[PATCH 1/2] Revert "vmalloc: back off when the current task is killed"

From: Johannes Weiner <hannes@cmpxchg.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Alan Cox <alan@llwyncelyn.cymru>, Christoph Hellwig <hch@lst.de>,
	Michal Hocko <mhocko@suse.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com
Subject: [PATCH 1/2] Revert "vmalloc: back off when the current task is killed"
Date: Wed, 4 Oct 2017 14:59:06 -0400	[thread overview]
Message-ID: <20171004185906.GB2136@cmpxchg.org> (raw)
In-Reply-To: <20171004185813.GA2136@cmpxchg.org>

This reverts commit 5d17a73a2ebeb8d1c6924b91e53ab2650fe86ffb and
commit 171012f561274784160f666f8398af8b42216e1f.

5d17a73a2ebe ("vmalloc: back off when the current task is killed")
made all vmalloc allocations from a signal-killed task fail. We have
seen crashes in the tty driver from this, where a killed task exiting
tries to switch back to N_TTY, fails n_tty_open because of the vmalloc
failing, and later crashes when dereferencing tty->disc_data.

Arguably, relying on a vmalloc() call to succeed in order to properly
exit a task is not the most robust way of doing things. There will be
a follow-up patch to the tty code to fall back to the N_NULL ldisc.

But the justification to make that vmalloc() call fail like this isn't
convincing, either. The patch mentions an OOM victim exhausting the
memory reserves and thus deadlocking the machine. But the OOM killer
is only one, improbable source of fatal signals. It doesn't make sense
to fail allocations preemptively with plenty of memory in most cases.

The patch doesn't mention real-life instances where vmalloc sites
would exhaust memory, which makes it sound more like a theoretical
issue to begin with. But just in case, the OOM access to memory
reserves has been restricted on the allocator side in cd04ae1e2dc8
("mm, oom: do not rely on TIF_MEMDIE for memory reserves access"),
which should take care of any theoretical concerns on that front.

Revert this patch, and the follow-up that suppresses the allocation
warnings when we fail the allocations due to a signal.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/vmalloc.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 8a43db6284eb..673942094328 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1695,11 +1695,6 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 	for (i = 0; i < area->nr_pages; i++) {
 		struct page *page;
 
-		if (fatal_signal_pending(current)) {
-			area->nr_pages = i;
-			goto fail_no_warn;
-		}
-
 		if (node == NUMA_NO_NODE)
 			page = alloc_page(alloc_mask|highmem_mask);
 		else
@@ -1723,7 +1718,6 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 	warn_alloc(gfp_mask, NULL,
 			  "vmalloc: allocation failure, allocated %ld of %ld bytes",
 			  (area->nr_pages*PAGE_SIZE), area->size);
-fail_no_warn:
 	vfree(area->addr);
 	return NULL;
 }
-- 
2.14.1
