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[2003:cb:c739:2400:78ac:64bb:a39e:2578]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3678aa3caccsm676393f8f.35.2024.07.02.04.02.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Jul 2024 04:02:30 -0700 (PDT) Message-ID: <0b549ff0-b0b6-4fc8-aa6f-0d76157575b3@redhat.com> Date: Tue, 2 Jul 2024 13:02:28 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 07/13] huge_memory: Allow mappings of PUD sized pages To: Alistair Popple Cc: dan.j.williams@intel.com, vishal.l.verma@intel.com, dave.jiang@intel.com, logang@deltatee.com, bhelgaas@google.com, jack@suse.cz, jgg@ziepe.ca, catalin.marinas@arm.com, will@kernel.org, mpe@ellerman.id.au, npiggin@gmail.com, dave.hansen@linux.intel.com, ira.weiny@intel.com, willy@infradead.org, djwong@kernel.org, tytso@mit.edu, linmiaohe@huawei.com, peterx@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, jhubbard@nvidia.com, hch@lst.de, david@fromorbit.com References: <874j98gjfg.fsf@nvdebian.thelocal> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: 9kk9s7f8h3crrmp9o11ykhqh8io8ps78 X-Rspamd-Queue-Id: D263F100022 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1719918156-431723 X-HE-Meta: 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 GSkVqGj/ QCZEUhfSzaYO0lseFFB/yoHK4ekPbX2s2P8CiUB5VfO4qZtuoxZyTkd3AJ3nHYCRoa7vWD/aNTSpR4HH18rw8rHjs7VECezWH5+EP75mCUTJwTDR7EciOpaMSFZHtSa9TgLHVmiszWIuMTjzyWdJ4k9voVk7ugIhntq7O+o5Rq+YOVo4edXH3parHwug5Q5cB19m30KnnDWow/7x8He9KtkJ3zo+wZTZa/wpnstisBMh9lN2RKEV8FY6HVLrytM4cMvNa2Zv2xpyacN/LepLWfcZz4Uw5OLdiILwxYFkobior9AufiKhhcF64CcpBkmUEXTqdnWuq5Gfn5SI42s6MXNFVmtw510AzwFzjcFj3fDd3dLxCPbKFzZlQlxCf6K9AJqcID2gN4A5kQQnwCDL8A1+v2ZF3/ckDzIg3mzG6RbKwljg+0JBfN1333dMhlUejYFh2 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 02.07.24 12:19, Alistair Popple wrote: > > David Hildenbrand writes: > >> On 27.06.24 02:54, Alistair Popple wrote: >>> Currently DAX folio/page reference counts are managed differently to >>> normal pages. To allow these to be managed the same as normal pages >>> introduce dax_insert_pfn_pud. This will map the entire PUD-sized folio >>> and take references as it would for a normally mapped page. >>> This is distinct from the current mechanism, vmf_insert_pfn_pud, >>> which >>> simply inserts a special devmap PUD entry into the page table without >>> holding a reference to the page for the mapping. >> >> Do we really have to involve mapcounts/rmap for daxfs pages at this >> point? Or is this only "to make it look more like other pages" ? > > The aim of the series is make FS DAX and other ZONE_DEVICE pages look > like other pages, at least with regards to the way they are refcounted. > > At the moment they are not refcounted - instead their refcounts are > basically statically initialised to one and there are all these special > cases and functions requiring magic PTE bits (pXX_devmap) to do the > special DAX reference counting. This then adds some cruft to manage > pgmap references and to catch the 2->1 page refcount transition. All > this just goes away if we manage the page references the same as other > pages (and indeed we already manage DEVICE_PRIVATE and COHERENT pages > the same as normal pages). > > So I think to make this work we at least need the mapcounts. > We only really need the mapcounts if we intend to do something like folio_mapcount() == folio_ref_count() to detect unexpected folio references, and if we have to have things like folio_mapped() working. For now that was not required, that's why I am asking. Background also being that in a distant future folios will be decoupled more from other compound pages, and only folios (or "struct anon_folio" / "struct file_folio") would even have mapcounts. For example, most stuff we map (and refcount!) via vm_insert_page() really must stop involving mapcounts. These won't be "ordinary" mapcount-tracked folios in the future, they are simply some refcounted pages some ordinary driver allocated. For FS-DAX, if we'll be using the same "struct file_folio" approach as for ordinary pageache memory, then this is the right thing to do here. >> I'm asking this because: >> >> (A) We don't support mixing PUD+PMD mappings yet. I have plans to change >> that in the future, but for now you can only map using a single PUD >> or by PTEs. I suspect that's good enoug for now for dax fs? > > Yep, that's all we support. > >> (B) As long as we have subpage mapcounts, this prevents vmemmap >> optimizations [1]. Is that only used for device-dax for now and are >> there no plans to make use of that for fs-dax? > > I don't have any plans to. This is purely focussed on refcounting pages > "like normal" so we can get rid of all the DAX special casing. > >> (C) We managed without so far :) > > Indeed, although Christoph has asked repeatedly ([1], [2] and likely > others) that this gets fixed and I finally got sick of it coming up > everytime I need to touch something with ZONE_DEVICE pages :) > > Also it removes the need for people to understand the special DAX page > recounting scheme and ends up removing a bunch of cruft as a bonus: > > 59 files changed, 485 insertions(+), 869 deletions(-) I'm not challenging the refcounting scheme. I'm purely asking about mapcount handling, which is something related but different. > > And that's before I clean up all the pgmap reference handling. It also > removes the pXX_trans_huge and pXX_leaf distinction. So we managed, but > things could be better IMHO. > Again, all nice things. >> Having that said, with folio->_large_mapcount things like >> folio_mapcount() are no longer terribly slow once we weould PTE-map a >> PUD-sized folio. >> >> Also, all ZONE_DEVICE pages should currently be marked PG_reserved, >> translating to "don't touch the memmap". I think we might want to >> tackle that first. Missed to add a pointer to [2]. > > Ok. I'm keen to get this series finished and I don't quite get the > connection here, what needs to change there? include/linux/page-flags.h "PG_reserved is set for special pages. The "struct page" of such a page should in general not be touched (e.g. set dirty) except by its owner. Pages marked as PG_reserved include: ... - Device memory (e.g. PMEM, DAX, HMM) " I think we already entered that domain with other ZONE_DEVICE pages being returned from vm_normal_folio(), unfortunately. But that really must be cleaned up for these pages to not look special anymore. Agreed that it likely is something that is not blocking this series. -- Cheers, David / dhildenb