From: Vlastimil Babka <vbabka@suse.cz>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, Michal Hocko <mhocko@kernel.org>,
netdev@vger.kernel.org, Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH v2] fs/select: add vmalloc fallback for select(2)
Date: Tue, 27 Sep 2016 10:06:24 +0200 [thread overview]
Message-ID: <91be8fd4-6600-d58d-d77a-d06ebed79f7e@suse.cz> (raw)
In-Reply-To: <20160926170105.517f74cd67ecdd5ef73e1865@linux-foundation.org>
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.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2016-09-27 8:06 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-22 16:43 [PATCH v2] fs/select: add vmalloc fallback for select(2) Vlastimil Babka
2016-09-22 16:49 ` Eric Dumazet
2016-09-22 16:56 ` Vlastimil Babka
2016-09-22 17:07 ` Eric Dumazet
2016-09-22 17:55 ` Vlastimil Babka
2016-09-23 9:42 ` David Laight
2016-09-23 9:58 ` Vlastimil Babka
2016-09-23 13:35 ` David Laight
2016-09-26 10:01 ` Vlastimil Babka
2016-09-26 15:02 ` David Laight
2016-09-25 18:50 ` Andi Kleen
2016-09-27 0:01 ` Andrew Morton
2016-09-27 1:38 ` Eric Dumazet
2016-09-27 8:13 ` Vlastimil Babka
2016-09-27 13:34 ` Eric Dumazet
2016-09-27 8:06 ` Vlastimil Babka [this message]
2016-09-27 8:45 ` [PATCH v3] " Vlastimil Babka
2016-09-27 10:22 ` Michal Hocko
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=91be8fd4-6600-d58d-d77a-d06ebed79f7e@suse.cz \
--to=vbabka@suse.cz \
--cc=akpm@linux-foundation.org \
--cc=eric.dumazet@gmail.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@kernel.org \
--cc=netdev@vger.kernel.org \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).