Re: [PATCH v1 07/11] mm/huge_memory: convert split_huge_pages_pid() from follow_page() to folio_walk

[Zi Yan <ziy@nvidia.com>]

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On 6 Aug 2024, at 6:24, David Hildenbrand wrote:

> On 06.08.24 12:03, David Hildenbrand wrote:
>> On 06.08.24 11:56, David Hildenbrand wrote:
>>> On 06.08.24 11:46, Ryan Roberts wrote:
>>>> On 02/08/2024 16:55, David Hildenbrand wrote:
>>>>> Let's remove yet another follow_page() user. Note that we have to do the
>>>>> split without holding the PTL, after folio_walk_end(). We don't care
>>>>> about losing the secretmem check in follow_page().
>>>>
>>>> Hi David,
>>>>
>>>> Our (arm64) CI is showing a regression in split_huge_page_test from mm selftests from next-20240805 onwards. Navigating around a couple of other lurking bugs, I was able to bisect to this change (which smells about right).
>>>>
>>>> Newly failing test:
>>>>
>>>> # # ------------------------------
>>>> # # running ./split_huge_page_test
>>>> # # ------------------------------
>>>> # # TAP version 13
>>>> # # 1..12
>>>> # # Bail out! Still AnonHugePages not split
>>>> # # # Planned tests != run tests (12 != 0)
>>>> # # # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
>>>> # # [FAIL]
>>>> # not ok 52 split_huge_page_test # exit=1
>>>>
>>>> It's trying to split some pmd-mapped THPs then checking and finding that they are not split. The split is requested via /sys/kernel/debug/split_huge_pages, which I believe ends up in this function you are modifying here. Although I'll admit that looking at the change, there is nothing obviously wrong! Any ideas?
>>>
>>> Nothing jumps at me as well. Let me fire up the debugger :)
>>
>> Ah, very likely the can_split_folio() check expects a raised refcount
>> already.
>
> Indeed, the following does the trick! Thanks Ryan, I could have sworn
> I ran that selftest as well.
>
> TAP version 13
> 1..12
> ok 1 Split huge pages successful
> ok 2 Split PTE-mapped huge pages successful
> # Please enable pr_debug in split_huge_pages_in_file() for more info.
> # Please check dmesg for more information
> ok 3 File-backed THP split test done
>
> ...
>
>
> @Andrew, can you squash the following?
>
>
> From e5ea585de3e089ea89bf43d8447ff9fc9b371286 Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@redhat.com>
> Date: Tue, 6 Aug 2024 12:08:17 +0200
> Subject: [PATCH] fixup: mm/huge_memory: convert split_huge_pages_pid() from
>  follow_page() to folio_walk
>
> We have to teach can_split_folio() that we are not holding an additional
> reference.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/linux/huge_mm.h | 4 ++--
>  mm/huge_memory.c        | 8 ++++----
>  mm/vmscan.c             | 2 +-
>  3 files changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e25d9ebfdf89..ce44caa40eed 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -314,7 +314,7 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
>  		unsigned long len, unsigned long pgoff, unsigned long flags,
>  		vm_flags_t vm_flags);
>  -bool can_split_folio(struct folio *folio, int *pextra_pins);
> +bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
>  int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>  		unsigned int new_order);
>  static inline int split_huge_page(struct page *page)
> @@ -470,7 +470,7 @@ thp_get_unmapped_area_vmflags(struct file *filp, unsigned long addr,
>  }
>   static inline bool
> -can_split_folio(struct folio *folio, int *pextra_pins)
> +can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
>  {
>  	return false;
>  }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 697fcf89f975..c40b0dcc205b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3021,7 +3021,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>  }
>   /* Racy check whether the huge page can be split */
> -bool can_split_folio(struct folio *folio, int *pextra_pins)
> +bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
>  {
>  	int extra_pins;
>  @@ -3033,7 +3033,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
>  		extra_pins = folio_nr_pages(folio);
>  	if (pextra_pins)
>  		*pextra_pins = extra_pins;
> -	return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - 1;
> +	return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - caller_pins;
>  }
>   /*
> @@ -3201,7 +3201,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>  	 * Racy check if we can split the page, before unmap_folio() will
>  	 * split PMDs
>  	 */
> -	if (!can_split_folio(folio, &extra_pins)) {
> +	if (!can_split_folio(folio, 1, &extra_pins)) {
>  		ret = -EAGAIN;
>  		goto out_unlock;
>  	}
> @@ -3537,7 +3537,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
>  		 * can be split or not. So skip the check here.
>  		 */
>  		if (!folio_test_private(folio) &&
> -		    !can_split_folio(folio, NULL))
> +		    !can_split_folio(folio, 0, NULL))
>  			goto next;
>   		if (!folio_trylock(folio))

The diff below can skip a folio with private and extra pin(s) early instead
of trying to lock and split it then failing at can_split_folio() inside
split_huge_page_to_list_to_order().

Maybe worth applying on top of yours?


diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a218320a9233..ce992d54f1da 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3532,13 +3532,10 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
                        goto next;

                total++;
-               /*
-                * For folios with private, split_huge_page_to_list_to_order()
-                * will try to drop it before split and then check if the folio
-                * can be split or not. So skip the check here.
-                */
-               if (!folio_test_private(folio) &&
-                   !can_split_folio(folio, 0, NULL))
+
+               if (!can_split_folio(folio,
+                                    folio_test_private(folio) ? 1 : 0,
+                                    NULL))
                        goto next;

                if (!folio_trylock(folio))

Best Regards,
Yan, Zi

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