Re: [PATCH v3] fs/select: add vmalloc fallback for select(2)

[Michal Hocko]

On Tue 27-09-16 10:45:36, Vlastimil Babka wrote:
> 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@xxxxxxxxx: fix failure path logic]
> [akpm@xxxxxxxxxxxxxxxxxxxx: use proper type for size]
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Yes this makes sense to me. It could be argued that this could be
simplified to not rely on high order allocations at all but this is
simple enough (and backportable to stable trees) and should work
reasonably well.

So FWIW
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

I would even argue to use __GFP_NORETRY for size > PAGE_SIZE because
giving a userspace an access to high order pages which can invoke OOM
killer is not a great idea. Something for a separate patch though.

> ---
>  fs/select.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/select.c b/fs/select.c
> index 8ed9da50896a..3d4f85defeab 100644
> --- 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>
>  
> @@ -554,7 +555,7 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
>  	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)];
> @@ -581,7 +582,14 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
>  	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;
>  	}
> @@ -618,7 +626,7 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
>  
>  out:
>  	if (bits != stack_fds)
> -		kfree(bits);
> +		kvfree(bits);
>  out_nofds:
>  	return ret;
>  }
> -- 
> 2.10.0
> 
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-- 
Michal Hocko
SUSE Labs