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SubjectRe: [PATCH 1/2] Revert "vmalloc: back off when the current task is killed"
On Wed 04-10-17 14:59:06, Johannes Weiner wrote:
> 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>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 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

--
Michal Hocko
SUSE Labs

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