2 approaches:
> 
> 1. Rename folio_migration_expected_refs to folio_migration_expected_base_refs, to
> to clarify it does not account for other potential contributors.
> or folio_unmapped_base_refs?
> 2. Accounting all possible contributors to expected refs:
> folio_expected_refs(mapping, folio)
> {	
> 	int refs = 1;
> 
> 	if (mapping) {
> 		if (folio_test_anon(folio))
> 			refs += folio_test_swapcache(folio) ?
> 				folio_nr_pages(folio) : 0;
> 		else
> 			refs += folio_nr_pages(folio);
> 
> 		if (folio_test_private(folio))
> 			refs++;
> 	}
> 	refs += folio_mapcount(folio); // takes mapped folio into account and evaluate as no-op for unmapped folios during migration
> 	return refs;
> }
> 
> Please let me know if this approach is acceptable or if you have
> other suggestions for improvement.

A couple of points:

1) Can we name it folio_expected_ref_count()

2) Can we avoid passing in the mapping? Might not be expensive to look it
    up again. Below I avoid calling folio_mapping().

3) Can we delegate adding the additional reference to the caller? Will make it
    easier to use elsewhere (e.g., not additional reference because we are holding
    the page table lock).

4) Can we add kerneldoc, and in particular document the semantics?

Not sure if we should inline this function or put it into mm/utils.c


I'm thinking of something like (completely untested):

  
diff --git a/include/linux/mm.h b/include/linux/mm.h
index a205020e2a58b..a0ad4ed9a75ff 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2112,6 +2112,61 @@ static inline bool folio_maybe_mapped_shared(struct folio *folio)
  	return folio_test_large_maybe_mapped_shared(folio);
  }
  
+/**
+ * folio_expected_ref_count - calculate the expected folio refcount
+ * @folio: the folio
+ *
+ * Calculate the expected folio refcount, taking references from the pagecache,
+ * swapcache, PG_private and page table mappings into account. Useful in
+ * combination with folio_ref_count() to detect unexpected references (e.g.,
+ * GUP or other temporary references).
+ *
+ * Does currently not consider references from the LRU cache. If the folio
+ * was isolated from the LRU (which is the case during migration or split),
+ * the folio was already isolated from the LRU and the LRU cache does not apply.
+ *
+ * Calling this function on an unmapped folio -- !folio_mapped() -- that is
+ * locked will return a stable result.
+ *
+ * Calling this function on a mapped folio will not result in a stable result,
+ * because nothing stops additional page table mappings from coming (e.g.,
+ * fork()) or going (e.g., munmap()).
+ *
+ * Calling this function without the folio lock will also not result in a
+ * stable result: for example, the folio might get dropped from the swapcache
+ * concurrently.
+ *
+ * However, even when called without the folio lock or on a mapped folio,
+ * this function can be used to detect unexpected references early (for example.
+ * if it makes sense to even lock the folio and unmap it).
+ *
+ * The caller must add any reference (e.g., from folio_try_get()) it might be
+ * holding itself to the result.
+ *
+ * Returns the expected folio refcount.
+ */
+static inline int folio_expected_ref_count(const struct folio *folio)
+{
+	const int order = folio_order(folio);
+	int ref_count = 0;
+
+	if (WARN_ON_ONCE(folio_test_slab(folio)))
+		return 0;
+
+	if (folio_test_anon(folio)) {
+		/* One reference per page from the swapcache. */
+		ref_count += folio_test_swapcache(folio) << order;
+	} else if (!((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS)) {
+		/* One reference per page from the pagecache. */
+		ref_count += !!folio->mapping << order;
+		/* One reference from PG_private. */
+		ref_count += folio_test_private(folio);
+	}
+
+	/* One reference per page table mapping. */
+	return ref_count + folio_mapcount(folio);;
+}
+
  #ifndef HAVE_ARCH_MAKE_FOLIO_ACCESSIBLE
  static inline int arch_make_folio_accessible(struct folio *folio)
  {
-- 
2.49.0


The PAGE_MAPPING_FLAGS can likely go away soon (I have patches for that),
then we only have to test for folio->mapping.

-- 
Cheers,

David / dhildenb

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