6.1-stable review patch. If anyone has any objections, please let me know.
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From: David Howells dhowells@redhat.com
[ Upstream commit c8070b78751955e59b42457b974bea4a4fe00187 ]
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 dhowells@redhat.com cc: Christoph Hellwig hch@infradead.org cc: David Hildenbrand david@redhat.com cc: Lorenzo Stoakes lstoakes@gmail.com cc: Andrew Morton akpm@linux-foundation.org cc: Jens Axboe axboe@kernel.dk cc: Al Viro viro@zeniv.linux.org.uk cc: Matthew Wilcox willy@infradead.org cc: Jan Kara jack@suse.cz cc: Jeff Layton jlayton@kernel.org cc: Jason Gunthorpe jgg@nvidia.com cc: Logan Gunthorpe logang@deltatee.com cc: Hillf Danton hdanton@sina.com cc: Christian Brauner brauner@kernel.org cc: Linus Torvalds torvalds@linux-foundation.org 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 lstoakes@gmail.com Reviewed-by: Christoph Hellwig hch@lst.de Acked-by: David Hildenbrand david@redhat.com Link: https://lore.kernel.org/r/20230526214142.958751-2-dhowells@redhat.com Signed-off-by: Jens Axboe axboe@kernel.dk Stable-dep-of: bddf10d26e6e ("uprobes: Reject the shared zeropage in uprobe_write_opcode()") Signed-off-by: Sasha Levin sashal@kernel.org --- 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 b18416f4500fe..7995ce2b9676a 100644 --- a/Documentation/core-api/pin_user_pages.rst +++ b/Documentation/core-api/pin_user_pages.rst @@ -113,6 +113,12 @@ 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 971186f0b7b07..03357c196e0ba 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1610,6 +1610,28 @@ 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) @@ -1620,8 +1642,8 @@ 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 e31d00443c4e6..b1daaa9d89aab 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); @@ -128,6 +129,13 @@ 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 @@ -177,6 +185,8 @@ 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_ptr(folio)); @@ -217,6 +227,13 @@ bool __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, @@ -3149,6 +3166,9 @@ 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) @@ -3225,6 +3245,9 @@ EXPORT_SYMBOL_GPL(pin_user_pages_fast_only); * * 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, @@ -3260,6 +3283,9 @@ 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, @@ -3282,6 +3308,9 @@ 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)