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Message-ID: <799691d6-d9dd-43fd-805e-7dce6efff3e6@redhat.com> Date: Mon, 2 Jun 2025 14:20:52 +0200 From: David Hildenbrand <david@...hat.com> To: Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org Cc: gerald.schaefer@...ux.ibm.com, dan.j.williams@...el.com, jgg@...pe.ca, willy@...radead.org, linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev, linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de, zhang.lyra@...il.com, debug@...osinc.com, bjorn@...nel.org, balbirs@...dia.com, lorenzo.stoakes@...cle.com, linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev, linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org, linux-cxl@...r.kernel.org, dri-devel@...ts.freedesktop.org, John@...ves.net Subject: Re: [PATCH 07/12] mm: Remove redundant pXd_devmap calls On 02.06.25 11:33, David Hildenbrand wrote: >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -1398,10 +1398,7 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr, >> } >> >> entry = pmd_mkhuge(pfn_t_pmd(pfn, prot)); >> - if (pfn_t_devmap(pfn)) >> - entry = pmd_mkdevmap(entry); >> - else >> - entry = pmd_mkspecial(entry); >> + entry = pmd_mkspecial(entry); >> if (write) { > > > I just stumbled over this, and I think there is something off here in > the PMD/PUD case. > > vmf_insert_folio_pmd() does a folio_get() + folio_add_file_rmap_pmd(). > > But then, we go ahead and turn this into a special mapping by setting it > pmd_mkdevmap()/pmd_mkspecial(). > > Consequently, vm_normal_page_pmd() would ignore them, not following the > rules documented for vm_normal_page() and behaving differently than > vmf_insert_page_mkwrite()->insert_page(). > > > folio_add_file_rmap_pmd() should never set these things special/devmap > in the first place :/ > > What am I missing? > > Note that fs/dax.c calls vmf_insert_folio_pmd() for PMDs and > vmf_insert_page_mkwrite() for PTEs. > > Consequently, PTEs will never be marked special (corner case, shared > zeropage), but PMDs would always. > > Hm? What an ugly piece of crap this pmd handling code is. Just noting because I stumbled over that myself: pmd_mkdevmap() does *not* imply pmd_special(). ... in contrast to pte_mkdevmap(), which will imply pte_special(). -- Cheers, David / dhildenb
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