linux-kernel - Re: [PATCHv2 1/2] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages

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Date:   Tue, 4 Jun 2019 00:08:08 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Pingfan Liu <kernelfans@...il.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Keith Busch <keith.busch@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 1/2] mm/gup: fix omission of check on FOLL_LONGTERM in
 get_user_pages_fast()

On Mon, Jun 03, 2019 at 04:56:10PM -0700, Ira Weiny wrote:
> On Mon, Jun 03, 2019 at 09:42:06AM -0700, Christoph Hellwig wrote:
> > > +#if defined(CONFIG_CMA)
> > 
> > You can just use #ifdef here.
> > 
> > > +static inline int reject_cma_pages(int nr_pinned, unsigned int gup_flags,
> > > +	struct page **pages)
> > 
> > Please use two instead of one tab to indent the continuing line of
> > a function declaration.
> > 
> > > +{
> > > +	if (unlikely(gup_flags & FOLL_LONGTERM)) {
> > 
> > IMHO it would be a little nicer if we could move this into the caller.
> 
> FWIW we already had this discussion and thought it better to put this here.
> 
> https://lkml.org/lkml/2019/5/30/1565

I don't see any discussion like this.  FYI, this is what I mean,
code might be easier than words:


diff --git a/mm/gup.c b/mm/gup.c
index ddde097cf9e4..62d770b18e2c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2197,6 +2197,27 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
 	return ret;
 }
 
+#ifdef CONFIG_CMA
+static int reject_cma_pages(struct page **pages, int nr_pinned)
+{
+	int i = 0;
+
+	for (i = 0; i < nr_pinned; i++)
+		if (is_migrate_cma_page(pages[i])) {
+			put_user_pages(pages + i, nr_pinned - i);
+			return i;
+		}
+	}
+
+	return nr_pinned;
+}
+#else
+static inline int reject_cma_pages(struct page **pages, int nr_pinned)
+{
+	return nr_pinned;
+}
+#endif /* CONFIG_CMA */
+
 /**
  * get_user_pages_fast() - pin user pages in memory
  * @start:	starting user address
@@ -2237,6 +2258,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 		ret = nr;
 	}
 
+	if (nr && unlikely(gup_flags & FOLL_LONGTERM))
+		nr = reject_cma_pages(pages, nr);
+
 	if (nr < nr_pages) {
 		/* Try to get the remaining pages with get_user_pages */
 		start += nr << PAGE_SHIFT;