[RFC PATCH 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()
From: David Howells <dhowells@redhat.com>
Date: 2023-05-25 15:52:18
Also in:
linux-fsdevel, linux-mm, lkml
Subsystem:
memory management, memory management - gup (get user pages), the rest · Maintainers:
Andrew Morton, David Hildenbrand, Linus Torvalds
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 <dhowells@redhat.com> cc: Christoph Hellwig <hch@infradead.org> cc: David Hildenbrand <redacted> 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 <redacted> 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 --- 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)