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[2003:d8:2f1a:f500:4346:f17c:2bde:808c]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b4708d0ae0sm13394704f8f.33.2025.07.08.07.31.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 08 Jul 2025 07:31:08 -0700 (PDT) Message-ID: <9e8771ef-e734-4d56-aa20-c3fdba0fd5ab@redhat.com> Date: Tue, 8 Jul 2025 16:31:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 07/11] mm/memremap: Add folio_split support To: Balbir Singh , linux-mm@kvack.org, akpm@linux-foundation.org Cc: dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org, Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Shuah Khan , Barry Song , Baolin Wang , Ryan Roberts , Matthew Wilcox , Peter Xu , Zi Yan , Kefeng Wang , Jane Chu , Alistair Popple , Donet Tom References: <20250306044239.3874247-1-balbirs@nvidia.com> <20250306044239.3874247-8-balbirs@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: ij3zn8n44mf6bq9fbj6rpgiydbnpbr7j X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 5A3531C0004 X-HE-Tag: 1751985075-142038 X-HE-Meta: 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 nlxJc7n7 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 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 06.03.25 05:42, Balbir Singh wrote: > When a zone device page is split (via huge pmd folio split). The > driver callback for folio_split is invoked to let the device driver > know that the folio size has been split into a smaller order. > > The HMM test driver has been updated to handle the split, since the > test driver uses backing pages, it requires a mechanism of reorganizing > the backing pages (backing pages are used to create a mirror device) > again into the right sized order pages. This is supported by exporting > prep_compound_page(). > > Signed-off-by: Balbir Singh > --- > include/linux/memremap.h | 7 +++++++ > include/linux/mm.h | 1 + > lib/test_hmm.c | 35 +++++++++++++++++++++++++++++++++++ > mm/huge_memory.c | 5 +++++ > mm/page_alloc.c | 1 + > 5 files changed, 49 insertions(+) > > diff --git a/include/linux/memremap.h b/include/linux/memremap.h > index 11d586dd8ef1..2091b754f1da 100644 > --- a/include/linux/memremap.h > +++ b/include/linux/memremap.h > @@ -100,6 +100,13 @@ struct dev_pagemap_ops { > */ > int (*memory_failure)(struct dev_pagemap *pgmap, unsigned long pfn, > unsigned long nr_pages, int mf_flags); > + > + /* > + * Used for private (un-addressable) device memory only. > + * This callback is used when a folio is split into > + * a smaller folio Confusing. When a folio is split, it is split into multiple folios. So when will this be invoked? > + */ > + void (*folio_split)(struct folio *head, struct folio *tail); head and tail are really suboptimal termonology. They refer to head and tail pages, which is not really the case with folios (in the long run). > }; > > #define PGMAP_ALTMAP_VALID (1 << 0) > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 98a67488b5fe..3d0e91e0a923 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1415,6 +1415,7 @@ static inline struct folio *virt_to_folio(const void *x) > void __folio_put(struct folio *folio); > > void split_page(struct page *page, unsigned int order); > +void prep_compound_page(struct page *page, unsigned int order); > void folio_copy(struct folio *dst, struct folio *src); > int folio_mc_copy(struct folio *dst, struct folio *src); > > diff --git a/lib/test_hmm.c b/lib/test_hmm.c > index a81d2f8a0426..18b6a7b061d7 100644 > --- a/lib/test_hmm.c > +++ b/lib/test_hmm.c > @@ -1640,10 +1640,45 @@ static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf) > return ret; > } > > + > +static void dmirror_devmem_folio_split(struct folio *head, struct folio *tail) > +{ > + struct page *rpage = BACKING_PAGE(folio_page(head, 0)); > + struct folio *new_rfolio; > + struct folio *rfolio; > + unsigned long offset = 0; > + > + if (!rpage) { > + folio_page(tail, 0)->zone_device_data = NULL; > + return; > + } > + > + offset = folio_pfn(tail) - folio_pfn(head); > + rfolio = page_folio(rpage); > + new_rfolio = page_folio(folio_page(rfolio, offset)); > + > + folio_page(tail, 0)->zone_device_data = folio_page(new_rfolio, 0); > + > + if (folio_pfn(tail) - folio_pfn(head) == 1) { > + if (folio_order(head)) > + prep_compound_page(folio_page(rfolio, 0), > + folio_order(head)); > + folio_set_count(rfolio, 1); > + } > + clear_compound_head(folio_page(new_rfolio, 0)); > + if (folio_order(tail)) > + prep_compound_page(folio_page(new_rfolio, 0), > + folio_order(tail)); > + folio_set_count(new_rfolio, 1); > + folio_page(new_rfolio, 0)->mapping = folio_page(rfolio, 0)->mapping; > + tail->pgmap = head->pgmap; Most of this doesn't look like it should be the responsibility of this callback. Setting up a new folio after the split (messing with compound pages etc) really should not be the responsibility of this callback. So no, this looks misplaced. > +} > + > static const struct dev_pagemap_ops dmirror_devmem_ops = { > .page_free = dmirror_devmem_free, > .migrate_to_ram = dmirror_devmem_fault, > .page_free = dmirror_devmem_free, > + .folio_split = dmirror_devmem_folio_split, > }; > > static int dmirror_device_init(struct dmirror_device *mdevice, int id) > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 995ac8be5709..518a70d1b58a 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3655,6 +3655,11 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, > MTHP_STAT_NR_ANON, 1); > } > > + if (folio_is_device_private(origin_folio) && > + origin_folio->pgmap->ops->folio_split) > + origin_folio->pgmap->ops->folio_split( > + origin_folio, release); Absolutely ugly. I think we need a wrapper for the > + > /* > * Unfreeze refcount first. Additional reference from > * page cache. > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 17ea8fb27cbf..563f7e39aa79 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -573,6 +573,7 @@ void prep_compound_page(struct page *page, unsigned int order) > > prep_compound_head(page, order); > } > +EXPORT_SYMBOL_GPL(prep_compound_page); Hmmm, that is questionable. We don't want arbitrary modules to make use of that. Another sign that you are exposing the wrong functionality/interface (folio_split) to modules. -- Cheers, David / dhildenb