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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/3/26 05:15, Hongfu Li wrote:
> From: Hongfu Li <lihongfu@kylinos.cn>
> 
> 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@kylinos.cn>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
> 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@linux.dev/
> 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@linux.dev/
> ---
>  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.

-- 
Cheers,

David