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bh=VN2ZrpZ7x9fEyIq82GnRy+FAZ1eh7E2WAh2aAKifPL4=; b=LIL53DOrLmOsYXtzVAqCspPRYgBf7HQN6lE48ZklwuwQjinO5ofSGNnfQNiLu3Cp1hl5QD k0dCQAM8wVvoGerdnUQcK8Mm28YZZBOpK+HBnoEdFRYw31U3FAtFka5BuSdh74unbtOmNc TNGEsSzd+cHbnRrP+3IEAy8U6JWh94M= 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-554-FDn4oHLSPcWua6K8xvmmRA-1; Thu, 25 May 2023 11:51:11 -0400 X-MC-Unique: FDn4oHLSPcWua6K8xvmmRA-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 9B8FF85A5AA; Thu, 25 May 2023 15:51:10 +0000 (UTC) Received: from warthog.procyon.org.uk (unknown [10.39.192.68]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3BEA6492B0A; Thu, 25 May 2023 15:51:08 +0000 (UTC) From: David Howells To: Christoph Hellwig , David Hildenbrand 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: [RFC PATCH 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() Date: Thu, 25 May 2023 16:51:00 +0100 Message-Id: <20230525155102.87353-2-dhowells@redhat.com> In-Reply-To: <20230525155102.87353-1-dhowells@redhat.com> References: <20230525155102.87353-1-dhowells@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 X-Rspam-User: X-Stat-Signature: p814jgoxeeeaxnimh3j9idkn5pp347a8 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 2218420013 X-HE-Tag: 1685029877-267902 X-HE-Meta: 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 Dem0VyWB 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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 the ZERO_PAGE unpinned if it extracts a pointer to it from the page tables and make unpin_user_page*() correspondingly ignore the ZERO_PAGE when unpinning. We don't want to risk overrunning the zero page's refcount as we're only allowed ~2 million pins on it - something that userspace can conceivably trigger. Signed-off-by: David Howells cc: Christoph Hellwig cc: David Hildenbrand 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 --- mm/gup.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) diff --git a/mm/gup.c b/mm/gup.c index bbe416236593..d2662aa8cf01 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 (page == ZERO_PAGE(0) || + !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 (page == ZERO_PAGE(0)) + return page_folio(ZERO_PAGE(0)); + /* * 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 (folio == page_folio(ZERO_PAGE(0))) + 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 (page == ZERO_PAGE(0)) + 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 the 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) @@ -3161,6 +3181,9 @@ 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 the 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)