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Message-ID: <20190408023746.16916-1-sjhuang@iluvatar.ai> Date: Mon, 8 Apr 2019 10:37:45 +0800 From: Huang Shijie <sjhuang@...vatar.ai> To: <akpm@...ux-foundation.org> CC: <william.kucharski@...cle.com>, <ira.weiny@...el.com>, <palmer@...ive.com>, <axboe@...nel.dk>, <keescook@...omium.org>, <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>, Huang Shijie <sjhuang@...vatar.ai> Subject: [PATCH 1/2] mm/gup.c: fix the wrong comments When CONFIG_HAVE_GENERIC_GUP is defined, the kernel will use its own get_user_pages_fast(). In the following scenario, we will may meet the bug in the DMA case: ..................... get_user_pages_fast(start,,, pages); ...... sg_alloc_table_from_pages(, pages, ...); ..................... The root cause is that sg_alloc_table_from_pages() requires the page order to keep the same as it used in the user space, but get_user_pages_fast() will mess it up. So change the comments, and make it more clear for the driver users. Signed-off-by: Huang Shijie <sjhuang@...vatar.ai> --- mm/gup.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 22acdd0f79ff..fb11ff90ba3b 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1129,10 +1129,6 @@ EXPORT_SYMBOL(get_user_pages_locked); * with: * * get_user_pages_unlocked(tsk, mm, ..., pages); - * - * It is functionally equivalent to get_user_pages_fast so - * get_user_pages_fast should be used instead if specific gup_flags - * (e.g. FOLL_FORCE) are not required. */ long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages, struct page **pages, unsigned int gup_flags) @@ -2147,6 +2143,10 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write, * If not successful, it will fall back to taking the lock and * calling get_user_pages(). * + * Note this routine may fill the pages array with entries in a + * different order than get_user_pages_unlocked(), which may cause + * issues for callers expecting the routines to be equivalent. + * * Returns number of pages pinned. This may be fewer than the number * requested. If nr_pages is 0 or negative, returns 0. If no pages * were pinned, returns -errno. -- 2.17.1
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