Re: [PATCH v4] mm: Use a folio in the softleaf_is_device_private path

[David Hildenbrand (Arm)]

On 8/17/26 17:52, David Hildenbrand (Arm) wrote:
> On 8/3/26 05:15, Hongfu Li wrote:
>> From: Hongfu Li <lihongfu@xxxxxxxxxx>
>>
>> Use the folio APIs in the device_private migration path of do_swap_page(),
>> replacing four calls to compound_head() with one.
>>
>> Signed-off-by: Hongfu Li <lihongfu@xxxxxxxxxx>
>> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
>> ---
>> v4:
>> - Drop the do_non_swap_page() extraction (patch 2/2 from v2)
>> - Added Reviewed-by: Lorenzo Stoakes
>> - Link to v2: https://lore.kernel.org/all/20260723030752.16979-1-hongfu.li@xxxxxxxxx/
>> v3:
>> - Please ignore v3.
>> v2:
>> - Drop fault_folio, reuse the existing folio variable instead
>> - Refresh commit messages
>> - Update comment to use "folio" instead of "page" in the
>>   device_private path
>> - Add Acked-by: David Hildenbrand
>> - Add patch 2/2 to extract do_non_swap_page() from do_swap_page()
>> - Link to v1: https://lore.kernel.org/all/20260721013347.65698-1-hongfu.li@xxxxxxxxx/
>> ---
>>  mm/memory.c | 11 ++++++-----
>>  1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 56b244552f13..bbb289ba09d3 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4873,18 +4873,19 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>  				goto unlock;
>>  
>>  			/*
>> -			 * Get a page reference while we know the page can't be
>> +			 * Get a folio reference while we know the folio can't be
>>  			 * freed.
>>  			 */
>> -			if (trylock_page(vmf->page)) {
>> +			folio = page_folio(vmf->page);
>> +			if (folio_trylock(folio)) {
>>  				struct dev_pagemap *pgmap;
>>  
>> -				get_page(vmf->page);
>> +				folio_get(folio);
>>  				pte_unmap_unlock(vmf->pte, vmf->ptl);
>>  				pgmap = page_pgmap(vmf->page);
>>  				ret = pgmap->ops->migrate_to_ram(vmf);
>> -				unlock_page(vmf->page);
>> -				put_page(vmf->page);
>> +				folio_unlock(folio);
>> +				folio_put(folio);
>>  			} else {
>>  				pte_unmap(vmf->pte);
>>  				softleaf_entry_wait_on_locked(entry, vmf->ptl);
> 
> Okay, so this breaks the HMM selftests:
> 
> # ok 30 hmm.hmm_device_private.migrate_anon_huge_fault
> # #  RUN           hmm.hmm_device_private.migrate_partial_unmap_fault ...
> [   14.589125] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
> [   14.589178] Modules linked in: test_hmm
> [   14.589203] CPU: 25 UID: 0 PID: 653 Comm: hmm-tests Not tainted 7.2.0-rc5+ #14 PREEMPT 
> [   14.589243] Hardware name: linux,dummy-virt (DT)
> [   14.589268] pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
> [   14.589300] pc : folio_unlock+0x78/0x88
> [   14.589326] lr : folio_unlock+0x78/0x88
> [   14.589349] sp : ffff80008284bb90
> [   14.589368] x29: ffff80008284bb90 x28: ffff0000cc8ce180 x27: 0000000000000000
> [   14.589406] x26: 0000000000000000 x25: ffff0000ca395dc0 x24: 0000000000000000
> [   14.589447] x23: 0000000020100073 x22: ffff0000c23d35c0 x21: ffffffffbfff8000
> [   14.589485] x20: ffffffffbfff8000 x19: ffff80008284bca0 x18: 0000000000000010
> [   14.589523] x17: 3030303030303030 x16: 2066666666666666 x15: 6630303030303030
> [   14.589562] x14: 0000000000000000 x13: 29296f696c6f6628 x12: 64656b636f6c5f74
> [   14.589603] x11: fffffffffffc7100 x10: fffffffffffc70b0 x9 : ffffb0504ff88f70
> [   14.589640] x8 : 3fffffffffffdfff x7 : ffff000c79b24bc0 x6 : 00000000000001a7
> [   14.589680] x5 : ffff000c79b24bc0 x4 : 0000000000000000 x3 : 0000000000000000
> [   14.589718] x2 : 0000000000000000 x1 : ffff0000cc8ce180 x0 : 000000000000003f
> [   14.589756] Call trace:
> [   14.589769]  folio_unlock+0x78/0x88 (P)
> [   14.589792]  do_swap_page+0xf40/0x1580
> [   14.589815]  __handle_mm_fault+0x648/0xe68
> [   14.589838]  handle_mm_fault+0x9c/0x330
> [   14.589858]  do_page_fault+0x268/0x7e0
> [   14.589882]  do_translation_fault+0x5c/0x80
> [   14.589904]  do_mem_abort+0x48/0xa0
> [   14.589927]  el0_da+0x38/0xd0
> [   14.589948]  el0t_64_sync_handler+0xd0/0xe8
> [   14.589970]  el0t_64_sync+0x198/0x1a0
> [   14.589990] Code: d65f03c0 d000eec1 913c4021 94015993 (d4210000) 
> 
> I can only speculate that we need another
> 
> 	folio = page_folio(vmf->page);
> 
> after the migrate_to_ram() call.
> 
> Let me try that real quick.
> 

Yeah, that fixes it:

diff --git a/mm/memory.c b/mm/memory.c
index 4134ac607ee0..83d1a2be957c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4937,6 +4937,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
                                pte_unmap_unlock(vmf->pte, vmf->ptl);
                                pgmap = page_pgmap(vmf->page);
                                ret = pgmap->ops->migrate_to_ram(vmf);
+                               folio = page_folio(vmf->page);
                                folio_unlock(folio);
                                folio_put(folio);
                        } else {


-- 
Cheers,

David