Hi,
On 09/23/2016 03:24 AM, Nicholas Piggin wrote:
On Fri, 23 Sep 2016 14:42:53 +0800
"Hillf Danton" <hillf.zj@xxxxxxxxxxxxxxx> 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.
How about something like this? (untested)
Eric isn't wrong about vmalloc sucking :)
Thanks,
Nick
---
fs/select.c | 57 +++++++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 43 insertions(+), 14 deletions(-)
diff --git a/fs/select.c b/fs/select.c
index 8ed9da5..3b4834c 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -555,6 +555,7 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
void *bits;
int ret, max_fds;
unsigned int size;
+ size_t nr_bytes;
struct fdtable *fdt;
/* Allocate small arguments on the stack to save memory and be faster */
long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
@@ -576,21 +577,39 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
* since we used fdset we need to allocate memory in units of
* long-words.
*/
- size = FDS_BYTES(n);
+ ret = -ENOMEM;
bits = stack_fds;
- if (size > sizeof(stack_fds) / 6) {
- /* Not enough space in on-stack array; must use kmalloc */
+ size = FDS_BYTES(n);
+ nr_bytes = 6 * size;
+
+ if (unlikely(nr_bytes > PAGE_SIZE)) {
+ /* Avoid multi-page allocation if possible */
ret = -ENOMEM;
- bits = kmalloc(6 * size, GFP_KERNEL);
- if (!bits)
- goto out_nofds;
+ fds.in = kmalloc(size, GFP_KERNEL);
+ fds.out = kmalloc(size, GFP_KERNEL);
+ fds.ex = kmalloc(size, GFP_KERNEL);
+ fds.res_in = kmalloc(size, GFP_KERNEL);
+ fds.res_out = kmalloc(size, GFP_KERNEL);
+ fds.res_ex = kmalloc(size, GFP_KERNEL);
+
+ if (!(fds.in && fds.out && fds.ex &&
+ fds.res_in && fds.res_out && fds.res_ex))
+ goto out;
+ } else {
+ if (nr_bytes > sizeof(stack_fds)) {
+ /* Not enough space in on-stack array */
+ if (nr_bytes > PAGE_SIZE * 2)
The 'if' looks extraneous?
Also, I wonder if we can just avoid some allocations altogether by
checking by if the user fd_set pointers are NULL? That can avoid failures :)
Thanks,
-Jason
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