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Message-ID: <13ea6653-d33d-4f76-88c8-a17f9989d069@redhat.com> Date: Mon, 2 Jun 2025 12:31:43 +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 00/12] mm: Remove pXX_devmap page table bit and pfn_t type On 29.05.25 08:32, Alistair Popple wrote: > Changes from v2 of the RFC[1]: > > - My ZONE_DEVICE refcount series has been merged as commit 7851bf649d42 (Patch series > "fs/dax: Fix ZONE_DEVICE page reference counts", v9.) which is included in > v6.15 so have rebased on top of that. > > - No major changes required for the rebase other than fixing up a new user of > the pfn_t type (intel_th). > > - As a reminder the main benefit of this series is it frees up a PTE bit > (pte_devmap). > > - This may be a bit late to consider for inclusion in v6.16 unless it can get > some more reviews before the merge window closes. I don't think missing v6.16 > is a huge issue though. > > - This passed xfstests for a XFS filesystem with DAX enabled on my system and > as many of the ndctl tests that pass on my system without it. > > Changes for v2: > > - This is an update to my previous RFC[2] removing just pfn_t rebased > on today's mm-unstable which includes my ZONE_DEVICE refcounting > clean-up. > > - The removal of the devmap PTE bit and associated infrastructure was > dropped from that series so I have rolled it into this series. > > - Logically this series makes sense to me, but the dropping of devmap > is wide ranging and touches some areas I'm less familiar with so > would definitely appreciate any review comments there. > > [1] - https://lore.kernel.org/linux-mm/cover.95ff0627bc727f2bae44bea4c00ad7a83fbbcfac.1739941374.git-series.apopple@nvidia.com/ > [2] - https://lore.kernel.org/linux-mm/cover.a7cdeffaaa366a10c65e2e7544285059cc5d55a4.1736299058.git-series.apopple@nvidia.com/ > > All users of dax now require a ZONE_DEVICE page which is properly > refcounted. This means there is no longer any need for the PFN_DEV, PFN_MAP > and PFN_SPECIAL flags. Furthermore the PFN_SG_CHAIN and PFN_SG_LAST flags > never appear to have been used. It is therefore possible to remove the > pfn_t type and replace any usage with raw pfns. > > The remaining users of PFN_DEV have simply passed this to > vmf_insert_mixed() to create pte_devmap() mappings. It is unclear why this > was the case but presumably to ensure vm_normal_page() does not return > these pages. These users can be trivially converted to raw pfns and > creating a pXX_special() mapping to ensure vm_normal_page() still doesn't > return these pages. > > Now that there are no users of PFN_DEV we can remove the devmap page table > bit and all associated functions and macros, freeing up a software page > table bit. > $ git grep FS_DAX_LIMITED fs/Kconfig: depends on ZONE_DEVICE || FS_DAX_LIMITED fs/Kconfig:config FS_DAX_LIMITED fs/dax.c: if (IS_ENABLED(CONFIG_FS_DAX_LIMITED)) fs/dax.c: if (IS_ENABLED(CONFIG_FS_DAX_LIMITED)) fs/dax.c: if (IS_ENABLED(CONFIG_FS_DAX_LIMITED)) include/linux/pfn_t.h: * PFN_SPECIAL - for CONFIG_FS_DAX_LIMITED builds to allow XIP, but not mm/memremap.c: if (IS_ENABLED(CONFIG_FS_DAX_LIMITED)) { Can we remove that? Especially the interaction with PFN_SPECIAL looks concerning. -- Cheers, David / dhildenb
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