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h=Date:From:To:Cc:Subject:References:In-Reply-To; b=oLN6x1RZlJ6qDs/H+h+Ay61SstBW7/YNPXc3SxbBy86CKNihdgkjOEtYXmxyDnQ+t YCfbtJMKKSRG7TK4/mO5PhDvvOER+hUWg/nmGYqu5fvUNa0jdcwcXHeoUiXpNHSmav zmc2Ahq5b81Iz6OGeB3J2uPQDtGpOhRhfbwnTbqG2DkqH/BqGejtFZmnoIHVT3rmjq 79vxQyRlFuMsF6WXXVA2Mj9dWl14eIYDh8+E84tUisTp692XcuouYzbXonB1aIR4SC Xna3tqsQyWClChM6RTfSaLYTYlifyVvlBRfhwJ4YvpBmXMr4lvMSPyhTVcFazGqByP kF8qS4PiZ39XA== Date: Tue, 18 Aug 2026 09:56:43 +0100 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Hongfu Li , 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 Message-ID: References: <20260818021326.5910-1-hongfu.li@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 214CB2000A X-Rspam-User: X-Stat-Signature: t3bwh8wfcue63m4cnaz8yuuam44tq48q X-Rspamd-Server: rspam06 X-HE-Tag: 1787043422-913906 X-HE-Meta: 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 wFOc/Jy2 Tv+dI7IGIEMHSPi9r32R8+oCc6MYWxH4cq+RYI+05Zzn/1IbkS2czSJdnKI+imWEA7LsRp6SsGO/8/b+uH8d8DydBPwQi0i+LsMhV0IwEnYRGaJhX9dXxux2S+4yX7+w1G4441XBiBpl+duAd2I9t6eQ8SeGTXICmwiFpGcJtJqGXMu85QCb8JHwy5Log5Rvg7lmLQjnhdJGoLFdcWtI/oPx55eeau0hE0nxCLF6cd/IhAFJdalIvUyF7/X1OlcVcg/S841Q8yMXT+Is/GFUhJAu9sUcWarhpLsKeydF5uZR1AAM= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. > > -- > Cheers, > > David -- Cheers, Lorenzo