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

David Hildenbrand
2024-08-07T09:57:25Z

On 06.08.24 17:36, Zi Yan wrote:
> 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))

Hmm, it does look a bit odd. It's not something from the caller (caller_pins), but a
folio property. Likely should be handled differently.

In vmscan code, we only call can_split_folio() on anon folios where
folio_test_private() does not apply.

But indeed, in split_huge_page_to_list_to_order() we'd have to fail if
folio_test_private() still applies after

Not sure if that is really better:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c40b0dcc205b..7cb743047566 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3026,11 +3026,14 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
int extra_pins;

/* Additional pins from page cache */
- if (folio_test_anon(folio))
+ if (folio_test_anon(folio)) {
extra_pins = folio_test_swapcache(folio) ?
folio_nr_pages(folio) : 0;
- else
+ } else {
extra_pins = folio_nr_pages(folio);
+ if (unlikely(folio_test_private(folio)))
+ extra_pins++;
+ }
if (pextra_pins)
*pextra_pins = extra_pins;
return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - caller_pins;
@@ -3199,9 +3202,11 @@ 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
+ * split PMDs. filemap_release_folio() will try to free buffer; if that
+ * fails, filemap_release_folio() fails.
*/
- if (!can_split_folio(folio, 1, &extra_pins)) {
+ if (WARN_ON_ONCE(folio_test_private(folio)) ||
+ !can_split_folio(folio, 1, &extra_pins)) {
ret = -EAGAIN;
goto out_unlock;
}
@@ -3531,13 +3536,7 @@ 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, 0, NULL))
goto next;

if (!folio_trylock(folio))

It assumes that folio_set_private() is impossible after filemap_release_folio() succeeded
and we're still holding the folio lock. Then we could even get rid of the WARN_ON_ONCE().

--
Cheers,

David / dhildenb