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    SubjectRe: [PATCH v2] fs/select: add vmalloc fallback for select(2)
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    On 09/27/2016 02:01 AM, Andrew Morton wrote:
    > On Thu, 22 Sep 2016 18:43:59 +0200 Vlastimil Babka <vbabka@suse.cz> 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. Also the allocation is temporary for the duration of the
    >> syscall, so it's unlikely to stress vmalloc too much.
    >>
    >> 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.
    >>
    >> ...
    >>
    >> --- 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>
    >>
    >> @@ -558,6 +559,7 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
    >> struct fdtable *fdt;
    >> /* Allocate small arguments on the stack to save memory and be faster */
    >> long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
    >> + unsigned long alloc_size;
    >>
    >> ret = -EINVAL;
    >> if (n < 0)
    >> @@ -580,8 +582,12 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
    >> bits = stack_fds;
    >> if (size > sizeof(stack_fds) / 6) {
    >> /* Not enough space in on-stack array; must use kmalloc */
    >> + alloc_size = 6 * size;
    >
    > Well. `size' is `unsigned'. The multiplication will be done as 32-bit
    > so there was no point in making `alloc_size' unsigned long.

    Uh, right. Thanks.

    > So can we tighten up the types in this function? size_t might make
    > sense, but vmalloc() takes a ulong.

    Let's do size_t then, as the conversion to ulong is safe.


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