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Tue, 18 Aug 2026 09:50:10 +0000 X-Migadu-Flow: FLOW_OUT Message-ID: <73cbb2f3-98ad-498d-8f33-ff7e39fe6ac0@linux.dev> Date: Tue, 18 Aug 2026 17:50:00 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: hongfu.li@linux.dev, akpm@linux-foundation.org, liam@infradead.org, lihongfu@kylinos.cn, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, rppt@kernel.org, surenb@google.com, vbabka@kernel.org Subject: Re: [PATCH v4] mm: Use a folio in the softleaf_is_device_private path To: "Lorenzo Stoakes (ARM)" , "David Hildenbrand (Arm)" References: <20260818021326.5910-1-hongfu.li@linux.dev> From: Hongfu Li In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 4BD9E40008 X-Stat-Signature: 7wd6aycpdmsyxqukmmz5gdhrbgbzmpbe X-HE-Tag: 1787046616-37613 X-HE-Meta: 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 Q/3hhOWb 3mGmDipv9Z/A8+7zIacYziH6OYtLGPE4cWDRYbSVPhcaZ4wd7GLBjDNTpkNa3Do4LZShvKA3zVRwz5bgt36Auc3wMV0gus+UV3i7LSHOuEuwQkzFAG5UObI9LoW6YIdmDjy3xCkURl3KKUBebElxARWEXpMINoTNZglauWfI9176IbLHJ3wAhMbyYrqEiWzkfwBzmApmFJ6KPTchmnWIzKDfC2YS0K5TVGCTKSzkFG2Vd7QnYZAYKowfyvJ5LL6FV9r37UYTQ8HqMxqZuuSwICK4jfGbwfUOUfUl8O+3GlZCfumeqXFPjm1I9vHi/x85M8B0PPMjy9YRnTOoDZRZeLd9C3ZAF88E78MJc Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/18/26 4:56 PM, Lorenzo Stoakes (ARM) wrote: > On Tue, Aug 18, 2026 at 09:30:40AM +0200, David Hildenbrand (Arm) wrote: >> On 8/18/26 04:13, Hongfu Li wrote: >>>>> 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? >>>> I suspect a split. >>>> >>>>> 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. >>>> I think, conceptually, calling into something that consumes a page (vmf->page) >>>> always needs care when operating on folios. >>>> >>>> Passing the vmf to some callback might be the odd thing here, because the >>>> vmf->page contract is not really clear. >>> I'm very sorry for introducing this regression. >>> Thank you for identifying and testing this issue. >>> >>> I will double-check this code path and apply your fix to run further >>> tests locally. >>> >>> Would it make sense to add a comment here like the below, to make this >>> subtle behavior clearer for future readers? >>> >>> diff --git a/mm/memory.c b/mm/memory.c >>> index 4134ac607ee0..2b6d6c863ecb 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -4937,6 +4937,12 @@ 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); >>> + /* >>> + * migrate_to_ram() can split a large folio, updating >>> + * vmf->page to a different folio. Re-fetch folio for >>> + * correct unlock/put. >>> + */ >> "migrate_to_ram() might have split the folio." >> >> Should be sufficient I guess. > Agreed, no need for a schloppy long comment here! > > /* migrate_to_ram() might have split the folio. */ > folio = page_folio(vmf->page); > > Should be fine. > > The commit message should be updated to reflect it too and mention that we are > going from 4 -> 2 compound_head()'s now. Thanks a lot for the suggestion. A shorter comment is indeed better. Will use your suggested one‑line comment, add the folio re‑fetch, and update commit message for v5. >> -- >> Cheers, >> >> David > -- > Cheers, Lorenzo -- Best regards, Hongfu