linux-kernel - Re: [PATCH v1] mm: convert folio_estimated_sharers() to folio_likely_mapped

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Message-ID: <dd0ad9f2-2d7a-45f3-9ba3-979488c7dd27@redhat.com>
Date: Mon, 25 Mar 2024 09:57:48 +0100
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org, akpm@...uxfoundation.org
Cc: linux-mm@...ck.org, Barry Song <v-songbaohua@...o.com>,
 Vishal Moola <vishal.moola@...il.com>, Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH v1] mm: convert folio_estimated_sharers() to
 folio_likely_mapped_shared()

On 27.02.24 21:15, David Hildenbrand wrote:
> Callers of folio_estimated_sharers() only care about "mapped shared vs.
> mapped exclusively", not the exact estimate of sharers. Let's consolidate
> and unify the condition users are checking. While at it clarify the
> semantics and extend the discussion on the fuzziness.
> 
> Use the "likely mapped shared" terminology to better express what the
> (adjusted) function actually checks.
> 
> Whether a partially-mappable folio is more likely to not be partially
> mapped than partially mapped is debatable. In the future, we might be able
> to improve our estimate for partially-mappable folios, though.
> 
> Note that we will now consistently detect "mapped shared" only if the
> first subpage is actually mapped multiple times. When the first subpage
> is not mapped, we will consistently detect it as "mapped exclusively".
> This change should currently only affect the usage in
> madvise_free_pte_range() and queue_folios_pte_range() for large folios: if
> the first page was already unmapped, we would have skipped the folio.
> 
> Cc: Barry Song <v-songbaohua@...o.com>
> Cc: Vishal Moola (Oracle) <vishal.moola@...il.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---

The following fixup on top to make kerneldoc happy:


 From 3e472636d266e3acba3755ed5712992adbc2151d Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@...hat.com>
Date: Mon, 25 Mar 2024 09:23:03 +0100
Subject: [PATCH] folio_likely_mapped_shared() kerneldoc fixup

Fixup "mm: convert folio_estimated_sharers() to
folio_likely_mapped_shared()".

Signed-off-by: David Hildenbrand <david@...hat.com>
---
  include/linux/mm.h | 32 +++++++++++++++++---------------
  1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index afe27ff3fa94..fb3724723448 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2183,23 +2183,25 @@ static inline size_t folio_size(struct folio *folio)
   * at most once into an MM, and they cannot be partially mapped.
   *
   * For other folios, the result can be fuzzy:
- * (a) For partially-mappable large folios (THP), the return value can wrongly
- *     indicate "mapped exclusively" (false negative) when the folio is
- *     only partially mapped into at least one MM.
- * (b) For pagecache folios (including hugetlb), the return value can wrongly
- *     indicate "mapped shared" (false positive) when two VMAs in the same MM
- *     cover the same file range.
- * (c) For (small) KSM folios, the return value can wrongly indicate "mapped
- *     shared" (false negative), when the folio is mapped multiple times into
- *     the same MM.
+ *    #. For partially-mappable large folios (THP), the return value can wrongly
+ *       indicate "mapped exclusively" (false negative) when the folio is
+ *       only partially mapped into at least one MM.
+ *    #. For pagecache folios (including hugetlb), the return value can wrongly
+ *       indicate "mapped shared" (false positive) when two VMAs in the same MM
+ *       cover the same file range.
+ *    #. For (small) KSM folios, the return value can wrongly indicate "mapped
+ *       shared" (false negative), when the folio is mapped multiple times into
+ *       the same MM.
   *
   * Further, this function only considers current page table mappings that
- * are tracked using the folio mapcount(s). It does not consider:
- * (1) If the folio might get mapped in the (near) future (e.g., swapcache,
- *     pagecache, temporary unmapping for migration).
- * (2) If the folio is mapped differently (VM_PFNMAP).
- * (3) If hugetlb page table sharing applies. Callers might want to check
- *     hugetlb_pmd_shared().
+ * are tracked using the folio mapcount(s).
+ *
+ * This function does not consider:
+ *    #. If the folio might get mapped in the (near) future (e.g., swapcache,
+ *       pagecache, temporary unmapping for migration).
+ *    #. If the folio is mapped differently (VM_PFNMAP).
+ *    #. If hugetlb page table sharing applies. Callers might want to check
+ *       hugetlb_pmd_shared().
   *
   * Return: Whether the folio is estimated to be mapped into more than one MM.
   */
-- 
2.43.2


-- 
Cheers,

David / dhildenb