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bh=1h88QXkQdFyWOO8na2GWe9TzIac4MB0oRM4R5bwyOic=; b=TgRH0QtEkUDqRxpcTA1aUf9Qd4ze8dvcRzCQWcd1DYYr6Zh9qcPOiYkDvsdx+jMIXjANZg cCGeYXDKcdZf2Qj/0H2KFOlLq+6Ok1WoYz7zXwGrpu2AyK4qyvoEQWhe1o7AoGDRFi11fW kcRMMGUpL7N937NaQI+0yN44AbZR9ms= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-488-yJ1R9c3TMlO_XregAeFROg-1; Fri, 26 May 2023 17:41:51 -0400 X-MC-Unique: yJ1R9c3TMlO_XregAeFROg-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id E2CD7185A78B; Fri, 26 May 2023 21:41:49 +0000 (UTC) Received: from warthog.procyon.org.uk (unknown [10.39.192.68]) by smtp.corp.redhat.com (Postfix) with ESMTP id 66AD2492B0A; Fri, 26 May 2023 21:41:47 +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 v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() Date: Fri, 26 May 2023 22:41:40 +0100 Message-Id: <20230526214142.958751-2-dhowells@redhat.com> In-Reply-To: <20230526214142.958751-1-dhowells@redhat.com> References: <20230526214142.958751-1-dhowells@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 X-Rspamd-Queue-Id: 27B0C40010 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: m8mx1i6ues4h4pwpj383dyb68fyzcp35 X-HE-Tag: 1685137315-202189 X-HE-Meta: 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 qq2/VfLp 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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 Reviewed-by: Lorenzo Stoakes Reviewed-by: Christoph Hellwig Acked-by: David Hildenbrand --- 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)