From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: [patch 038/155] mm/gup: split get_user_pages_remote() into two routines Date: Wed, 01 Apr 2020 21:05:10 -0700 Message-ID: <20200402040510.BXlTrn0dg%akpm@linux-foundation.org> References: <20200401210155.09e3b9742e1c6e732f5a7250@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Return-path: Received: from mail.kernel.org ([198.145.29.99]:51966 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725789AbgDBEFN (ORCPT ); Thu, 2 Apr 2020 00:05:13 -0400 In-Reply-To: <20200401210155.09e3b9742e1c6e732f5a7250@linux-foundation.org> Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: akpm@linux-foundation.org, corbet@lwn.net, dan.j.williams@intel.com, david@fromorbit.com, hch@infradead.org, ira.weiny@intel.com, jack@suse.cz, jgg@ziepe.ca, jglisse@redhat.com, jhubbard@nvidia.com, kirill.shutemov@linux.intel.com, linux-mm@kvack.org, mhocko@suse.com, mike.kravetz@oracle.com, mm-commits@vger.kernel.org, shuah@kernel.org, torvalds@linux-foundation.org, vbabka@suse.cz, viro@zeniv.linux.org.uk, willy@infradead.org =46rom: John Hubbard Subject: mm/gup: split get_user_pages_remote() into two routines Patch series "mm/gup: track FOLL_PIN pages", v6. This activates tracking of FOLL_PIN pages. This is in support of fixing the get_user_pages()+DMA problem described in [1]-[4]. FOLL_PIN support is now in the main linux tree. However, the patch to use FOLL_PIN to track pages was *not* submitted, because Leon saw an RDMA test suite failure that involved (I think) page refcount overflows when huge pages were used. This patch definitively solves that kind of overflow problem, by adding an exact pincount, for compound pages (of order > 1), in the 3rd struct page of a compound page. If available, that form of pincounting is used, instead of the GUP_PIN_COUNTING_BIAS approach. Thanks again to Jan Kara for that idea. Other interesting changes: * dump_page(): added one, or two new things to report for compound pages: head refcount (for all compound pages), and map_pincount (for compound pages of order > 1). * Documentation/core-api/pin_user_pages.rst: removed the "TODO" for the huge page refcount upper limit problems, and added notes about how it works now. Also added a note about the dump_page() enhancements. * Added some comments in gup.c and mm.h, to explain that there are two ways to count pinned pages: exact (for compound pages of order > 1) and fuzzy (GUP_PIN_COUNTING_BIAS: for all other pages). =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D General notes about the tracking patch: This is a prerequisite to solving the problem of proper interactions between file-backed pages, and [R]DMA activities, as discussed in [1], [2], [3], [4] and in a remarkable number of email threads since about 2017. :) In contrast to earlier approaches, the page tracking can be incrementally applied to the kernel call sites that, until now, have been simply calling get_user_pages() ("gup"). In other words, opt-in by changing from this: get_user_pages() (sets FOLL_GET) put_page() to this: pin_user_pages() (sets FOLL_PIN) unpin_user_page() =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Future steps: * Convert more subsystems from get_user_pages() to pin_user_pages(). The first probably needs to be bio/biovecs, because any filesystem testing is too difficult without those in place. * Change VFS and filesystems to respond appropriately when encountering dma-pinned pages. * Work with Ira and others to connect this all up with file system leases. [1] Some slow progress on get_user_pages() (Apr 2, 2019): https://lwn.net/Articles/784574/ [2] DMA and get_user_pages() (LPC: Dec 12, 2018): https://lwn.net/Articles/774411/ [3] The trouble with get_user_pages() (Apr 30, 2018): https://lwn.net/Articles/753027/ [4] LWN kernel index: get_user_pages() https://lwn.net/Kernel/Index/#Memory_management-get_user_pages This patch (of 12): An upcoming patch requires reusing the implementation of get_user_pages_remote(). Split up get_user_pages_remote() into an outer routine that checks flags, and an implementation routine that will be reused. This makes subsequent changes much easier to understand. There should be no change in behavior due to this patch. Link: http://lkml.kernel.org/r/20200211001536.1027652-2-jhubbard@nvidia.com Signed-off-by: John Hubbard Acked-by: Kirill A. Shutemov Reviewed-by: Jan Kara Cc: Ira Weiny Cc: J=C3=A9r=C3=B4me Glisse Cc: "Matthew Wilcox (Oracle)" Cc: Al Viro Cc: Christoph Hellwig Cc: Dan Williams Cc: Dave Chinner Cc: Jason Gunthorpe Cc: Jonathan Corbet Cc: Michal Hocko Cc: Mike Kravetz Cc: Shuah Khan Cc: Vlastimil Babka Signed-off-by: Andrew Morton --- mm/gup.c | 56 +++++++++++++++++++++++++++++++---------------------- 1 file changed, 33 insertions(+), 23 deletions(-) --- a/mm/gup.c~mm-gup-split-get_user_pages_remote-into-two-routines +++ a/mm/gup.c @@ -1557,6 +1557,37 @@ static __always_inline long __gup_longte } #endif /* CONFIG_FS_DAX || CONFIG_CMA */ =20 +#ifdef CONFIG_MMU +static long __get_user_pages_remote(struct task_struct *tsk, + struct mm_struct *mm, + unsigned long start, unsigned long nr_pages, + unsigned int gup_flags, struct page **pages, + struct vm_area_struct **vmas, int *locked) +{ + /* + * Parts of FOLL_LONGTERM behavior are incompatible with + * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on + * vmas. However, this only comes up if locked is set, and there are + * callers that do request FOLL_LONGTERM, but do not set locked. So, + * allow what we can. + */ + if (gup_flags & FOLL_LONGTERM) { + if (WARN_ON_ONCE(locked)) + return -EINVAL; + /* + * This will check the vmas (even if our vmas arg is NULL) + * and return -ENOTSUPP if DAX isn't allowed in this case: + */ + return __gup_longterm_locked(tsk, mm, start, nr_pages, pages, + vmas, gup_flags | FOLL_TOUCH | + FOLL_REMOTE); + } + + return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas, + locked, + gup_flags | FOLL_TOUCH | FOLL_REMOTE); +} + /* * get_user_pages_remote() - pin user pages in memory * @tsk: the task_struct to use for page fault accounting, or @@ -1619,7 +1650,6 @@ static __always_inline long __gup_longte * should use get_user_pages because it cannot pass * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault. */ -#ifdef CONFIG_MMU long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm, unsigned long start, unsigned long nr_pages, unsigned int gup_flags, struct page **pages, @@ -1632,28 +1662,8 @@ long get_user_pages_remote(struct task_s if (WARN_ON_ONCE(gup_flags & FOLL_PIN)) return -EINVAL; =20 - /* - * Parts of FOLL_LONGTERM behavior are incompatible with - * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on - * vmas. However, this only comes up if locked is set, and there are - * callers that do request FOLL_LONGTERM, but do not set locked. So, - * allow what we can. - */ - if (gup_flags & FOLL_LONGTERM) { - if (WARN_ON_ONCE(locked)) - return -EINVAL; - /* - * This will check the vmas (even if our vmas arg is NULL) - * and return -ENOTSUPP if DAX isn't allowed in this case: - */ - return __gup_longterm_locked(tsk, mm, start, nr_pages, pages, - vmas, gup_flags | FOLL_TOUCH | - FOLL_REMOTE); - }