linux-kernel - Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero

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Message-Id: <20190904114820.42d9c4daf445ded3d0da52ab@linux-foundation.org>
Date:   Wed, 4 Sep 2019 11:48:20 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     zhong jiang <zhongjiang@...wei.com>, mhocko@...nel.org,
        anshuman.khandual@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero

On Wed, 4 Sep 2019 13:24:58 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:

> On 9/4/19 12:26 PM, zhong jiang wrote:
> > With the help of unsigned_lesser_than_zero.cocci. Unsigned 'nr_pages"'
> > compare with zero. And __get_user_pages_locked will return an long value.
> > Hence, Convert the long to compare with zero is feasible.
> 
> It would be nicer if the parameter nr_pages was long again instead of unsigned
> long (note there are two variants of the function, so both should be changed).

nr_pages should be unsigned - it's a count of pages!

The bug is that __get_user_pages_locked() returns a signed long which
can be a -ve errno.

I think it's best if __get_user_pages_locked() is to get itself a new
local with the same type as its return value.  Something like:

--- a/mm/gup.c~a
+++ a/mm/gup.c
@@ -1450,6 +1450,7 @@ static long check_and_migrate_cma_pages(
 	bool drain_allow = true;
 	bool migrate_allow = true;
 	LIST_HEAD(cma_page_list);
+	long ret;
 
 check_again:
 	for (i = 0; i < nr_pages;) {
@@ -1511,17 +1512,18 @@ check_again:
 		 * again migrating any new CMA pages which we failed to isolate
 		 * earlier.
 		 */
-		nr_pages = __get_user_pages_locked(tsk, mm, start, nr_pages,
+		ret = __get_user_pages_locked(tsk, mm, start, nr_pages,
 						   pages, vmas, NULL,
 						   gup_flags);
 
-		if ((nr_pages > 0) && migrate_allow) {
+		nr_pages = ret;
+		if (ret > 0 && migrate_allow) {
 			drain_allow = true;
 			goto check_again;
 		}
 	}
 
-	return nr_pages;
+	return ret;
 }
 #else
 static long check_and_migrate_cma_pages(struct task_struct *tsk,