linux-kernel - Re: [PATCH v1 1/2] mm: let pte_lockptr() consume a pte

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Message-ID: <498c936f-fa30-4670-9bbc-4cd8b7995091@redhat.com>
Date: Mon, 29 Jul 2024 10:46:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, Peter Xu <peterx@...hat.com>,
 Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1 1/2] mm: let pte_lockptr() consume a pte_t pointer

On 29.07.24 09:48, Qi Zheng wrote:
> 
> 
> On 2024/7/26 02:39, David Hildenbrand wrote:
>> pte_lockptr() is the only *_lockptr() function that doesn't consume
>> what would be expected: it consumes a pmd_t pointer instead of a pte_t
>> pointer.
>>
>> Let's change that. The two callers in pgtable-generic.c are easily
>> adjusted. Adjust khugepaged.c:retract_page_tables() to simply do a
>> pte_offset_map_nolock() to obtain the lock, even though we won't actually
>> be traversing the page table.
>>
>> This makes the code more similar to the other variants and avoids other
>> hacks to make the new pte_lockptr() version happy. pte_lockptr() users
>> reside now only in  pgtable-generic.c.
>>
>> Maybe, using pte_offset_map_nolock() is the right thing to do because
>> the PTE table could have been removed in the meantime? At least it sounds
>> more future proof if we ever have other means of page table reclaim.
> 
> Agree, this helps us recheck the pmd entry.
> 
>>
>> It's not quite clear if holding the PTE table lock is really required:
>> what if someone else obtains the lock just after we unlock it? But we'll
>> leave that as is for now, maybe there are good reasons.
>>
>> This is a preparation for adapting hugetlb page table locking logic to
>> take the same locks as core-mm page table walkers would.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>    include/linux/mm.h   |  7 ++++---
>>    mm/khugepaged.c      | 21 +++++++++++++++------
>>    mm/pgtable-generic.c |  4 ++--
>>    3 files changed, 21 insertions(+), 11 deletions(-)
> 
> Since pte_lockptr() no longer has a pmd parameter, it is best to modify
> the comments above __pte_offset_map_lock() as well:
> 
> ```
> This helps the caller to avoid a later pte_lockptr(mm, *pmd), which
> might by that time act on a changed *pmd ...
> ```

Right, thanks a lot for the review!

The following on top;


 From a46b16aa9bfa02ffb425d364d7f00129a8e803ad Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@...hat.com>
Date: Mon, 29 Jul 2024 10:43:34 +0200
Subject: [PATCH] fixup: mm: let pte_lockptr() consume a pte_t pointer

Let's adjust the comment, passing a pte to pte_lockptr() and dropping
a detail about changed *pmd, which no longer applies.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
  mm/pgtable-generic.c | 10 +++++-----
  1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index 13a7705df3f87..f17465b43d344 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -350,11 +350,11 @@ pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
   * pte_offset_map_nolock(mm, pmd, addr, ptlp), above, is like pte_offset_map();
   * but when successful, it also outputs a pointer to the spinlock in ptlp - as
   * pte_offset_map_lock() does, but in this case without locking it.  This helps
- * the caller to avoid a later pte_lockptr(mm, *pmd), which might by that time
- * act on a changed *pmd: pte_offset_map_nolock() provides the correct spinlock
- * pointer for the page table that it returns.  In principle, the caller should
- * recheck *pmd once the lock is taken; in practice, no callsite needs that -
- * either the mmap_lock for write, or pte_same() check on contents, is enough.
+ * the caller to avoid a later pte_lockptr(mm, pte): pte_offset_map_nolock()
+ * provides the correct spinlock pointer for the page table that it returns.
+ * In principle, the caller should recheck *pmd once the lock is taken; in
+ * practice, no callsite needs that - either the mmap_lock for write, or
+ * pte_same() check on contents, is enough.
   *
   * Note that free_pgtables(), used after unmapping detached vmas, or when
   * exiting the whole mm, does not take page table lock before freeing a page
-- 
2.45.2


-- 
Cheers,

David / dhildenb