4.4-stable review patch. If anyone has any objections, please let me know.
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From: Vlastimil Babka vbabka@suse.cz
commit 2d19309cf86883f634a4f8ec55a54bda87db19bf upstream.
The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows with the number of fds passed. We had a customer report page allocation failures of order-4 for this allocation. This is a costly order, so it might easily fail, as the VM expects such allocation to have a lower-order fallback.
Such trivial fallback is vmalloc(), as the memory doesn't have to be physically contiguous and the allocation is temporary for the duration of the syscall only. There were some concerns, whether this would have negative impact on the system by exposing vmalloc() to userspace. Although an excessive use of vmalloc can cause some system wide performance issues - TLB flushes etc. - a large order allocation is not for free either and an excessive reclaim/compaction can have a similar effect. Also note that the size is effectively limited by RLIMIT_NOFILE which defaults to 1024 on the systems I checked. That means the bitmaps will fit well within single page and thus the vmalloc() fallback could be only excercised for processes where root allows a higher limit.
Note that the poll(2) syscall seems to use a linked list of order-0 pages, so it doesn't need this kind of fallback.
[eric.dumazet@gmail.com: fix failure path logic] [akpm@linux-foundation.org: use proper type for size] Link: http://lkml.kernel.org/r/20160927084536.5923-1-vbabka@suse.cz Signed-off-by: Vlastimil Babka vbabka@suse.cz Acked-by: Michal Hocko mhocko@suse.com Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Eric Dumazet eric.dumazet@gmail.com Cc: David Laight David.Laight@ACULAB.COM Cc: Hillf Danton hillf.zj@alibaba-inc.com Cc: Nicholas Piggin npiggin@gmail.com Cc: Jason Baron jbaron@akamai.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/select.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)
--- a/fs/select.c +++ b/fs/select.c @@ -29,6 +29,7 @@ #include <linux/sched/rt.h> #include <linux/freezer.h> #include <net/busy_poll.h> +#include <linux/vmalloc.h>
#include <asm/uaccess.h>
@@ -550,7 +551,7 @@ int core_sys_select(int n, fd_set __user fd_set_bits fds; void *bits; int ret, max_fds; - unsigned int size; + size_t size, alloc_size; struct fdtable *fdt; /* Allocate small arguments on the stack to save memory and be faster */ long stack_fds[SELECT_STACK_ALLOC/sizeof(long)]; @@ -577,7 +578,14 @@ int core_sys_select(int n, fd_set __user if (size > sizeof(stack_fds) / 6) { /* Not enough space in on-stack array; must use kmalloc */ ret = -ENOMEM; - bits = kmalloc(6 * size, GFP_KERNEL); + if (size > (SIZE_MAX / 6)) + goto out_nofds; + + alloc_size = 6 * size; + bits = kmalloc(alloc_size, GFP_KERNEL|__GFP_NOWARN); + if (!bits && alloc_size > PAGE_SIZE) + bits = vmalloc(alloc_size); + if (!bits) goto out_nofds; } @@ -614,7 +622,7 @@ int core_sys_select(int n, fd_set __user
out: if (bits != stack_fds) - kfree(bits); + kvfree(bits); out_nofds: return ret; }