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b=B2decGWw0u8E7Av6H5ZFQnlYMzFfuJAbTInHOvgpu0D2kdZTJG0o3Zst6tb0ogmX2 LM8dUeQolCxKnhlSnplrBUfpPQUDqoA1zXGC5hKj2qKOzBsdNkgonr/zDWXueKKYtf lBXXECedtgjcVqnpWWGwV4XFM+VTUBM1A5/YoQY1wjBfFERJpCYulqxvw0jSCuytQ8 aK2rJaaI1iR3RVBlYwsnArjbskXGWTf6AbD9uLyxHv8JeTnbQE0LM05iGu9gT1KFtQ EbYbf1b4/090HVMcAjjg5YUSf8ymLDoVuYHPPJMbBpgO4Dgm08g/GPDheJWjuTG7Sn 5WHUjgW6JR4Iw== Date: Mon, 17 Aug 2026 17:39:15 +0100 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Hongfu Li , akpm@linux-foundation.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hongfu Li Subject: Re: [PATCH v4] mm: Use a folio in the softleaf_is_device_private path Message-ID: References: <20260803031554.30421-1-hongfu.li@linux.dev> <3ae2f1e1-c784-44ed-9b2c-5edf5699d627@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: 94u9yd1u36bnypektq775ysyg9wtgprd X-Rspamd-Queue-Id: 2E5EE180004 X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1786984774-85087 X-HE-Meta: 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 FuTiQ9RB pjvzBMqHeuk1shtbghm74gagLk06/5mNlwXTF4ZivaIAAmuKbSIs4cCEiJCE5XL2kVtpk+KZYdRbCXwmErPEK2wMpMDXudrfSQoGhzc4eZzbtYRYyMlbo3m4/kie+TBaOcmRfqewRuQ8Qtko+hd1M37qdSoTG3VBNjFIODgvJqlxOARUrmC9PoaYzDMmRqqKvPyGFPcCblQWwUUwEnerpqruf3p/TMzgNtoke2mxNVkJrKK0nkN+o1Vg1YIG2VPyLMkchTDbjjlBM/zKUFFDVBgMXoCUTEl4XKP/Ws6ZM3MLtuonxZtqIf4/+4n0/vJR52hwH Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 17, 2026 at 05:57:57PM +0200, David Hildenbrand (Arm) wrote: > On 8/17/26 17:52, David Hildenbrand (Arm) wrote: > > On 8/3/26 05:15, Hongfu Li wrote: > >> From: Hongfu Li > >> > >> 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 > >> Acked-by: David Hildenbrand (Arm) > >> Reviewed-by: Lorenzo Stoakes (ARM) > >> --- > >> 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. > > > > 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); So after migrate_to_ram() the folio might have been split or otherwise somehow the page doesn't belong to the same locked, refcount-incremented folio it did before? That's kinda a footgun... but this documents it at least. I think a comment explaining how this can happen would be helpful though as this doesn't seem intuitive. > folio_unlock(folio); > folio_put(folio); > } else { > > > -- > Cheers, > > David -- Cheers, Lorenzo