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bh=1h88QXkQdFyWOO8na2GWe9TzIac4MB0oRM4R5bwyOic=; b=Stg6TyP2re1BjKxYs0u1ScJmO+AIWkAsLtJUM3Nc7orFPoBkrXp/QTyG4voqK/N3a/5QuS m94lTKeDqp+xf/R3w8/B/oqdyandOs5U9rGcAU50xHYOXO+tIzyWdYxUwzE5asYu3JF3s6 C7hCwTe9KHObFYQnJkeNUwTj3bsBbF8= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-515-mcN1S3BFMF-76eFF94-8_A-1; Fri, 26 May 2023 07:29:15 -0400 X-MC-Unique: mcN1S3BFMF-76eFF94-8_A-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 760E3381D208; Fri, 26 May 2023 11:29:14 +0000 (UTC) Received: from warthog.procyon.org.uk (unknown [10.39.192.68]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0526B140E95D; Fri, 26 May 2023 11:29:11 +0000 (UTC) From: David Howells To: Christoph Hellwig , David Hildenbrand , Lorenzo Stoakes Cc: David Howells , Jens Axboe , Al Viro , Matthew Wilcox , Jan Kara , Jeff Layton , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , Christian Brauner , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton Subject: [PATCH v3 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() Date: Fri, 26 May 2023 12:28:57 +0100 Message-Id: <20230526112859.654506-2-dhowells@redhat.com> In-Reply-To: <20230526112859.654506-1-dhowells@redhat.com> References: <20230526112859.654506-1-dhowells@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 73C7C40014 X-Stat-Signature: x7kaaxk6x4yi9fd5qa6phnxd1panb57d X-Rspam-User: X-HE-Tag: 1685100560-542155 X-HE-Meta: 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 n6mhaE1F 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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: Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a pointer to it from the page tables and make unpin_user_page*() correspondingly ignore a ZERO_PAGE when unpinning. We don't want to risk overrunning a zero page's refcount as we're only allowed ~2 million pins on it - something that userspace can conceivably trigger. Add a pair of functions to test whether a page or a folio is a ZERO_PAGE. Signed-off-by: David Howells cc: Christoph Hellwig cc: David Hildenbrand cc: Lorenzo Stoakes cc: Andrew Morton cc: Jens Axboe cc: Al Viro cc: Matthew Wilcox cc: Jan Kara cc: Jeff Layton cc: Jason Gunthorpe cc: Logan Gunthorpe cc: Hillf Danton cc: Christian Brauner cc: Linus Torvalds cc: linux-fsdevel@vger.kernel.org cc: linux-block@vger.kernel.org cc: linux-kernel@vger.kernel.org cc: linux-mm@kvack.org --- Notes: ver #3) - Move is_zero_page() and is_zero_folio() to mm.h for dependency reasons. - Add more comments and adjust the docs. ver #2) - Fix use of ZERO_PAGE(). - Add is_zero_page() and is_zero_folio() wrappers. - Return the zero page obtained, not ZERO_PAGE(0) unconditionally. Documentation/core-api/pin_user_pages.rst | 6 +++++ include/linux/mm.h | 26 +++++++++++++++++-- mm/gup.c | 31 ++++++++++++++++++++++- 3 files changed, 60 insertions(+), 3 deletions(-) diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst index 9fb0b1080d3b..d3c1f6d8c0e0 100644 --- a/Documentation/core-api/pin_user_pages.rst +++ b/Documentation/core-api/pin_user_pages.rst @@ -112,6 +112,12 @@ pages: This also leads to limitations: there are only 31-10==21 bits available for a counter that increments 10 bits at a time. +* Because of that limitation, special handling is applied to the zero pages + when using FOLL_PIN. We only pretend to pin a zero page - we don't alter its + refcount or pincount at all (it is permanent, so there's no need). The + unpinning functions also don't do anything to a zero page. This is + transparent to the caller. + * Callers must specifically request "dma-pinned tracking of pages". In other words, just calling get_user_pages() will not suffice; a new set of functions, pin_user_page() and related, must be used. diff --git a/include/linux/mm.h b/include/linux/mm.h index 27ce77080c79..3c2f6b452586 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1910,6 +1910,28 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma, return page_maybe_dma_pinned(page); } +/** + * is_zero_page - Query if a page is a zero page + * @page: The page to query + * + * This returns true if @page is one of the permanent zero pages. + */ +static inline bool is_zero_page(const struct page *page) +{ + return is_zero_pfn(page_to_pfn(page)); +} + +/** + * is_zero_folio - Query if a folio is a zero page + * @folio: The folio to query + * + * This returns true if @folio is one of the permanent zero pages. + */ +static inline bool is_zero_folio(const struct folio *folio) +{ + return is_zero_page(&folio->page); +} + /* MIGRATE_CMA and ZONE_MOVABLE do not allow pin pages */ #ifdef CONFIG_MIGRATION static inline bool is_longterm_pinnable_page(struct page *page) @@ -1920,8 +1942,8 @@ static inline bool is_longterm_pinnable_page(struct page *page) if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE) return false; #endif - /* The zero page may always be pinned */ - if (is_zero_pfn(page_to_pfn(page))) + /* The zero page can be "pinned" but gets special handling. */ + if (is_zero_page(page)) return true; /* Coherent device memory must always allow eviction. */ diff --git a/mm/gup.c b/mm/gup.c index bbe416236593..ad28261dcafd 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -51,7 +51,8 @@ static inline void sanity_check_pinned_pages(struct page **pages, struct page *page = *pages; struct folio *folio = page_folio(page); - if (!folio_test_anon(folio)) + if (is_zero_page(page) || + !folio_test_anon(folio)) continue; if (!folio_test_large(folio) || folio_test_hugetlb(folio)) VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page); @@ -131,6 +132,13 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags) else if (flags & FOLL_PIN) { struct folio *folio; + /* + * Don't take a pin on the zero page - it's not going anywhere + * and it is used in a *lot* of places. + */ + if (is_zero_page(page)) + return page_folio(page); + /* * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a * right zone, so fail and let the caller fall back to the slow @@ -180,6 +188,8 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags) static void gup_put_folio(struct folio *folio, int refs, unsigned int flags) { if (flags & FOLL_PIN) { + if (is_zero_folio(folio)) + return; node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs); if (folio_test_large(folio)) atomic_sub(refs, &folio->_pincount); @@ -224,6 +234,13 @@ int __must_check try_grab_page(struct page *page, unsigned int flags) if (flags & FOLL_GET) folio_ref_inc(folio); else if (flags & FOLL_PIN) { + /* + * Don't take a pin on the zero page - it's not going anywhere + * and it is used in a *lot* of places. + */ + if (is_zero_page(page)) + return 0; + /* * Similar to try_grab_folio(): be sure to *also* * increment the normal page refcount field at least once, @@ -3079,6 +3096,9 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast); * * FOLL_PIN means that the pages must be released via unpin_user_page(). Please * see Documentation/core-api/pin_user_pages.rst for further details. + * + * Note that if a zero_page is amongst the returned pages, it will not have + * pins in it and unpin_user_page() will not remove pins from it. */ int pin_user_pages_fast(unsigned long start, int nr_pages, unsigned int gup_flags, struct page **pages) @@ -3110,6 +3130,9 @@ EXPORT_SYMBOL_GPL(pin_user_pages_fast); * * FOLL_PIN means that the pages must be released via unpin_user_page(). Please * see Documentation/core-api/pin_user_pages.rst for details. + * + * Note that if a zero_page is amongst the returned pages, it will not have + * pins in it and unpin_user_page*() will not remove pins from it. */ long pin_user_pages_remote(struct mm_struct *mm, unsigned long start, unsigned long nr_pages, @@ -3143,6 +3166,9 @@ EXPORT_SYMBOL(pin_user_pages_remote); * * FOLL_PIN means that the pages must be released via unpin_user_page(). Please * see Documentation/core-api/pin_user_pages.rst for details. + * + * Note that if a zero_page is amongst the returned pages, it will not have + * pins in it and unpin_user_page*() will not remove pins from it. */ long pin_user_pages(unsigned long start, unsigned long nr_pages, unsigned int gup_flags, struct page **pages, @@ -3161,6 +3187,9 @@ EXPORT_SYMBOL(pin_user_pages); * pin_user_pages_unlocked() is the FOLL_PIN variant of * get_user_pages_unlocked(). Behavior is the same, except that this one sets * FOLL_PIN and rejects FOLL_GET. + * + * Note that if a zero_page is amongst the returned pages, it will not have + * pins in it and unpin_user_page*() will not remove pins from it. */ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages, struct page **pages, unsigned int gup_flags)